Lecture 5 - Friday, January 31, 2003 R. Jones Chapter 8
Cellular Reproduction-Mitosis and Meiosis
For these topics it is essential that you consult the figures in chapter 8. Without these images as reference cell reproduction can be difficult to envisage and understand.
1.Mitosis. Cells enter mitosis after having undergone a round of DNA synthesis during the S-phase of the Cell Cycle. During S each of the cell's chromosomes is duplicated. Prophase is the first stage of mitosis when chromosomes condense, the nuclear membrane disintegrates, the nucleolus disappears, centrioles (in cells that have them, e.g almost all animal cells but not plants) replicate and move to establish the poles of the cell. The spindle apparatus begins to form and some of the microtubules assist in centriole movement.
2. At the end of prophase a spindle apparatus is visible with microtubules connecting the poles of the cell at the centrioles, as well as microtubules attaching to the kinetochores of the chromatids. Remember, each chromosome is made up of two chromatids attached at the region called the centromere. The centromere of each chromatid has a kinetochore, a region to which the mcirotubules from only one pole become attached.
3. Metaphase is the distinguishing feature of KARYOKINESIS. In mitosis, each chromosome assumes its place at the equator of the cell, sometimes referred to as the metaphase plate. Chromosomes line up in a way allowing microtubules from opposite poles to attach to each of the daughter chromatids, i.e., one chromatid of the chromosome is attached to microtubules from one pole and its sister chromatid is attached to a microtubule from the other pole.
4.Anaphase: Pulling of the chromatids to the poles separates daughter chromatids from each other delivering each to opposite poles. As soon as chromatid separation occurs they are now called chromosomes. The chromosomes are pulled to the poles ending anaphase.
5. Telophase: Nuclear envelope begins to reform, spindle apparatus disassembles, chrmosomes unwind, the nucleolus reforms and CYTOKINESIS, the process of cellular replication begins
6. In animal cells cytokinesis occurs by the action of cytoskeletal components that pinch the cell into two equal halves at the equator. A cosntriction forms that eventually pinches the cell into two totally separate cells.
7. In plant cells, the presence of the rigid cell wall prevents cytokinesis by this mechanism. In plants a cell wall is laid down between the newly-forming nuclei. The cell wall forms from vesicles containing aggregates of cell wall material. This so-called cell plate grows until it cuts off two new cells.
8. The result of karyokinesis and cytokinesis is two daugthter cells that have the same exact genetic make-up as the parent cell and the same exact number of chromosomes. This is achieved because the chromosome at the beginning of mitosis consisted of two chromatids that become chromomes as soon as they are separated in anaphase. If the daughter cells of the replication that we have just described are themselves to undergo mitosis, they must now duplicate thier own chromosomes and this occurs as the cell progresses along the cell cycle to the S phase. After the S phase each chromosome is now duplicated and ready for another round of mitosis.
9. Meiosis is the way in which diploid (2N) somatic cells halve their chromsome number and become haploid or (N). This is the process that gives rise to gametes (egg and sperm in animals), but the process of meiosis does not give rise to gametes in plants. More on this later.
10. To understand meisosis you must remember that the diploid cell contains 2 sets of homologous pairs of chromosomes, one from each parent. During meiosis the homologous chromosomes pair up and undergo crossing over that results in the exchnage of genetic information between the sets of homologous pairs.
11. Meiosis consists of two succesive round of replication called meisosis I and meiosisi II. Each round of replication is divided up into stages of prophase, metaphase, anaphase and telophase.
12. In prophase I of meiosis many of the changes in the nucleus and cytoskeleton are similar to those found in mitosis. The exception is that homologous chromosomes pair up and form a very tight association, a process called synapsis. During synapasis chromosomes (remember, each is made up of two chromatids!) exchange portions of their chromatids, a process called crossing over. This occurs frequently, e.g. it's been estmated that each pair of human chromsomes undergoes 2-3 crossing over events during every prophase I of meiosis.
13. In metaphase I chromosomes align at the equator on the cell as homolgous pairs, one homolog facing one pole of the cell, the other facing the other pole. The way in which parental chromsomes are arranged at the metaphase plate is random and the number of possibilities of their organization is 2 to the power of the number of chromsomes, so in humans there are 2 to the power of 23 possible arrangements of human chromsomes (i.e. only 1 in 8,388,608 possibilities that two eggs or sperm will receive an exact maternal or paternal complement of chromosomes.
14. Anaphase I: The microtubules pulls one set of each homolgous pair of chromosomes to the pole and telophase ensures that two separate groups of chromosomes accumulate at the poles. Each pole has one set of the homologous pairs of chromosomes.
15. Cytokinesis occurs as telophase ends and this separates the chromosomes into two cells, EACH OF WHICH NOW HAS HALF OF THE NUMBER OF CHROMOSOMES THAT WERE FOUND IN THE PARENT CELL.
16. The second round of Meiosis, Meiosis II is indistinguishable from Mitosis. In some organisms, telophase of meiosis I is followed by interphase but in many cells, prophase II of meiosis follows on directly from telophase I. In prophase II a new spindle apparatus in formed, each chromosome becomes attached to the spindle apparatus in metaphase II, thesister chromatids separate at anaphase II and the new chromosomes are moved to the poles of the two daughter cells that begin to form in telophase II. IN MEISOSIS II, TWO CELLS FORM FROM EACH HAPLOID CELL PRODUCED IN MEISOSIS I. ACOMBINATION OF MEISOSIS I AND II THEREFORE PRODUCES 4 HAPLOID (N) CELLS FROM ONE DIPLOID (2N) CELL.