AP Biology Chapter 10 Meiosis

Key Concepts: Sexual reproduction entails meiosis, gamete formation, and fertilization Germ cells of male and female animals produce gametes Diploid cells have two of each type of chromosome, one from each parent Meiosis divides the chromosome number in half

Key Concepts: Gametes are haploid (n) The union of gametes at fertilization results in a diploid number (2n) During meiosis, chromosomes may exchange random segments by crossing over Meiosis leads to variations in traits among offspring

Comparison of Asexual and Sexual Reproduction Single parent  offspring inherit same genes as parent (number and type) Sexual Produces genetic variation among offspring Involves Meiosis Gamete formation Fertilization

Chromosomes come in pairs, like socks One homolog Homologous chromosomes (unreplicated)

Homologous Chromosomes Cell has two of each chromosome One chromosome in each pair from ♀ (mom) and one from ♂ (dad) Paternal and maternal chromosomes carry the same genes but different alleles.

Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits. This variation is traits is the basis for evolutionary change

Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells anther ovary Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number Human Karyotype

Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei form

A Chromosome Refresher…

Germ Cell at Interphase A diploid cell (2n) DNA duplicated Ready for first division of meiosis

Meiosis I

Meiosis I - Overview newly forming microtubules Prophase I Meiosis I Metaphase I spindle equator one pair of homologous chromosomes Anaphase I Telophase I Stepped Art

Meiosis I Prophase I Metaphase I Homologs pair Crossing over occurs Spindle forms Nuclear envelope disappears Metaphase I Pairs of homologous chromosomes are at spindle equator Random arrangement

Crossing Over Crossing over

Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring Either the maternal of paternal member of a homologous pair can end up at either pole The chromosome in a gamete are a mix of chromosomes from the two parents

Meiosis I – continued… Anaphase I Telophase I Duplicated chromosomes pulled apart from homologue toward opposite spindle poles Sister chromatids remain attached Telophase I Chromosomes arrive at poles Usually followed by cytokinesis Cells divides into two cells

Meiosis II - overview NO DNA duplication between I and II Sister chromatids are separated Result is four haploid daughter cells

Meiosis II Prophase II Anaphase II Telophase II Metaphase II New spindle formed Chromosomes move towards equator Metaphase II Duplicated chromosomes positioned at the equator Anaphase II Sister chromatids are moved apart Telophase II Four nuclei with haploid chromosome number (n) resulting in four cells

combinations possible 1 2 3 combinations possible Possible outcomes of the random alignment of only three pairs of chromosomes at metaphase I (without crossing over!!) As a result of random alignment, the number of possible combinations in a gamete is 2n, where n=# of chromosome types or or or Stepped Art

Results of Recombination Random alignment

Oogenesis

Spermatogenesis

Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Adds to variation among offspring

In Conclusion Life cycle of sexually reproduction includes meiosis, gamete formation, and fertilization A germ cell (2n) has two of each type of chromosome inherited from each parent. Each pair is homologous. Chromosomes are duplicated in interphase. Meiosis consists of two divisions

In Conclusion Meiosis produces haploid cells In meiosis I, each chromosome moves away from homologous partner. In meiosis II, sister chromatids are separated Meiosis produces haploid cells Crossing over, random allocation of chromosomes, and the chance of gametes meeting at fertilization contribute to variation in traits among offspring