A. Chapter 15 Summary

          Although the cell cycle may seem quite elementary, because it is such an essential part of biology it is important to review and be sure that you completely understand it. Chapter 15 focuses on eukaryotic cell cycle, which like all things eukaryotic is much more intricate and complex than anything prokaryotic. There are two different ways that a eukaryotic cell can replicate, through mitosis or meiosis.

            One of the main differences between prokaryotic cell division and Eukaryotic cell division is chromosomes. A chromosome is created at the start of cell division when a strand of DNA condenses. Cytogenetics is the field of genetics that involves the microscopic examination of chromosomes and cell division. Cytogenetics is possible because chromosomes, unlike unwound strands of DNA, are compact enough to be seen under a light microscope. Chromosomes always come in sets. For example, humans have sets of 23 chromosomes. But other eukaryotes have different numbers of chromosomes in each set; fruit flies only have 4. In most eukaryotic cells are diploid, meaning there are two different sets chromosomes, one from mom and one from dad. Homologues chromosomes are the same kind of chromosome (they have the same number in the set) but they are different because one is from each parent. A few cells are haploid, which means that they only contain one set of chromosomes.

          Cell division does not just happen whenever. But instead there is an entire cycle the at preps the cell and makes sure that everything runs smoothly. The cell cycle includes the phases G1, S, G2, and M. In order for everything to work out correctly, the cell has set up checkpoints along the way. When everything is good, the cell sends out a cyclin which binds to a cyclin-dependent kinase. When they bind together then the cells sends out a series of reactions that generally gives a go ahead. The first stage of the cell cycle is G1. This is known as a resting stage. It is where the cell grows and commits to dividing. There is a checkpoint towards the end of G1 known as the restriction point which determines if conditions are favorable for cell division and if the DNA is damaged. The next stage is S, for synthesis. DNA is replicated and referred to as a pair of sister chromatids. For example the fruit flies created 8 chromatids, but technically since they paired up into chromosomes there are still 4 chromosomes. G2 is the second gap which is used to produce the necessary proteins for dividing. It also contains another checkpoint which looks for DNA damage, determines if all of the DNA is replicated, and monitors the levels of proteins needed for M phase. The M phase is the final stage which is when the cell finally replicates and becomes two identical cells during a process called mitosis. Cytokinesis is the actual separation of the cytoplasm, this may be considered the C phase or part of the M phase.

          Mitotic cell division includes two main steps, mitosis and cytokinesis. It is important because it increases population, produces multicellular organisms, and maintains organisms. Interphase is not a part of mitosis, but rather everything that happens before it to make sure the cell is ready. Interphase includes G1, S, and G2. Using the example of the fruit fly, during interphase the 4 chromosomes are replicated to produce 8 sister chromatids. The chromatids that are identical pair up and connect at a centromere. Prophase is the first step of mitosis. The nuclear envelope begins to dissociate. The chromatids condense into highly compacted structures that are readily visible by light microscopy. Next is prometaphase where the nuclear envelope disappears. The centrosomes move apart and mark the two poles. Spindle fibers begin to grow and attach to the sister chromatids at the kinetichore proteins at the centromeres. Two kinetichores attach to each pair of chromatids, one attaching to each side, from opposite poles. In Metaphase all of the pairs of sister chromatids line up in the middle of the cell along the metaphase plate and the pairs break apart. The kinetichore tubules shorten, pulling each sister chromatid closer the pole it was attached to in Anaphase. The poles also get further apart from each other. In telophase the chromatids reach their respective poles and decondense. The nuclear envelope reforms around them. Cytokinesis differs in animal cells and plant cells. In animal cells, a cleavage furrow is created and then completely separates the two cell. In plant cells, a cell plate is first created and then turned into a cell wall separating the two daughter cells.

          The other form of eukaryotic cell division is meiosis. This is where things get a little confusing. Meiosis is the process by which haploid cells are produced from a cell that was originally diploid. Meiosis occurs in germ cells because when they join and create a zygote, the zygote should only have two pairs of chromosomes. If meiosis did not occur and two normal cells joined together to create a zygote, the zygote would have four sets of chromosomes, which is too many and with every generation it would increase. Meiosis creates four, genetically unique haploid cells from one diploid cell.

            The first stage of meiosis, Prophase 1 is the critical difference from mitosis. In prophase 1, homologous pairs of sister chromatids synapse to form bivalents, and crossing over occurs. Crossing over is when the two adjacent chromatids from different chromosomes exchange genetic material. Similar to mitosis, the chromosomes also condense and the nuclear envelope disappears. The next difference in Metaphase I is that the bivalents line up at the metaphase plate randomly. Meiosis II is almost identical to mitosis.

B. Useful Materials

Prevention and Correction Mechanisms behind Anaphase Synchrony: Implications for the Genesis of Aneuploidy

This journal article discusses different prevention and correction mechanisms that make sure that that the spindle fibers connect and disconnect to the right kinetichores correctly. This is a very important step because if one of the many checkpoints misses an incorrect attachment it would lead to aneuploidy. Aneuploidy is discussed in section 4 and is an alteration in the number of chromosomes. Aneuploidy could affect many generations down the line. In the abstract, the author mentions the kinetichore attachment checkpoint which is right before anaphase and makes sure that the spindle fibers are all attached and if they are to the right kinetichore. The NoCut checkpoint is also included and it delays cell cleavage during cytokinesis until chromosomes can free the spindle mid-region.

Added: 2/13/11   Source: PubMed