Nuclear Division
The Cell Cycle
The cell cycle consists of four phases:
· G1 (gap phase 1)- also called the growth phase during which the biosynthetic activities of the cell, which slow down considerably during M phase, resume at a high rate. G1 is marked by synthesis of various enzymes needed in S phase, mainly those needed for DNA replication. Duration of G1 is highly variable, even among different cells of the same species.
· S phase (synthesis)- starts when DNA synthesis commences and when complete, all of the chromosomes have been replicated, i.e., each chromosome with two sister chromatids. The amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same. Rates of RNA transcription and protein synthesis, with the exception of histones, are very low during this phase.
· G2 phase (gap phase 2)- lasts until the cell enters mitosis. Again, significant biosynthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.
· M phase (mitosis)- consists of the stages of nuclear division. This is the phase of the cell cycle in which the spindle apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart.
· C (cytokinesis)- the phase of the cell cycle when the cytoplasm divides, creating two daughter cells.
G1, S and G2 together constitute interphase, the portion of the cell cycle between cell divisions.
Duration of the cell cycle varies depending on cell type. Animal embryos can complete their cell cycle in less than 20 minutes; fruit fly (drosophila) embryo cells have the shortest animal cell cycle of 8 minutes. The duration of the cell cycle varies. In embryonic cells G1 and G2 are greatly reduced (approx. 8-20 min). Mature mammalian cells divide every 24 h. The actual M phase takes only about 1hour. Most mature cells need time to grow and so their cycles are much longer than those of embryonic tissues. Typically, a dividing mammalian cell completes its cell cycle in about 24hrs, but some cells, such as certain liver cells, have cycles lasting over a year.
During the cycle, growth occurs throughout G1 and G2 phases, as well as during the S phase. Most of the variation in the length of the cell cycle between organisms or cell types occurs in the G1 phase. Cells often pause in G1 before DNA replication and enter a resting state called G0; cells may remain in this phase for days to years before resuming cell division. Muscle and nerve cells remain permanently at G0.
During G1, cells undergo the major portion of their growth. During the S phase, each chromosome replicates to produce two sister chromatids that, remain attached to each other at the centromere. (The centromere is a visible point on the chromosome that contains repeated DNA sequences that bind specific proteins. These proteins make up the kinetochore to which microtubules attach during cell division). During the G2 phase chromosomes begin the process of condensation (tight coiling). The cell starts building the microtubule spindle out of the protein tubulin. The cell grows throughout interphase. During G1 and G2 proteins are synthesized and organelles are produced. In animal cells only, a pair of microtubule-organizing centres called the centrioles replicate, producing one for each pole of the cell.
Mitosis
Mitosis- nuclear division in which the duplicated chromosomes separate to form two genetically identical daughter nuclei.
Significance of mitosis:
1. Genetic stability: two nuclei having same number of chromosomes as the parent cell are produced. There is no genetic variation, i.e. clones are produced. Daughter cells can be different only if mutation occurs during DNA replication.
2. Growth: by increasing the number of cells within an organism. This is the bases of growth in unicellular organisms.
3. Cell replacement: to replace dying cells
4. Regeneration: of whole parts of body e.g. legs in crustacea.
5. Asexual reproduction occurs by mitosis in organisms like yeast and members of Protoctista.
The stages of mitosis include Prophase, Metaphase, Anaphase and Telophase
Prophase:
1. Condensed chromosomes become visible with the light microscope.
2. Condensation process of chromosomes continues, they appear as two sister chromatids held together at the centromere.
3. rRNA synthesis stops
4. Cytoskeleton disassemble and assembly of spindle apparatus. In animal cells the pair of centrioles move apart, a bridge of microtubules called the spindle apparatus is formed between them. In plant cells a bridge of microtubules forms across the cell but there are no centrioles. The aster forms in animal cells, is a radial array of microtubules extending from the centriole to the plasma membrane, possibly serving for retraction of the spindle. Plant cells do not form the aster as they have rigid cell walls. The line of division of the cell is perpendicular to the spindle fibres.
5. The nuclear envelope breaks down (reabsorbed by the ER)
6. Golgi and ER are dispersed
Prometaphase:
1. The condensed chromosomes attach to the spindle by their kinetochores. Bipolar attachment is important for the successful separation of the two chromatids into two daughter cells.
2. The chromosomes move to the center of the cell. All chromosomes arrange themselves on the equator of the cell with the sister chromatids oriented to opposite poles by their kinetochore microtubules
Metaphase:
1. All chromosomes are aligned at equator of the cell, called the metaphase plate (more of a plane but indicate axis of cell division).
2. Chromosomes are attached to opposite poles and are under tension.
Anaphase:
1. Cohesin, the proteins holding centromeres of sister chromatids are degraded, freeing the individual chromatids.
2. Chromosomes are pulled to opposite poles.
3. Spindle poles move apart as microtubular spindle fibers physically anchored to opposite poles slide past each other, away from the cell center
Telophase:
1. The spindle apparatus disassemble to tubulin monomers (will eventually be used to construct cytoskeleton of daughter cells).
2. A nuclear envelope forms around each set of chromosomes.
3. The chromosomes uncoil to permit gene expression
Telophase is the reverse of prophase, bringing the cell back to a state of interphase.
Cytokinesis
It is the division of the cytoplasm of a cell after nuclear division.
Animal cells: cytokinesis is achieved by means of a constricting belt of actin filaments. As these filaments slide past one another, the diameter of the belt decreases, pinching the cell and creating a cleavage furrow around the cell circumference. The constriction continues until eventually it slices all the way across the cell. At this point the cell divides in two
Plant cells: the cell assembles membrane components in their interior, at right angle to the spindle apparatus. This is called the cell plate, it continues to grow outwards until it reaches the interior surface of the plasma membrane and fuses with it. Cellulose is then laid down on the new membrane, creating two new cell walls. The space between the daughter cells becomes impregnated with pectin forming the middle lamella.
In most fungi and some groups of protists, the nuclear membrane does not dissolve during mitosis. All the events of mitosis occur within the nucleus. Cytokinesis occurs after nuclear division.
Meiosis
It is a form of cell division that leads to the production of gametes, which are egg cells and sperm cells. Gametes contain half the number of chromosomes of adult body cells. Adult body cells (somatic cells) are diploid (2n), containing 2 sets of chromosomes. Gametes are haploid (n), containing only 1 set of chromosomes.
Significance of meiosis is generating genetic diversity through:
1. synapsis and crossing over at chiasmata during prophase I;
2. random alignment of maternal and paternal chromosomes at the equator during metaphase I and independent assortment of chromosomes during anaphase I;
3. random alignment of chromosomes at the equator during metaphase II and independent assortment of chromatids during anaphase II.
Germ-line cells- During zygote development, cells that are set aside from the somatic cells that will eventually undergo meiosis to produce gametes.
Somatic cells- any cells of a multicellular organism except those that are destined to form gametes. Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages
Meiosis in a diploid organism consists of one round of DNA replication and two rounds of divisions called meiosis I and meiosis II. The resulting cells are haploid gametes and the process is called gametogenesis. Each round containing prophase, metaphase, anaphase and telophase stages.
Meiosis is characterized by 4 features:
1. synapsis and crossing over
2. sister chromatids remain joined at their centromeres throughout meiosis I
3. Kinetochores of sister chromatids attach to the same pole in meiosis I
4. DNA replication is suppressed between meiosis I and meiosis II.
Meiosis I
Prophase I
1. The chromosomes begin to condense.
2. Nuclear membrane breaks down.
3. Microtubules are formed into a spindle.
4. Homologous chromosomes synapsis (pair). The complex formed is called synaptonemal complex, consisting of the homologues paired closely with a lattice of meiosis specific cohesion between them. This structure is also called a tetrad or bivalent.
5. During synapsis recombination (or crossing over) occurs at recombination nodules. This involves the homologues exchanging chromosomal material. The sites of crossing over are called chiasmata.
Metaphase I
1. The pairs of homologous chromosomes align along the metaphase plate. A kinetochore microtubule from one pole of the cell attaches to one homologue of a chromosome, while a kinetochore microtubule from the other cell pole attaches to the other homologue of a pair.
2. The orientation of each pair on the spindle axis is random. Therefore, independent assortment of chromosomes
Anaphase I
The kinetochore microtubules shorten, and homologous pairs are pulled apart. One duplicated homologue goes to one pole of the cell, while the other duplicated homologue goes to the other pole. Sister chromatids do not separate.
Telophase I
The separated homologues form a cluster at each pole of the cell, and the nuclear envelope re-forms around each daughter cell nucleus. Cytokinesis may occur. The resulting two cells have half the number of chromosomes as the original cell and are therefore haploid. Each chromosome is still in the duplicated state and consists of two sister chromatids. Sister chromatids are not identical as crossing over has occurred.
The second meiotic division occurs after an interval of variable length but is typically brief. Meiosis II is like mitotic division without DNA replication
Meiosis II
Prophase II- a new spindle apparatus is formed in each cell and the nuclear envelope breaks down.
Metaphase II- Chromosomes (made up of sister chromatids joined at the centromere) align along the metaphase plate in each cell. Kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids.
Anaphase II- When microtubules shorten in anaphase II, the centromeres split, and sister chromatids are pulled to opposite poles of the cell.
Telophase II- The nuclear membrane reforms around four different clusters of chromosomes. After cytokinesis, four haploid cells result.
No cells are alike due to:
1. The random alignment of homologous pairs (maternal and paternal) at metaphase I and independent assortment of chromosomes during anaphase I
2. The crossing over during prophase I
3. Random alignment of chromosomes at the equator during metaphase II and independent assortment of chromatids during anaphase II
Polyploidy- the state of three or more sets of chromosomes in each cell.