Chapter 15 Summary

The Eukaryotic Cell Cycle

This section mainly talks about the cell cycle itself. The cell cycle has 4 phases, G1, S, G2, and M. During the G1 phase, the cells prepares to divide. It absorbs enough nutrients fro the cell to last through the entire process. During the S phase, chromosomes are duplicated and the two replicas are joined by a protein called a centromere. During the G2 phase, the cell makes proteins required for dividing and sorting. The final and the shortest stage is the M phase. The M phase is essentially the division of one nucleus into two nuclei. The cell cycle contains 3 checkpoints (G1, G2, and metaphase) to make sure the cell has the right balance of everything and it isn't defected. 

Mitotic Cell Divison

In Mitosis, the cell is divided to produce two identical daughter cells. The starting product and the final product are most diploid (2n). Mitotic cell division is essential for sexual reproduction. Mitosis consists mainly of 4 steps, prophase, metaphase, anaphase, and telophase. In prophase, the nucleus starts to disappear and the DNA starts to condense, forming chromsomes (consists of two identical sister chromatids, replicates in the S phase). During metaphase the chromosomes line up along the center, single file. During anaphase, each half of a chromsome is pulled to the opposite ends, creating a diploid number of chromosomes on each end. During telophase, the nucleus reappears, the DNA un-condenses, and the cells divides completely. 

Meiosis and Sexual Reproduction

Meiosis is the formation of 4 haploid gametes from a cell. Gametes are essential for sexual reproduction. Meiosis consists of 2 cycles. In teh first cycle, homologous pairs of chromosomes line up along the center (one from mom, one from dad). This is because the final product has to be a haploid cell, so there can be only one chromosome from each homologous pair. After the homologous pairs are separated into two cells, the cells enter the second cycle. In teh second cycle, sister chromatids of the chromosomes are separated, forming gametes (single copy of each homologous pair). 

Variation in Chromosome Structure and Number

This section basically talks about the different structures a chromosome can have (ex. X and Y). These shapes are caused by the points at which the sister chromosomes attach to each other. For example, if the attach in the center, they form a X shape. Chromosomes also come in specific numbers. The human cell is diploid (46), meaning we get one set of chromosomes from mom (23) and one set from dad (23). Each species has a different "ploid" number and a different number of gene pairs. 

Useful Links

1. Meiotic Recombination and Male Infertility - This journal mentions that male infertility is caused by a defect in their process of meiosis which is caused by mutations. These mutations can affect the "ploidy" number in gametes. 

2. This image compares the process of mitosis to meiosis. It is easy to understand and outlines each step clearly.

3. This image is interesting because it compares meiosis in males and females. Males produce 4 mature gametes in one cycle, however females produce only 1 mature ovum.