UT Knoxville BIOL 140 - Chapter 12 (59 pages)

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Chapter 12 Meiosis 2011 Pearson Education Inc Introduction During sexual reproduction a sperm and an egg unite to form a new individual This process is called fertilization Meiosis is nuclear division that precedes the formation of gametes egg and sperm and results in a halving of chromosome number 2011 Pearson Education Inc Chromosomes Come in Distinct Types Each organism has a characteristic number of chromosomes The karyotype is the number and types of chromosomes present in an organism Sex chromosomes determine the sex of the individual all other chromosomes are autosomes Humans have 46 chromosomes in every cell except their gametes 1 pair of sex chromosomes 22 pairs of autosomes 2011 Pearson Education Inc 2011 Pearson Education Inc Homologous Chromosomes Have the Same Genes Chromosomes of the same type are called homologous chromosomes or homologs Chromosomes carry genes 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 Homologs carry the same genes in the same locations but each one may contain different alleles 2011 Pearson Education Inc 2011 Pearson Education Inc The Concept of Ploidy The haploid number n indicates the number of distinct types of chromosomes present A cell s ploidy n 2n 3n etc indicates the number of each type of chromosome present 2011 Pearson Education Inc Ploidy Varies among Organisms Organisms whose cells contain just one of each type of chromosome are called haploid n Those whose cells contain two versions of each type of chromosome are termed diploid 2n Diploid cells have one paternal chromosome and one maternal chromosome Organisms with three or more versions of each type of chromosome are called polyploid 3n 4n etc 2011 Pearson Education Inc 2011 Pearson Education Inc An Overview of Meiosis Meiosis reduces chromosome number by half In diploid organisms the products of meiosis are haploid Just before meiosis begins each chromosome in the