Cell cycle and cell division
- 1. Cell Cycle and Cell Division Mrs. Praveen Garg VITS College, Satna
- 2. Introduction • The cell cycle can be thought of as the life cycle of a cell. • In other words, it is the series of growth and development steps. • To divide, a cell must complete several important tasks: it must grow, copy its genetic material (DNA), and physically split into two daughter cells. • In eukaryotic cells, or cells with a nucleus, the stages of the cell cycle are divided into two major phases: interphase and the mitotic (M) phase. • During interphase, the cell grows and makes a copy of its DNA. • During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells.
- 3. Interphase • G1 phase: During G1 also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps. • S phase: In S phase, the cell synthesizes a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during M phase. • G2 phase: During the second gap phase, or G2 phase the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins. • The G1, S, G2 phase together are known as interphase. The prefix inter- means between, reflecting that interphase takes place between one mitotic (M) phase and the next.
- 5. M Phase • During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. • M phase involves two division processes: mitosis and cytokinesis. • In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. • Mitosis takes place in four stages: prophase, metaphase, anaphase, and telophase. • In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. • Cytokinesis usually begins just as mitosis is ending, with a little overlap. Importantly, cytokinesis takes place differently in animal and plant cells.
- 7. • This is also known as mitosis division. • It occurs in somatic cells. • Mitosis is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. • Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. • The division of the mother cell into two daughter cells genetically identical to each other. • These stages are prophase, prometaphase, metaphase, anaphase, and telophase. • During mitosis, the chromosomes, which have already duplicated, condense and attach to spindle fibers that pull one copy of each chromosome to opposite sides of the cell. • The result is two genetically identical daughter nuclei. The rest of the cell may then continue to divide by cytokinesis to produce two daughter cells. • Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Mitotic Cell division
- 9. • Plectonemic coiling takes place during mitosis, in which sister chromatids are tightly coiled upon each other and are not easy to separate. • Paranemic coiling found in meiosis where the two strands are loosely coiled, making them easier to separate. • The degree of coiling of the chromonemal fibrils during cell division depends on the length of the chromosome.
- 10. Meiosis Division • Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells. • It involves two rounds of division that result in four cells with only one copy of each chromosome (haploid). • Prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. • Later on, during fertilization, the haploid cells produced by meiosis from a male and female will fuse to create a cell with two copies of each chromosome again, the zygote.
- 11. • In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell. • Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. • The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle.
- 12. • Interphase is divided into three phases: • Growth 1 (G1) phase: In this phase, the cell synthesizes proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA. • Synthesis (S) phase: The genetic material is replicated; each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. • Growth 2 (G2) phase: G2 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G2 phase of the mitotic cell cycle.
- 14. • Meiosis I • The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated • P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs • M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell • A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell • T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells •
- 16. • Meiosis II • The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I) • P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before) • M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator • A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles • T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells. • The final outcome of meiosis is the production of four haploid daughter cells
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