Cell Division: Mitosis and Meiosis (OCR)

2 TYPES OF CELL DIVISION Mitosis division of somatic (body) cells Meiosis division of gametes (sex cells) MITOSIS  MITOSIS Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase,...

2 TYPES OF CELL DIVISION Mitosis division of somatic (body) cells Meiosis division of gametes (sex cells) MITOSIS  MITOSIS Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle.  MITOSIS Phases of Mitosis ⚫ Interphase ⚫ Prophase ⚫ Metaphase ⚫ Anaphase ⚫ Telophase MITOSIS: INTERPHASE  MITOSIS: INTERPHASE The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules.  MITOSIS: INTERPHASE Interphase is composed of G1 phase (cell growth), followed by S phase (DNA synthesis), followed by G2 phase (cell growth). At the end of interphase comes the mitotic phase, which is made up of mitosis and cytokinesis and leads to the formation of two daughter cells.  MITOSIS: INTERPHASE - GAP 0 The G0 phase (referred to the G zero phase) or resting phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle.  MITOSIS: INTERPHASE - GAP 1 The g1 phase, or Gap 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis. G1 phase ends when the cell moves into the S phase of interphase.  MITOSIS: INTERPHASE - SYNTHESIS The S stage stands for "Synthesis". This is the stage when DNA replication occurs. S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.  MITOSIS: INTERPHASE - GAP 2 During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide. MITOSIS: MITOTIC STAGE  MITOSIS: PROPHASE Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Chromosome are thickening, shorten and paired while becoming more visible. 2 chromatids joined by a centromere Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle.  MITOSIS: METAPHASE Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome. Become attached to spindle fibres by centromeres Homologous chromosomes do not associate  MITOSIS: ANAPHASE The paired chromosomes separate at the kinetochores and move AWAY to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules. Spindle fibres contract pulling chromatids to the opposite poles of the cell  MITOSIS: TELOPHASE Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage. Chromosomes uncoil, spindle fibres disintegrate, centrioles replicate and nuclear membrane forms finally cell divides MITOSIS: CYTOKINESIS  MITOSIS: CYTOKINESIS In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells. MEIOSIS  MEIOSIS The process that produces haploid gametes is meiosis. Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of the organisms. During meiosis, homologous chromosomes separate, and haploid cells form that have only one chromosome from each pair. Two cell divisions occur during meiosis, and a total of four haploid cells are produced. The two cell divisions are called meiosis I and meiosis II.  MEIOSIS Phases of Meiosis ⚫ Interphase ⚫ Prophase II ⚫ Prophase I ⚫ Metaphase II ⚫ Metaphase I ⚫ Anaphase II ⚫ Anaphase I ⚫ Telophase II ⚫ Telophase I ⚫ Cytokinesis ⚫ Cytokinesis ⚫ Interphase MEIOSIS: INTERPHASE  MEIOSIS: INTERPHASE Prior to undergoing meiosis, a cell goes through an interphase period in which it grows, replicates its chromosomes, and checks all of its systems to ensure that it is ready to divide. Everything that happens in Mitosis during interphase also happens in Meiosis including G0, G1, S and G2. MEIOSIS I: M-PHASE  MEIOSIS: PROPHASE I At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma).  MEIOSIS: PROPHASE I The nucleolus disappears during prophase I. In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.  MEIOSIS: METAPHASE I The centrioles are at opposite poles of the cell. The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).  MEIOSIS: ANAPHASE I Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle. Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.  MEIOSIS: TELOPHASE I The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids.  MEIOSIS: TELOPHASE I A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes.  MEIOSIS: TELOPHASE I Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes recondense in prophase II. MEIOSIS I MEIOSIS II: M-PHASE  MEIOSIS: PROPHASE II While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II. The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes. The nuclear envelope breaks down, and the spindle apparatus forms.  MEIOSIS: METAPHASE II Each of the daughter cells completes the formation of a spindle apparatus. Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate. For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole.  MEIOSIS: ANAPHASE II The centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle. The separated chromatids are now called chromosomes in their own right.  MEIOSIS: TELOPHASE II A nuclear envelope forms around each set of chromosomes. Cytokinesis takes place, producing four daughter cells (gametes, in animals), each with a haploid set of chromosomes. Because of crossing-over, some chromosomes are seen to have recombined segments of the original parental chromosomes.  MEIOSIS II NOTE: Other topic such as chromosomes, DNA and Gametes will be discuss under Genetics Topic NEXT TOPIC: CELL TRANSPORT