Assignent Solutionnurs

In this unit, we discussed the role of DNA in a cell, and the process of cell division. It’s challenging to remember the phases of mitosis and meiosis. Throughout this lesson, it is important to remember the number of chromitids in a human cell, and the organizational changes that are occurring in each phase.
The cell cycle (see lecture slides 16-20) is the normal life cycle of a cell. A typical human cell such as a skin cell or nerve cell (not sperm or egg) has 23 pairs of chromatids (23 x 2 = 46 chromatids). During cell division, When a cell is about to divide, the cell enters S phase of the cell cycle. “S” stands for synthesis, which means “to make”. What is the cell making? MORE DNA! In S phase, the cell copies all of its DNA. Therefore, 46 chromatids are copied, and at the end of S phase, the cell now has 46 x 2 = 92 chromatids. The cell then leaves S phase and enters G2, a phase of the cell cycle where all the newly copied DNA is checked for errors. At the end of G2, the cell is now ready to enter M phase, which is the phase where cell division occurs.
Mitosis is the type of cell division that a typical body cell (ex. a skin cell) would go through when it needs to make an identical copy of itself. Mitosis is divided into several phases: prophase, metaphase, anaphase, telophase, and cytokinesis (see lecture slides 21-26). During prophase through telophase, the cell has twice as much DNA as it normally does (92 chromatids), and is organizing the chromatids and getting the cell ready to split into two.
In the phase called cytokinesis, the 92 chromatid cell splits into two identical 46 chromatid cells. These cells are identical to the original cell that was in G1 of the cell cycle.
Summary of mitosis:
92 chromatid cell (leaving G2 of cell cycle) à enters mitosis à prophase through telophase (92 chromatids) à cytokinesis (divides into two cells 92 à 46 + 46 chromatids)
Meiosis is a special type of cell division that is only used to make sperm and egg cells. Sperm and egg cells have 23 unpaired chromatids. This is because a sperm’s DNA will combine with an egg’s DNA to produce 23 pairs of chromatids, resulting in a new offspring. Because sperm and egg require half the DNA of a typical body cell, their cell division must be different from typical mitosis.
Meiosis begins, just like mitosis, with a single cell that finished G2 and has 92 chromatids (see lecture slides 31-37). This 92 chromatid cell undergoes TWO rounds of cell division. The first round of cell division is called meiosis I and has the same phases as a regular mitosis (prophase I, metaphase I, anaphase I, telophase I). It ends with interkinesis (the first cell division into two cells 92 à 46 + 46 chromatids).
The two 46 chromatid cells then each undergo a second round of cell division (prophase II, metaphase II, anaphase II, telophase II). It ends with cytokinesis (the second cell division, where each of the two cells divide in half  46 à 23 + 23 chromatids).
Summary of meiosis:
92 chromatid cell (leaving G2 of cell cycle) à enters meiosis I à prophase through telophase (92 chromatids) à cytokinesis (divides into two cells 92 à 46 + 46 chromatids) à both 46 chromatid cells enters meiosis II à prophase through telophase ( 46 + 46 chromatids) à cytokinesis (each 46 chromatid cell divides into two cells  (46 à 23 + 23 chromatids) and (46 à 23 + 23  chromatids).
Let’s play a meiosis game: Open-up your lecture slides and look at slides 36 and 37. I will begin by supplying a number of chromatids and a description copied from the slide describing what is happening. You respond by identifying the phase of meiosis. (For example, if I say 46 + 46 chromatids and “Chromatids line-up at the equator”, you should reply “metaphase II”.) Then it is your turn: write down a chromosome number and a description of a different phase of meiosis. The person responding after you must identify the phase you are describing. Then they must choose another meiosis phase, and provide its chromosome number and description.