Meiosis Reduction Division Production of Gametes Vocabulary Meiosis

Meiosis Reduction Division Production of Gametes

Vocabulary Meiosis • Homologous Chromosomes - chromosomes in a biological cell that pair (synapse) during meiosis • Crossing over - when two chromosomes break and then reconnect but to the different end piece. The result is an exchange of genes, called genetic recombination • Chromatid - is one of two identical strands of DNA making up a chromosome that are joined at their centromeres • Centromeres – holds two chromatids together to make a chromosome • Tetrad - consists of two homologous chromosomes attached together each composed of two sister chromatids. • Haploid – (n) half the normal number of chromosomes • Diploid – (2 n) the normal number of chromosomes • Loci - the position of a gene on a chromosome

Gametes have a single set of chromosomes • Gametes (egg and sperm) are haploid, with only one set of chromosomes • Somatic (body) cells are diploid. • Meiosis reduces the chromosome number from diploid to haploid Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

The human life cycle • Meiosis creates gametes • Mitosis of the zygote produces

1. Chromosomes are duplicated in meiosis I (4 n) 2. Crossing over occurs (meiosis I) MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Microtubules attached to Spindle kinetochore Sites of crossing over Chromatin Sister chromatids Tetrad Figure 8. 14, part 1 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Metaphase plate Centromere (with kinetochore) ANAPHASE I Sister chromatids remain attached Homologous chromosomes separate

3. Then cell divides once to form two cells that are diploid (2 n) and then those two cells divide again to form four daughter cells which are haploid (n) gametes. (meiosis II) MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Figure 8. 14, part 2 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Haploid daughter cells forming

Meiosis number of chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Mitosis chromosome number Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

In meiosis I, homologous chromosomes are paired – While paired, they cross over and exchange genetic information (DNA) – homologous pairs are then separated, and two daughter cells are produced Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Meiosis II is essentially the same as mitosis without the doubling of the chromosomes – The two daughter cells divide – sister chromatids of each chromosome separate – result is four haploid daughter cells each with ½ the genetic information Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE ANAPHASE TELOPHASE 2 n Chromosome replication 2 n = 4 Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together 2 n Daughter cells of mitosis Figure 8. 15 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings MEIOSIS I METAPHASE I ANAPHASE I TELOPHASE I Daughter cells of meiosis I Haploid n=2 No further MEIOSIS II chromosomal replication; sister chromatids separate during anaphase II n n Daughter cells of meiosis II

Homologous chromosomes carry different versions of genes at corresponding loci • Each chromosome of a homologous pair comes from a different parent – Each chromosome thus differs at many points from the other member of the pair – Crossing over mixes the genes from the parents increasing variation Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Crossing over further increases genetic variability • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis • This increases genetic variability Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

MEIOSIS I END OF INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

MEIOSIS TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

Spermatogenesis vs Oogenesis • Spermatogenesis goes though meiosis and produces 4 sperm for every one follicle cell. • Oogenesis goes though meiosis and produces ONLY 1 mature egg cell and 3 polar bodies for every one follicle cell. – The polar bodies will be broken down and reabsorbed.

a SPERMATOGENESIS b OOGENESIS spermatogonium oogonium primary spermatocyte primary oocyte meiosis l secondary spermatocyte secondary oocyte meiosis ll polar body spermatids polar bodies (will be degraded) egg

Accidents during meiosis can alter chromosome number • Abnormal chromosome count is a result of nondisjunction – Either homologous pairs fail to separate during meiosis I Nondisjunction in meiosis I Normal meiosis II Gametes n+1 n– 1 Number of chromosomes Figure 8. 21 A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

– Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n+1 n– 1 n Number of chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings n Figure 8. 21 B

• Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n+1 Zygote 2 n + 1 Sperm cell n (normal) Figure 8. 21 C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings