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Meiosis

Introduction to this section:

Reading:
The power of a billion: India's genomics revolution
Nobel Prize Lecture: Telomerase discovery: the excitement of putting together pieces of the puzzle

Video 1: Haploid and Diploid Cells (2:31)

Video 2: Meiosis (1:48)

Video 3: Meiosis (5:27)

Figure 1: Meiosis

Study Guide (Transcripts/Summaries):
Cells that have two matching sets of chromosomes are referred to as diploid cells. Sexual reproduction is the combination of the genetics of two parents. Sexual reproduction creates new combinations and shuffles genetic variation from generation to generation. The typical diploid cell contains one set of chromosomes from each parent. Because each parent contributes a set of genes, each offspring has the potential to be unique in the traits that are inherited from their parents.

Gametes are haploid cells. Animals produce gametes from germ cells; for example, the male germ cells divide into primary spermatocytes, which undergo meiosis and form secondary spermatocytes and then spermatids. Fertilization is the fusion of two haploid gametes, and produces one diploid zygote.

Meiosis starts with DNA replication and then proceeds with two rounds of nuclear division. The cell starts meiosis I diploid (2n). During prophase I, the chromosomes condense, the nuclear envelope dissolves, and the spindles form. Homologous chromosomes pair up. Their close proximity allows for some exchange or crossing over between arms of the chromosomes. Crossing over creates new genetic variation. During metaphase I, the homologous pairs line up at the metaphase plate. During anaphase I, the homologous pairs are separated and pulled toward opposite poles. During telophase I, the nuclear envelope develops around the sister chromatids. At the end of meiosis I, because the homologues were separated, each new cell is now haploid (n), leaving each nucleus with only one of each homologous pair.

Meiosis II proceeds without DNA replication. During prophase II, the nuclear membrane dissolves, chromosomes condense, and spindles form. During metaphase II, sister chromatids line up at the metaphase plate. During anaphase II, the sister chromatids are separated. At the end of telophase II, there are four haploid cells that have a single copy of each homologous chromosome in their nucleus.

Sources:
Video 1: Diploid Haploid Cells | Genetics | Biology, https://www.youtube.com/watch?v=MU83VWAvUf4
Video 2: Meiosis, https://www.youtube.com/watch?v=tZVeMK6Kxdk
Video 3: Meiosis, https://www.youtube.com/watch?v=-DLGfd-Wpr4
Figure 1: Meiosis from: CK-12 Biology (say thanks), http://www.ck12.org/book/CK-12-Biology-Concepts/, Figure 2.71