Mg3
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3 CELL CYCLE, MITOSIS AND MEIOSIS
3.1 Cell cycle The life of a cell begins with its formation by cell division of a mother cell and ends with either the formation of daughter cells or with its death. The cell cycle can be regarded as the life cycle of an individual cell. It can divided into two phases:
·Mitosis (or cell division)—which results in the production of two daughter cells. ·Interphase—the interval between divisions during which the cell undergoes its functions and prepares for mitosis.
A typical cell spends most of its life in interphase. This portion of the cell cycle is divided into three phases, G1, S, and G2.
Major phases of the cell cycle, showing the alternation of interphase and mitosis (division).
G1SG2M. Interphase = G1+S+G2. G1—Cell grows and carries out normal metabolism. S—DNA replication. G2—Cell grows and prepares for mitosis.
3.2 Mitosis and meiosis 3.2.1 Overview of cell division Mitosis is the type of cell division that occurs in somatic cells and results in the production of two genetically identical daughter cells.
Meiosis occurs in gamete formation (e.g. sperm and ova). Each daughter cell contains half the genetic information of the parent cell and crossing-over ensures a reassortment of genetic material between the chromosomes of each homologous pair.
3.2.2 Mitosis 2n2n.
Prophase
There is condensation of chromosomes and the centrioles duplicate and migrate towards opposite poles of the cell. The nucleolus disappears. A spindle of microtubules is formed simultaneously. Dissolution of the nuclear membrane marks the end of prophase.
Metaphase
The chromosomes become attached to the spindle. The area of attachment is called the kinetochore. The chromosomes become arranged along the spindle, forming the equatorial plate.
Anaphase
Chromatids separated at the centromeres and are pulled to opposite poles by the spindle. The end of anaphase is marked by the clustering of two groups of identical chromatids of opposite poles of the cell.
Telophase
The chromosomes begin to uncoil. The nuclear membrane re-forms and nucleoli reappear. The cytoplasm is divided into two by the process of cytokinesis.
Two identical diploid daughter cells are formed as a result of mitosis.
3.2.3 Meiosis 2nn.
Meiosis Ⅰ Prophase Ⅰ There are five stages during which homologous chromosomes come together and exchange segments in homologous recombination:
·Leptotene
Spindle forms.
·Zygotene
Homologous chromosomes pair, shorten, and thicken, and form bivalents (pairs of homologous chromosomes).
·Pachytene
Chiasmata begin to form. Chiasma is the Xshaped connection visible between paired homologous chromosomes. These become points of “crossing-over” between the chromatids.
·Diplotene
Exchange of genetic material in chiasmata.
·Diakinesis
Recombinant chromosomes are formed and the nuclear membrane disappears.
homologous chromosomes pair
crossing-over
genetic recombination
Metaphase Ⅰ
Like mitotic metaphase, chromosomes become attached to a spindle.
Anaphase Ⅰ
The chromatids do not separate and the homologous chromosomes are pulled by the spindle fibers toward opposite poles of the cell.
So only half of the original number of chromosomes migrate toward each pole. The chromosomes migrating toward each pole thereby consist of one member of each pair of autosomes and one of the sex chromosomes, hence “reduction division”.
Telophase Ⅰ
Two genetically different daughter cells are formed. These cells contain only one member of each pair of homologous chromosomes.
Meiosis Ⅱ
The second division is like mitosis. The chromatids separate in anaphase Ⅱ.
Two processes in meiosis are vital in the generation of genetic diversity:
·Chiasmata formation (“crossingover”), which allows random exchange of genetic material between homologous chromosomes. ·Independent segregation of homologous chromosomes.
Meiosis generates four haploid germ cells from one diploid premeiotic cell.
3.2.4 Relationship between meiosis and gametogenesis The stage of meiosis can be related directly to stages in gametogenesis, the formation of gametes. In mature males the seminiferous tubules of the testes are populated by spermatogonia, which are diploid cells. After going through several mitotic divisions, the spermatogonia
produce primary spermatocytes. Each primary spermatocyte, which is also diploid, undergoes meiosis Ⅰ to produce a pair of secondary spermatocytes. These undergo meiosis Ⅱ, and each produces a pair of haploid spermatids. The spermatids then lose most of their cytoplasm and develop tails for
swimming as they become mature sperm cells. This process, known as spermatogenesis, continues throughout the life of the mature male.
In spermatogenesis, the primary spermatocyte divides by meiosis, giving rise to four spermatids; the spermatids differentiate, becoming mature sperm cells..
In spermatogenesis, each diploid primary spermatocyte produces four haploid sperm cells.
Oogenesis, the process in which female gametes are formed, differs in several important ways from spermatogenesis. Whereas the cycle of spermatogenesis is constantly recurring in males, much of female oogenesis is completed before birth. Diploid oogonia divide mitotically to produce primary
oocytes. Primary oocytes are suspended in prophase Ⅰ by the time the female is born. Meiosis continues only when a mature primary oocyte is ovulated. In meiosis Ⅰ the primary oocyte produces one secondary oocyte (containing the cytoplasm) and one polar body. The secondary oocyte then emerges from the
follicle and proceeds down the fallopian tube, with the polar body attached to it. Meiosis Ⅱ begins only if the secondary oocyte is fertilized by a sperm cell. If so , one haploid mature ovum, containing the cytoplasm, and another haploid polar body are produced. The polar bodies eventually disintegrate. After
fertilization, the nuclei of the sperm cell and ovum fuse, forming a diploid zygote. The zygote then begins its development into an embryo through a series of mitotic divisions.
In oogenesis, only one functional ovum is produced from each primary oocyte; the other three cells produced are polar bodies that degenerate.
In oogenesis one haploid ovum and three haploid polar bodies are produced meiotically from a diploid primary oocyte.
After we have learned this chapter we should be able to: 1.Identify the stages in the cell cycle. 2.Contrast the events of mitosis and meiosis. 3.Compare spermatogenesis with oogenesis.
3.1 Cell cycle The life of a cell begins with its formation by cell division of a mother cell and ends with either the formation of daughter cells or with its death. The cell cycle can be regarded as the life cycle of an individual cell. It can divided into two phases:
·Mitosis (or cell division)—which results in the production of two daughter cells. ·Interphase—the interval between divisions during which the cell undergoes its functions and prepares for mitosis.
A typical cell spends most of its life in interphase. This portion of the cell cycle is divided into three phases, G1, S, and G2.
Major phases of the cell cycle, showing the alternation of interphase and mitosis (division).
G1SG2M. Interphase = G1+S+G2. G1—Cell grows and carries out normal metabolism. S—DNA replication. G2—Cell grows and prepares for mitosis.
3.2 Mitosis and meiosis 3.2.1 Overview of cell division Mitosis is the type of cell division that occurs in somatic cells and results in the production of two genetically identical daughter cells.
Meiosis occurs in gamete formation (e.g. sperm and ova). Each daughter cell contains half the genetic information of the parent cell and crossing-over ensures a reassortment of genetic material between the chromosomes of each homologous pair.
3.2.2 Mitosis 2n2n.
Prophase
There is condensation of chromosomes and the centrioles duplicate and migrate towards opposite poles of the cell. The nucleolus disappears. A spindle of microtubules is formed simultaneously. Dissolution of the nuclear membrane marks the end of prophase.
Metaphase
The chromosomes become attached to the spindle. The area of attachment is called the kinetochore. The chromosomes become arranged along the spindle, forming the equatorial plate.
Anaphase
Chromatids separated at the centromeres and are pulled to opposite poles by the spindle. The end of anaphase is marked by the clustering of two groups of identical chromatids of opposite poles of the cell.
Telophase
The chromosomes begin to uncoil. The nuclear membrane re-forms and nucleoli reappear. The cytoplasm is divided into two by the process of cytokinesis.
Two identical diploid daughter cells are formed as a result of mitosis.
3.2.3 Meiosis 2nn.
Meiosis Ⅰ Prophase Ⅰ There are five stages during which homologous chromosomes come together and exchange segments in homologous recombination:
·Leptotene
Spindle forms.
·Zygotene
Homologous chromosomes pair, shorten, and thicken, and form bivalents (pairs of homologous chromosomes).
·Pachytene
Chiasmata begin to form. Chiasma is the Xshaped connection visible between paired homologous chromosomes. These become points of “crossing-over” between the chromatids.
·Diplotene
Exchange of genetic material in chiasmata.
·Diakinesis
Recombinant chromosomes are formed and the nuclear membrane disappears.
homologous chromosomes pair
crossing-over
genetic recombination
Metaphase Ⅰ
Like mitotic metaphase, chromosomes become attached to a spindle.
Anaphase Ⅰ
The chromatids do not separate and the homologous chromosomes are pulled by the spindle fibers toward opposite poles of the cell.
So only half of the original number of chromosomes migrate toward each pole. The chromosomes migrating toward each pole thereby consist of one member of each pair of autosomes and one of the sex chromosomes, hence “reduction division”.
Telophase Ⅰ
Two genetically different daughter cells are formed. These cells contain only one member of each pair of homologous chromosomes.
Meiosis Ⅱ
The second division is like mitosis. The chromatids separate in anaphase Ⅱ.
Two processes in meiosis are vital in the generation of genetic diversity:
·Chiasmata formation (“crossingover”), which allows random exchange of genetic material between homologous chromosomes. ·Independent segregation of homologous chromosomes.
Meiosis generates four haploid germ cells from one diploid premeiotic cell.
3.2.4 Relationship between meiosis and gametogenesis The stage of meiosis can be related directly to stages in gametogenesis, the formation of gametes. In mature males the seminiferous tubules of the testes are populated by spermatogonia, which are diploid cells. After going through several mitotic divisions, the spermatogonia
produce primary spermatocytes. Each primary spermatocyte, which is also diploid, undergoes meiosis Ⅰ to produce a pair of secondary spermatocytes. These undergo meiosis Ⅱ, and each produces a pair of haploid spermatids. The spermatids then lose most of their cytoplasm and develop tails for
swimming as they become mature sperm cells. This process, known as spermatogenesis, continues throughout the life of the mature male.
In spermatogenesis, the primary spermatocyte divides by meiosis, giving rise to four spermatids; the spermatids differentiate, becoming mature sperm cells..
In spermatogenesis, each diploid primary spermatocyte produces four haploid sperm cells.
Oogenesis, the process in which female gametes are formed, differs in several important ways from spermatogenesis. Whereas the cycle of spermatogenesis is constantly recurring in males, much of female oogenesis is completed before birth. Diploid oogonia divide mitotically to produce primary
oocytes. Primary oocytes are suspended in prophase Ⅰ by the time the female is born. Meiosis continues only when a mature primary oocyte is ovulated. In meiosis Ⅰ the primary oocyte produces one secondary oocyte (containing the cytoplasm) and one polar body. The secondary oocyte then emerges from the
follicle and proceeds down the fallopian tube, with the polar body attached to it. Meiosis Ⅱ begins only if the secondary oocyte is fertilized by a sperm cell. If so , one haploid mature ovum, containing the cytoplasm, and another haploid polar body are produced. The polar bodies eventually disintegrate. After
fertilization, the nuclei of the sperm cell and ovum fuse, forming a diploid zygote. The zygote then begins its development into an embryo through a series of mitotic divisions.
In oogenesis, only one functional ovum is produced from each primary oocyte; the other three cells produced are polar bodies that degenerate.
In oogenesis one haploid ovum and three haploid polar bodies are produced meiotically from a diploid primary oocyte.
After we have learned this chapter we should be able to: 1.Identify the stages in the cell cycle. 2.Contrast the events of mitosis and meiosis. 3.Compare spermatogenesis with oogenesis.
