Cell Cycle and Cell Division
SUMMARY**** According to the cell theory, cells arise from preexisting cells. The process by which this occurs is called cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.
The stages through which a cell passes from one division to the next is called the cell cycle. Cell cycle is divided into two phases called (i) Interphase – a period of preparation for cell division, and (ii) Mitosis (M phase) – the actual period of cell division. Interphase is further subdivided into G1 , S and G2 . G1 phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase. S phase marks the phase of DNA replication and chromosome duplication. G2 phase is the period of cytoplasmic growth. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing. Metaphase is marked by the alignment of chromosomes at the equatorial plate. During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase. Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.
In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent. Meiosis is divided into two phases – meiosis I and meiosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over. Meiosis I has a long prophase, which is divided further into five phases. These are leptotene, zygotene, pachytene, diplotene and diakinesis. During metaphase I the bivalents arrange on the equatorial plate. This is followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell. In telophase I, the nuclear membrane and nucleolus reappear. Meiosis II is similar to mitosis. During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.
The stages through which a cell passes from one division to the next is called the cell cycle. Cell cycle is divided into two phases called (i) Interphase – a period of preparation for cell division, and (ii) Mitosis (M phase) – the actual period of cell division. Interphase is further subdivided into G1 , S and G2 . G1 phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase. S phase marks the phase of DNA replication and chromosome duplication. G2 phase is the period of cytoplasmic growth. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing. Metaphase is marked by the alignment of chromosomes at the equatorial plate. During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase. Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.
In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent. Meiosis is divided into two phases – meiosis I and meiosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over. Meiosis I has a long prophase, which is divided further into five phases. These are leptotene, zygotene, pachytene, diplotene and diakinesis. During metaphase I the bivalents arrange on the equatorial plate. This is followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell. In telophase I, the nuclear membrane and nucleolus reappear. Meiosis II is similar to mitosis. During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.
Rudolf Virchow gave omnis cellula e cellula which means new cells arise from the pre-existing cells.
EXERCISES **1. What is the average cell cycle span for a mammalian cell? 2. Distinguish cytokinesis from karyokinesis. 3. Describe the events taking place during interphase. 4. What is Go (quiescent phase) of cell cycle? 5. Why is mitosis called equational division? 6. Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator. (ii) Centromere splits and chromatids separate. (iii) Pairing between homologous chromosomes takes place. (iv) Crossing over between homologous chromosomes takes place. 7. Describe the following: (a) synapsis; (b) bivalent; (c) chiasmata; Draw a diagram to illustrate your answer. 8. How does cytokinesis in plant cells differ from that in animal cells? 9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size. 10. Distinguish anaphase of mitosis from anaphase I of meiosis. 11. List the main differences between mitosis and meiosis. 12. What is the significance of meiosis? 13. Discuss with your teacher about: (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur. 14. Can there be mitosis without DNA replication in ‘S’ phase? 15. Can there be DNA replication without cell division? 16. Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell; (ii) amount of DNA content (C) per cell
Cell Cycle:
The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle.
The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle.
Phases of Cell Cycle:
Eukaryotic cell divide once in every 24 hours (duration may vary from organism to organism)Yeast completes cell cycle in 90 min.
Cell Cycle has two phasesi. Interphaseii. M Phase (Mitosis phase)
Interphase is the phase between two successive M phases. M phase is when the actual cell division or mitosis occurs.
In 24 hour duration, only one hour is for cell division and interphase lasts more than 95% of cell cycle.
The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis). The interphase (resting phase) is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
Interphase is divided into three further phases:* G1 phase (Gap 1)* S phase (Synthesis)* G2 phase (Gap 2)
G1 phase is the interval between mitosis and initiation of DNA replication. In G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
S or synthesis phase is the period during which DNA synthesis or replication takes place. The amount of DNA per cell doubles. If the initial amount of DNA is 2C then it increases to 4C. But no increase in the chromosome number; if the cell had diploid (2n) number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues.
Eukaryotic cell divide once in every 24 hours (duration may vary from organism to organism)Yeast completes cell cycle in 90 min.
Cell Cycle has two phasesi. Interphaseii. M Phase (Mitosis phase)
Interphase is the phase between two successive M phases. M phase is when the actual cell division or mitosis occurs.
In 24 hour duration, only one hour is for cell division and interphase lasts more than 95% of cell cycle.
The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis). The interphase (resting phase) is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
Interphase is divided into three further phases:* G1 phase (Gap 1)* S phase (Synthesis)* G2 phase (Gap 2)
G1 phase is the interval between mitosis and initiation of DNA replication. In G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA.
S or synthesis phase is the period during which DNA synthesis or replication takes place. The amount of DNA per cell doubles. If the initial amount of DNA is 2C then it increases to 4C. But no increase in the chromosome number; if the cell had diploid (2n) number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues.
Some cells (adult animals) do not exhibit division (e.g., heart cells) and many other cells divide only occasionally (to replace cells that have been lost because of injury or cell death).
Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic cells. Some haploid cells divide by mitosis, for example, male honey bees. Plants can show mitotic divisions in both haploid and diploid cells.
Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic cells. Some haploid cells divide by mitosis, for example, male honey bees. Plants can show mitotic divisions in both haploid and diploid cells.
Some cells (adult animals) do not exhibit division (e.g., heart cells) and many other cells divide only occasionally (to replace cells that have been lost because of injury or cell death).
Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic cells. Some haploid cells divide by mitosis, for example, male honey bees. Plants can show mitotic divisions in both haploid and diploid cells.
Cells that do not divide further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic cells. Some haploid cells divide by mitosis, for example, male honey bees. Plants can show mitotic divisions in both haploid and diploid cells.
M Phase:
The number of chromosomes in the parent and progeny cells is the same, it is also called as equational division. Mitosis has been divided into four stages (for convenience) of nuclear division (karyokinesis).
Cell division is a progressive process and no clear-cut lines between various stages.
Karyokinesis involves following four stages:
*Prophase *Metaphase*Anaphase *Telophase
The number of chromosomes in the parent and progeny cells is the same, it is also called as equational division. Mitosis has been divided into four stages (for convenience) of nuclear division (karyokinesis).
Cell division is a progressive process and no clear-cut lines between various stages.
Karyokinesis involves following four stages:
*Prophase *Metaphase*Anaphase *Telophase
Prophase:
Prophase is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase. In the S and G2 phases the new DNA molecules formed are not distinct but intertwined.
Prophase is the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.
Prophase is completed when:
*Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
*Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.
Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope
Prophase is the first stage of karyokinesis of mitosis follows the S and G2 phases of interphase. In the S and G2 phases the new DNA molecules formed are not distinct but intertwined.
Prophase is the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.
Prophase is completed when:
*Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
*Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.
Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope
Metaphase
The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell.
By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied.
At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell.
Metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
The key features of metaphase are:
*Spindle fibres attach to kinetochores of chromosomes. *Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.
The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell.
By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied.
At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell.
Metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
The key features of metaphase are:
*Spindle fibres attach to kinetochores of chromosomes. *Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.
Anaphase
At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles.
As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.
Thus, anaphase stage is characterised by the following key events:
*Centromeres split and chromatids separate. *Chromatids move to opposite poles.
At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles.
As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.
Thus, anaphase stage is characterised by the following key events:
*Centromeres split and chromatids separate. *Chromatids move to opposite poles.
Telophase
At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality.
The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles
This is the stage which shows the following key events:
*Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
*Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.
*Nucleolus, golgi complex and ER reform.
At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality.
The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles
This is the stage which shows the following key events:
*Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
*Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.
*Nucleolus, golgi complex and ER reform.
Cytokinesis
Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed
In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.
Plant cells however, are enclosed by a relatively inextensible cell wall, therefore, they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).
Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed
In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.
Plant cells however, are enclosed by a relatively inextensible cell wall, therefore, they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).
Significance of Mitosis
Mitosis or the equational division is usually restricted to the diploid cells only.
However, in some lower plants and in some social insects haploid cells also divide by mitosis.
It is very essential to understand the significance of this division in the life of an organism.
Mitosis usually results in the production of diploid daughter cells with identical genetic complement. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.
It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.
Mitosis or the equational division is usually restricted to the diploid cells only.
However, in some lower plants and in some social insects haploid cells also divide by mitosis.
It is very essential to understand the significance of this division in the life of an organism.
Mitosis usually results in the production of diploid daughter cells with identical genetic complement. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.
It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.
Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.
Meiosis
Sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes.
Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.
Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase.
Meiosis occurs during gametogenesis in plants and animals. This leads to the formation of haploid gametes.
The key features of meiosis are as follows:
*Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
*Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
*Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
*Four haploid cells are formed at the end of meiosis II.
-Meiotic events can be grouped under the following phases:
..Meiosis I .........Meiosis II..Prophase I........Prophase II..Metaphase I......Metaphase II..Anaphase I........Anaphase II..Telophase I.......TelophaseII
Sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes.
Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.
Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase.
Meiosis occurs during gametogenesis in plants and animals. This leads to the formation of haploid gametes.
The key features of meiosis are as follows:
*Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
*Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
*Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
*Four haploid cells are formed at the end of meiosis II.
-Meiotic events can be grouped under the following phases:
..Meiosis I .........Meiosis II..Prophase I........Prophase II..Metaphase I......Metaphase II..Anaphase I........Anaphase II..Telophase I.......TelophaseII
Meiosis I
Prophase I: Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
This is followed by the second stage of prophase I called zygotene. During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. However, these are more clearly visible at the next stage.
The first two stages of prophase I are relatively short-lived compared to the next stage that is pachytene. During this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes.
Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years. The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase.
Metaphase I: The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes.
Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres (Figure 10.3).
Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as dyad of cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived. There is no replication of DNA during interkinesis. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
Prophase I: Prophase of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
This is followed by the second stage of prophase I called zygotene. During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of complex structure called synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad. However, these are more clearly visible at the next stage.
The first two stages of prophase I are relatively short-lived compared to the next stage that is pachytene. During this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes.
Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures, are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years. The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down. Diakinesis represents transition to metaphase.
Metaphase I: The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes.
Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres (Figure 10.3).
Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as dyad of cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived. There is no replication of DNA during interkinesis. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
Meiosis II
Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis.
The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.
Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.
Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells
Prophase II: Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis.
The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.
Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.
Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells
Significance of Meiosis
Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
EXERCISES **Q1. What is the average cell cycle span for a mammalian cell? Ans. The average cell cycle span for a mammalian cell is approx 24 hours.
Q2. Distinguish cytokinesis from karyokinesis. Ans. Cytokinesis: a. Cytokinesis is the biological process involving the division of a cell's cytoplasm during mitosis or meiosisb. It is divided into four stages - prophase, metaphase, anaphase, telophase
Karyokinesis:a. Karyokinesis is the biological process involving the division of a cell's nucleus during mitosis or meiosisb. Stages such as prophase, metaphase, anaphase and telophase are not present in karyokinesis
Q3. Describe the events taking place during interphase. Ans. Interphase involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication is an orderly manner. Interphase is divided into three phases.G1 phaseS PhaseG2 Phase
G1Phase - It is the stage during which the cell grows and prepares its DNA for replication. In this phase, the cell is metabolically active.S Phase - It is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same. G2Phase - In this phase, the cell continues to grow and prepares itself for division. The proteins and RNA required for mitosis are synthesised during this stage.
Q4. What is Go (quiescent phase) of cell cycle? Ans. G0 or quiescent phase is the stage wherein cells remain metabolically active, but do not proliferate unless called to do so. Such cells are used for replacing the cells lost during injury.
Q5. Why is mitosis called equational division? Ans. Mitosis is the process of cell division wherein the chromosomes replicate and get equally distributed into two daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell i.e. diploid. Hence, mitosis is known as equational division.
Q6. Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator(ii) Centromere splits and chromatids separate. (iii) Pairing between homologous chromosomes takes place. (iv) Crossing over between homologous chromosomes takes place. Ans. (i) Chromosomes are moved to spindle equator = Metaphase(ii) Centromere splits and chromatids separate. = Anaphase(iii) Pairing between homologous chromosomes takes place = Zygotene of Meiosis I (iv) Crossing over between homologous chromosomes takes place = Pachytene of Meiosis I
Q7. Describe the following: (a) synapsis; (b) bivalent; (c) chiasmata; Draw a diagram to illustrate your answer. Ans. a. Synapsis: The pairing of homologous chromosomes is called synapsis. This occurs during the second stage of prophase I or zygotene.b. Bivalent: Bivalent or tetrad is a pair of synapsed homologous chromosomes. They are formed during the zygotene stage of prophase I of meiosis.c. Chiasmata: Chiasmata is the site where two sister chromatids have crossed over. It represents the site of cross-over. It has formed during the diplotene stage of prophase I of meiosis.
Q8. How does cytokinesis in plant cells differ from that in animal cells?Ans. Cytokinesis in plant cells:(i) The division of the cytoplasm takes place by cell plate formation.(ii) Cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls.Cytokinesis is animal cells:(i)The division of the cytoplasm takes place by cleavage. (ii)Cleavage starts at the periphery and then moves inward, dividing the cell into two parts. Q9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size. Ans. (a)Spermatogenesis or the formation of sperms in human beings occurs by the process of meiosis. It results in the formation of four equal-sized daughter cells. (b)Oogenesis or the formation of ovum in human beings occurs by the process of meiosis. It results in the formation of four daughter cells which are unequal in size.
Q10. Distinguish anaphase of mitosis from anaphase I of meiosis. Ans. Anaphase of Mitosis: Anaphase is the stage during which the centromere splits and the chromatids separate. The chromosomes move apart, toward the opposite poles. These chromosomes are genetically identicalAnaphase I of meiosis: During anaphase I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. Hence, in anaphase I, the chromosomes of each bivalent pair separate, while the sister chromatids remain together.
Q11. List the main differences between mitosis and meiosis. Ans. Mitosis1. In mitotic division, a single division results in two daughter cells. 2. Mitosis is known as equational division. This is because the daughter cells have the same diploid number of chromosomes as the parent. 3. Prophase is short and does not comprise any phase. diplotene, and diakinesis. 4. There is no pairing of chromosomes, crossing-over, or chiasmata-formation during prophase. 5. Synaptonemal complex is not formed. 6. Anaphase involves the separation of the chromatids of each chromosome. 7. Mitosis plays a significant role in the healing, repair, and growth of a cell. Meiosis1.Meiotic division involves two successive divisions –meiosis I and meiosis II. These divisions result in four daughter cells.2.Meiosis I is known as reductional division. This is because the chromosome number is reduced to half. Meiosis II is known as equational division. This is because the sister chromatids separate and the chromosome number remains the same.3.Prophase Iis very long and comprises 5 phases –leptotene, zygotene, pachytene,4.In the zygotene stage of prophase, the pairing of chromosomes occurs. During pachytene, the crossing-over occurs. The chiasmata are formed in the diplotene stage.5.Synaptonemal complex is formed during the zygotene stage of prophase I.6.During anaphase I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. During anaphase II, the chromatids separate as a result of the splitting of the centromere. 7.Meiosis brings about variation and maintains the chromosome number from generation to generation.
Q12. What is the significance of meiosis? Ans. Meiosis is the process involving the reduction in the amount of genetic material. It comprises two successive nuclear and cell divisions, with a single cycle of DNA replication. As a result, at the end of meiosis II, four haploid cells are formed.
Significance of meiosis
*Meiosis maintains the chromosome number from generation to generation. It reduces the chromosome number to half so that the process of fertilisation restores the original number in the zygote.
*Variations are caused by the cross-over andthe random distribution of homologous chromosomes between daughter cells. Variations play an important role in evolution.
*Chromosomal mutations are brought about by the introduction of certain abnormalities. These chromosomal mutations may be advantageous for an individual.
Q13. Discuss with your teacher about: (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur. Ans. i) In some insects and lower plants, fertilization is immediately followed by zygotic meiosis, which leads to the production of haploid organisms. This type of life cycle is known as haplontic life cycle.
ii) The phenomenon of polyploidy can be observed in some haploid cells in higher plants in which cell division does not occur. Polyploidy is a state in which cells contain multiple pairs of chromosomes than the basic set. Polyploidy can be artificially induced in plants by applying colichine to cell culture.
Q14. Can there be mitosis without DNA replication in ‘S’ phase? Ans.Mitotic cell division cannot take place without DNA replication in S phase. Two important events take place during S phase –one is the synthesis or duplication of DNA and the other is the duplication of the centriole. DNA duplication is important as it maintains the chromosome number in the daughter cells. Mitosis is an equational division. Therefore, the duplication of DNA is an important step.
Q15. Can there be DNA replication without cell division? Ans.There can be DNA replication without cell division. During cell division, the parent cell gets divided into two daughter cells. However, if there is a repeated replication of DNA without any cell division, then this DNA will keep accumulating inside the cell. This would increase the volume of the cell nucleus, thereby causing cell expansion. An example of DNA duplication without cell division is commonly observed in the salivary glands of Drosophila. The chromosome undergoing repeated DNA duplication is known as polytene chromosome.
Q16. Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell; (ii) amount of DNA content (C) per cell Ans. During meiosis, the number of chromosomes and the amount of DNA in a cell change.
(i)Number of chromosomes (N) per cell During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.
(ii)Amount of DNA content (C) per cell
During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present.
During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.
Q2. Distinguish cytokinesis from karyokinesis. Ans. Cytokinesis: a. Cytokinesis is the biological process involving the division of a cell's cytoplasm during mitosis or meiosisb. It is divided into four stages - prophase, metaphase, anaphase, telophase
Karyokinesis:a. Karyokinesis is the biological process involving the division of a cell's nucleus during mitosis or meiosisb. Stages such as prophase, metaphase, anaphase and telophase are not present in karyokinesis
Q3. Describe the events taking place during interphase. Ans. Interphase involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication is an orderly manner. Interphase is divided into three phases.G1 phaseS PhaseG2 Phase
G1Phase - It is the stage during which the cell grows and prepares its DNA for replication. In this phase, the cell is metabolically active.S Phase - It is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same. G2Phase - In this phase, the cell continues to grow and prepares itself for division. The proteins and RNA required for mitosis are synthesised during this stage.
Q4. What is Go (quiescent phase) of cell cycle? Ans. G0 or quiescent phase is the stage wherein cells remain metabolically active, but do not proliferate unless called to do so. Such cells are used for replacing the cells lost during injury.
Q5. Why is mitosis called equational division? Ans. Mitosis is the process of cell division wherein the chromosomes replicate and get equally distributed into two daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell i.e. diploid. Hence, mitosis is known as equational division.
Q6. Name the stage of cell cycle at which one of the following events occur: (i) Chromosomes are moved to spindle equator(ii) Centromere splits and chromatids separate. (iii) Pairing between homologous chromosomes takes place. (iv) Crossing over between homologous chromosomes takes place. Ans. (i) Chromosomes are moved to spindle equator = Metaphase(ii) Centromere splits and chromatids separate. = Anaphase(iii) Pairing between homologous chromosomes takes place = Zygotene of Meiosis I (iv) Crossing over between homologous chromosomes takes place = Pachytene of Meiosis I
Q7. Describe the following: (a) synapsis; (b) bivalent; (c) chiasmata; Draw a diagram to illustrate your answer. Ans. a. Synapsis: The pairing of homologous chromosomes is called synapsis. This occurs during the second stage of prophase I or zygotene.b. Bivalent: Bivalent or tetrad is a pair of synapsed homologous chromosomes. They are formed during the zygotene stage of prophase I of meiosis.c. Chiasmata: Chiasmata is the site where two sister chromatids have crossed over. It represents the site of cross-over. It has formed during the diplotene stage of prophase I of meiosis.
Q8. How does cytokinesis in plant cells differ from that in animal cells?Ans. Cytokinesis in plant cells:(i) The division of the cytoplasm takes place by cell plate formation.(ii) Cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls.Cytokinesis is animal cells:(i)The division of the cytoplasm takes place by cleavage. (ii)Cleavage starts at the periphery and then moves inward, dividing the cell into two parts. Q9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size. Ans. (a)Spermatogenesis or the formation of sperms in human beings occurs by the process of meiosis. It results in the formation of four equal-sized daughter cells. (b)Oogenesis or the formation of ovum in human beings occurs by the process of meiosis. It results in the formation of four daughter cells which are unequal in size.
Q10. Distinguish anaphase of mitosis from anaphase I of meiosis. Ans. Anaphase of Mitosis: Anaphase is the stage during which the centromere splits and the chromatids separate. The chromosomes move apart, toward the opposite poles. These chromosomes are genetically identicalAnaphase I of meiosis: During anaphase I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. Hence, in anaphase I, the chromosomes of each bivalent pair separate, while the sister chromatids remain together.
Q11. List the main differences between mitosis and meiosis. Ans. Mitosis1. In mitotic division, a single division results in two daughter cells. 2. Mitosis is known as equational division. This is because the daughter cells have the same diploid number of chromosomes as the parent. 3. Prophase is short and does not comprise any phase. diplotene, and diakinesis. 4. There is no pairing of chromosomes, crossing-over, or chiasmata-formation during prophase. 5. Synaptonemal complex is not formed. 6. Anaphase involves the separation of the chromatids of each chromosome. 7. Mitosis plays a significant role in the healing, repair, and growth of a cell. Meiosis1.Meiotic division involves two successive divisions –meiosis I and meiosis II. These divisions result in four daughter cells.2.Meiosis I is known as reductional division. This is because the chromosome number is reduced to half. Meiosis II is known as equational division. This is because the sister chromatids separate and the chromosome number remains the same.3.Prophase Iis very long and comprises 5 phases –leptotene, zygotene, pachytene,4.In the zygotene stage of prophase, the pairing of chromosomes occurs. During pachytene, the crossing-over occurs. The chiasmata are formed in the diplotene stage.5.Synaptonemal complex is formed during the zygotene stage of prophase I.6.During anaphase I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. During anaphase II, the chromatids separate as a result of the splitting of the centromere. 7.Meiosis brings about variation and maintains the chromosome number from generation to generation.
Q12. What is the significance of meiosis? Ans. Meiosis is the process involving the reduction in the amount of genetic material. It comprises two successive nuclear and cell divisions, with a single cycle of DNA replication. As a result, at the end of meiosis II, four haploid cells are formed.
Significance of meiosis
*Meiosis maintains the chromosome number from generation to generation. It reduces the chromosome number to half so that the process of fertilisation restores the original number in the zygote.
*Variations are caused by the cross-over andthe random distribution of homologous chromosomes between daughter cells. Variations play an important role in evolution.
*Chromosomal mutations are brought about by the introduction of certain abnormalities. These chromosomal mutations may be advantageous for an individual.
Q13. Discuss with your teacher about: (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur. Ans. i) In some insects and lower plants, fertilization is immediately followed by zygotic meiosis, which leads to the production of haploid organisms. This type of life cycle is known as haplontic life cycle.
ii) The phenomenon of polyploidy can be observed in some haploid cells in higher plants in which cell division does not occur. Polyploidy is a state in which cells contain multiple pairs of chromosomes than the basic set. Polyploidy can be artificially induced in plants by applying colichine to cell culture.
Q14. Can there be mitosis without DNA replication in ‘S’ phase? Ans.Mitotic cell division cannot take place without DNA replication in S phase. Two important events take place during S phase –one is the synthesis or duplication of DNA and the other is the duplication of the centriole. DNA duplication is important as it maintains the chromosome number in the daughter cells. Mitosis is an equational division. Therefore, the duplication of DNA is an important step.
Q15. Can there be DNA replication without cell division? Ans.There can be DNA replication without cell division. During cell division, the parent cell gets divided into two daughter cells. However, if there is a repeated replication of DNA without any cell division, then this DNA will keep accumulating inside the cell. This would increase the volume of the cell nucleus, thereby causing cell expansion. An example of DNA duplication without cell division is commonly observed in the salivary glands of Drosophila. The chromosome undergoing repeated DNA duplication is known as polytene chromosome.
Q16. Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell; (ii) amount of DNA content (C) per cell Ans. During meiosis, the number of chromosomes and the amount of DNA in a cell change.
(i)Number of chromosomes (N) per cell During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.
(ii)Amount of DNA content (C) per cell
During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, the chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present.
During mitosis, the number of chromosomes remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.
