Cell Division and Reproduction - CHAPTER 4

CHAPTER IV CELL DIVISION AND REPRODUCTION In the human body, as in virtually all large multicellular organisms, some cells continue to grow and divide throughout life, whereas others stop dividing at some stage of development and may not regain division ability again. Human nerve cells...

CHAPTER IV CELL DIVISION AND REPRODUCTION In the human body, as in virtually all large multicellular organisms, some cells continue to grow and divide throughout life, whereas others stop dividing at some stage of development and may not regain division ability again. Human nerve cells (see CHAPTERIV.PPT file Slide 2), for example, stop increasing in number by the second year of life and never regain the ability to divide. Most cells in the liver stop multiplying when it reaches its final size, around the time of puberty, but can start dividing again if less than 2/3 of the liver is removed. Since the cell division is a continuous process and the main features of this event is same for all eukaryotic cells (with the exception of germ cells “GAMETES”) it is also known as the CELL CYCLE (Slide 3). Hypothetically, if the cell cycle is timed as 24 hours, the three preparatory stages (G1, S and G2) take about 23 hours and the mitosis itself only about 1 hour. During the G1 phase the major event is the protein synthesis. The cell stars to multiply every single protein it has and also prepares the enzymes required for the DNA duplication (DNA Replication) which would be soon take place in the S phase. During the S phase, the major activity is the DNA Replication (Slide 4) and this continues until all chromosomal DNA is duplicated. While the DNA replication is proceeding, protein synthesis is still carried out in the background by the cell. At the end of the S phase, all the chromosomes become pairs of identical sister chromatids, joined at regions called centromeres (Slide 5). During the G2 phase the final remaining proteins and other macromolecules are synthesized and the cell reaches its maximum size. When the cell completes all the needed preparations for cell division, finally the M phase (Mitosis) starts (Slide 4). Mitosis distributes the genetic material into two equal sets in two daughter nuclei (KARYOKINESIS) and the entire cell is usually then divides (CYTOKINESIS) to give two daughter cells with one daughter nucleus to each. Those cells that naturally stop dividing (such as nerve cells) are “arrested” at some point in the G1 phase (or deviate out from the cycle at the G1 phase; which is called G0 phase, and remain arrested forever). Once a cell passes the G1 arrest point, the cycle seems to operate spontaneously and S, G2, M and the next G1 follows. The G1, S and G2 phases are collectively called INTERPHASE because these are the preparatory stages between two mitoses.  MITOSIS The M phase or mitosis itself is consisted of 4 phases – PROPHASE, METAPHASE, ANAPHASE and TELOPHASE. However, it is needed to mention at this stage that none of the phases have clear-cut boundaries. Rather, by examining a cell under the microscope, some characteristic appearances give us an idea about which phase of mitosis the cell is in. PROPHASE Although at the early stages of prophase the chromosomes do not looked doubled (due to the still presence of the nuclear envelope), at the end of the prophase and during the prometaphase (a transition phase between prophase and metaphase) the sister chromatid pairs become visible due to the disintegration of the nuclear envelope (Slide 6). Another characteristic event occurring during the prophase is the migration of the centrosomes (pairs of centriols) towards the opposite poles of the cell and the beginning of the formation of mitotic spindles from these structures. These spindles start to attach to the centromere regions (which are divided and called “kinetochores”) of the sister chromatids. METAPHASE One of the most important event in this phase is the arrangement of sister chromatid pairs to the equatorial plane of the cell which is called “metaphase plate”. At this phase of mitosis the already divided centromeres (kinetochores) are attached with individual spindle fibers and become ready to be pulled towards the two different poles of the cell (Slide 7). Also the animal cells at this phase tend to become completely spherical. ANAPHASE The separation of the sister chromatids starts at this phase and progresses (Slide 7). Because they are separated they are by their own right are called daughter chromosomes. This phase comes to an end when a constriction around the equator of the cell called “Cleavage Furrow” initiates (Slide 8). TELOPHASE The “Cleavage Furrow” deepens gradually at this phase and around the daughter chromosomes, arrived to the poles of the nearly-forming two cells, a nuclear envelope starts to form. Also within the nucleus the nucleolus is started to form at this stage (Slide 7). When the nuclear envelope formation is finished (KARYOKINESIS), the cytoplasmic division is finally finished (CYTOKINESIS) and two daughter cells are formed (Slide 8). Although the essential features of mitosis are similar in animal and plant cells (Slide 9), there are some differences. Firstly; since the presence of rigid cell walls, plant cells do not become spherical during metaphase in plant mitosis. Secondly; in plant cells cleavage furrow do not form, instead the two daughter cells become separated by the formation of a CELL PLATE between them (Slide 8). ASEXUAL AND SEXUAL REPRODUCTION Reproduction in broadest terms is the capacity to produce others of their own kind. Reproduction assures the continuation of each species by replacing individuals that grow old and die or that are killed or eaten by predators or by other individuals. Given the enormous diversity of living things and the universality of reproduction among them, it is not surprising that reproductive mechanisms vary widely from one group of organisms to another. Nevertheless, all reproduction occurs in one of two forms; ASEXUAL SEXUAL ASEXUAL REPRODUCTION In asexual reproduction new individuals are generated by a single parent. In unicellular organisms asexual reproduction is equivalent to cell division. Each of the two cells produced by the cell division, known as “daughter cells”, contains the same genetic program as the parent cell, because the parent cell makes an exact copy of its genetic program before it divides. In unicellular prokaryotes (bacteria and cyanobacteria) the asexual reproduction is known as BINARY FISSION and involves replication of the chromosomal DNA and other cellular contents first and then a basic cytokinesis event (Slide 10). In unicellular eukaryotes, which include protists, certain types of yeast and algae, cell division is preceded by MITOSIS. In multicellular organisms, mitotic cell divisions can produce new individuals in several different ways. BUDDING is a form of asexual reproduction in which the offspring originate as outgrows from the parents body (Slide 11 animal example, Slide 13 plant example). Sometimes, fallen-off body parts can also form new individuals and this type of asexual reproduction is called FRAGMENTATION (Slide 12 animal example, Slide 13 plant example). In laboratory environment, even a single cell from a plant can be grown to a whole plant (TOTIPOTENCY) by using tissue culture techniques (Slide 14). However, in the nature such asexual reproduction mechanisms do occur as well and the products are called “clones” (Slide 15). Frequently in our campus such clone poplar trees could easily be observed near mother trees. SEXUAL REPRODUCTION In sexual reproduction, a new individual is produced by the contribution of two parents (Slide 16). These parents, in their sex organs (GONADS), produce sex cells (GAMETES) and when these gametes unite (FERTILISATION) first a ZYGOTE, than after about 1 X 1013 mitosis later, a new individual is formed. Gametes differ from all other body cells (SOMATIC CELLS) in their DNA content. In humans, while the somatic cells contain two copies of each chromosome (2 X 22 + XX or XY), they are called DIPLOID, whereas the gametes contain only half of this chromosome content (22 + X or 22 + Y), they are called HAPLOID (Slide 17). The zygote that results from fertilization contains a mixture of DNA and therefore of genetic programs from the two parents contributing the gametes. Sexual reproduction, unlike asexual reproduction, creates genetic variation in new individuals and therefore is more widespread amongst the multicellular eukaryotic organisms. Formation of haploid gametes in gonads is an extremely important event which ensures the maintenance of constant chromosome number for a given species throughout generations. The biological process by which these haploid gametes are produced is called MEIOSIS. MEIOSIS In animals and plants, meiosis occurs only among specialized cells located in reproduction organs. These specialized cells accomplish the halving of chromosome number (Diploid to Haploid) by duplicating their entire set of chromosomes once (Interphase) and then dividing twice in succession (Slide 18). Each of the meiotic divisions (hence, designated as I or II) consists of the same four phases of cell division that occur in mitosis and the same cellular organelles (spindles, centrosomes, nuclear membrane etc.) and related structures (centromeres, chromosomes etc.) are involved. FIRST MEIOTIC DIVISION (DIVISION I)  INTERPHASE Interphase of meiosis is essentially the same with Interphase of mitosis (Slide 3). PROPHASE I Prophase I of meiosis is similar to the Prophase of mitosis (disintegration of the nuclear envelope and thus the duplicated sister chromatids becoming visible, migration of the centrosomes to the opposite poles of the cell and attachment of spindle fibers to the centromere regions of the sister chromatid pairs). However, there are some characteristic differences between Prophase I of meiosis and Prophase of mitosis. 1) In Prophase I of meiosis, homologous chromosomes (Slide 19) – pairs of the same chromosomes – (all 23 pairs in females and 22 pairs in males (since X and Y are not homologous)) come together in pairs of sister chromatids joined at the centromeres in a process called SYNAPSIS (Slide 20). 2) Because these synaptic pairs are composed of four sister chromatids, their coming together forms a structure called a TETRAD (Slide 20). The sister chromatids in the tetrad structure often exchange segments at points called CHISMATA and this segment exchange event is called CROSSING OVER. Prophase I comes to an end when these already-crossed-over synaptic pairs (tetrads) move toward the equatorial plane of the cell (the metaphase plate) (Slide 20). METAPHASE I During Metaphase I tetrads arrive at the Metaphase Plate but a significant difference between this phase and the Metaphase of mitosis occurs; In mitosis metaphase, the centromere which holds the two sister chromatids, divides whereas, at Metaphase I, this division does not occur therefore the sister chromatids remain attached. As a consequence of this only the homologous chromosomes -in the form of sister chromatid pairs- separate from each other (Slide 20) and one of the pairs of the homologous chromosomes is pulled towards one pole and the other to the other pole. ANAPHASE I Mitosis Anaphase and Anaphase I of meiosis is essentially the same except at this phase in meiosis, the sister chromatids are still attached together. TELOPHASE I The events taking place in Telophase I of meiosis is almost the same as Telophase of mitosis (Arriving of the sister chromatid pairs to the poles, Cleavage Furrow formation, nuclear membrane formation, beginning and completion of the cytokinesis). However, the result is different. Here, the two cells produced contain only the half of the number of chromosomes contained in the nucleus of the parent cell (counting each sister chromatid pair as one duplicated chromosome) thus two haploid cells are formed (Slide 21). In some organisms, a short rest period called “INTERKINESIS” take place between the first and second meiotic divisions but in other organisms the second meiotic division is initiated without further delay. SECOND MEIOTIC DIVISION (DIVISION II) The second meiotic division (Slide 21) is basically a mitotic division with the exception of the absence of Interphase. - During Prophase II, nuclear envelope disintegrates, sister chromatids become visible, centrosomes migrate to the opposite poles of the cell and spindle fibers attach to the centromere regions of the sister chromatid pairs. - During Metaphase II, the centromere regions divide and attachment of the spindle fibers to the centromeres completed. - During Anaphase II, the sister chromatids are pulled away from each other (separate), and started to move towards the poles of the cell. Cleavage Furrow formation is initiated towards the late portion of this phase. - During the Telophase II, sister chromatid pairs arrive to the poles of the cells, Cleavage Furrow formation progresses, nuclear membrane formation begins and completed and finally the Cleavage Furrow deepens further and the Cytokinesis occurs. The overall result of meiosis is the production of 4 Haploid gametic cells (Slides 22 and 23). For humans, there are 23 chromosome pairs. So for an egg or sperm, a chromosome either comes from the father or mother. For 23 pairs of chromosomes the total number of possible combinations is 2 23 or about 1/10.000.000. So, the chance that two eggs or sperms produced by an individual will have identical chromosome sets is very slight indeed. This probability further decreased by the genetic variability caused by crossing over and occurrence of mutations. - END OF CHAPTER IV-