Chapter 12

Chapter 12 - Chapter 12: Meiosis During sexual...

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Chapter 12: Meiosis During sexual reproduction, a sperm (male reproductive cell) and an egg (female reproductive cell) unite to form a new individual, a process called fertilization . Meiosis is nuclear division that precedes formation of gametes (egg and sperm) and results in the halving of the chromosome number. 12.1 How Does Meiosis Occur? Each organism has a characteristic number of chromosomes . The vast majority of plants and animals have more than one of each type of chromosome and are said to be diploid . A gene is a section of DNA that influences one or more hereditary traits in an individual. Different versions of a specific gene are called alleles . Chromosomes of the same type, with the same genes in the same locations, are homologous chromosomes , or homologs . Homologous chromosomes carry the same genes, but each homolog may contain different alleles. Sex chromosomes determine the sex of the individual; all other chromosomes are autosomes . The karyotype is the number and types of chromosomes present in an organism. Diploid – two versions of each type of chromosome. Diploid organisms have two alleles of each gene-one on each of the homologous pairs of chromosomes. Organisms such as bacteria are haploid because their cells contain just one of each type of chromosome. Haploid organisms do not contain homologous chromosomes. By convention, the letter n stands for the number of distinct (humans, n=23) types of chromosomes in a given cell and is called the haploid number . If sex chromosomes are present, they are counted as a single type in the haploid number. A cell’s ploidy indicates the number of each type of chromosome present. Ploidy refers to the number of each type of chromosome present; haploid number identifies how many different types of chromosomes occur. During meiosis, the number of chromosomes is reduced by half. An Overview of Meiosis Meiosis consists of two cell divisions, meiosis I and meiosis II. Just before meiosis begins, each chromosome in the diploid 2 n parent cell is replicated. When replication is complete, each chromosome consists of two identical sister chromatids attached at the centromere ( Figure 12.3a ). In
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This note was uploaded on 05/06/2008 for the course BIOL 100 taught by Professor Staff during the Spring '07 term at UMBC.

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Chapter 12 - Chapter 12: Meiosis During sexual...

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