Chapter 13: Meiosis. Pay particular attention to ... Section 13.1 How does meiosis occur? Especially figures 13.1, 13.2, 13.6, 13.7, 13.8, tables 13.1, 13.2 How Does Meiosis Occur - Meiosis is a nuclear division resulting in cells that have only one of each type of chromosome and half as many chromosomes as the parent cell. In animals it leads to the formation of eggs and sperm. - In diploid (2n) organisms, individuals have two versions of each type of chromosome. The two versions are called homologs. One homolog is inherited from the mother and one from the father. Haploid organisms (n) have just one of each type of chromosome. - Each chromosome is replicated well before meiosis begins. This occurs as DNA is replicated during the S phase of the cell cycle. At the start of meiosis I, each chromosome consists of a pair of sister chromatids, each with a double-helical DNA molecule identical to that of the other sister chromatid - Homologous pairs of chromosomes synapse early in meiosis I, forming a bivalent-two closely paired homologous chromosomes. Non-sister chromatids undergo crossing over. - The pair of homologous chromosomes, connected by at least one chiasma, is moved to the metaphase plate - At the end of meiosis I, the homologous chromosomes are separated and distributed to two daughter cells. The daughter cells are haploid, because each receives one of each type of chromosome. - During meiosis II, sister chromatids of the replicated chromosomes separate and are distributed to two daughter cells - From one diploid cell with replicated chromosomes, meiosis produces four haploid daughter cells with unreplicated chromosomes Figure 13.1
- Cells contain different types of chromosomes that often come in pairs. - Numbers and letters designate the types of Drosophila (fruit fly) chromosomes. - The X chromosome is sometimes called chromosome 1 Autosomes: a chromosome present in two copies in all individuals in a species Sex chromosome: a chromosome not present in two copies in both males and females - Humans have 24 different chromosome an X, a Y, and 22 autosomes named 1 to 22 - Females have two of each autosome and two X chromosomes. This is written as 46,XX where 46 is the number of all chromosomes and the two Xs are the sex chromosomes present in this number. - Males are 46,XY and have the same autosomes but one of each sex chromosome. These summaries of a species, number and types of chromosomes is known as its karyotype ("nucleus form=). - A gene is a section of DNA that influences some hereditary trait - Allele different versions of the same gene - The letter n stands for the number of chromosomes present in a gamete and is called the haploid number. Human sperm and egg cells, for example, are both n = 23 Figure 13.2 - Homologous chromosomes may contain different alleles of the same gene - Cells replicate each of their chromosomes before starting meiosis - At the beginning of meiosis, chromosomes are in