Chapter 13 Meiosis

Living Things Reproduce Results in similarities and differences between offspring and parents Facilitated by heredity or inheritance Passing on of characteristics from one generation to the next Genetics is the study

Reproduction Asexual produces offspring that are exact copies of parents Single parent Produces clones, groups of genetically identical individuals Variation from mutation Sexual produces offspring with unique combinations of genes Two parents Vary genetically

Maternal and Paternal Chromosomes 23 different types Size, banding, and centromere Karyotype Homologs Genes for same characters Autosomes and sex chromosomes

Identifying Cells n = number of chromosomes in a single set Cells with 2 sets are diploid (2n) In cells where synthesis has occurred, each appears as sister chromatids Somatic cells Cells with 1 set are haploid (n) Gametes 22 autosomes and 1 sex chromosome Egg is X and sperm is Y Assign information to cell at right

Human Life Cycle Egg and sperm fuse = fertilization = zygote 2 chromosomes sets Mitosis throughout life All cells but gametes Conserves chromosome numbers Meiosis forms gametes Reduce chromosome sets by how much?

Sexual Life Cycles Alternation of fertilization and meiosis is common Timing varies between species Alternation of generations

Cellular Division Mitosis n or 2n are capable Depends on life cycle type Meiosis 2n only Chromosomes can’t be further reduced

Meiosis (Overview) Similar to mitosis 2 cell divisions Produces 4 daughter cells with ½ number of chromosomes Only time homologs associated with each other Each with different alleles or different versions of genes at same loci, or location

Homologs pair Synapsis and crossing over of nonsister chromatids

Resembles mitosis Sister chromatids split

Comparing Cell Division MitosisMeiosis Conserves number of chromosomes Daughter cells genetically identical Reduce chromosome set from diploid to haploid Daughter cells genetically different Meiosis I Synapsis and crossing over Homologs on metaphse plate Separation of homologs

Genetic Variation Mutation is ultimate source of genetic diversity Creates alleles, which are what? Sexual reproduction reshuffles alleles = unique combos 3 mechanisms contribute Random fertilization Fusion of any 2 gametes with previous variation = zygote with more (2 23 ) 2 = 70 trillion Independent assortment Random orientation of homologs 2 23 = 8.4 million Crossing over Produces recombinant chromosomes, chromosomes with genes from 2 different parents 1-3 times per chromosome is normal

Genetic Variation Mechanisms Independent Assortment Crossing Over