Chapter 9.5 - Meiosis

College Prep BiologyMr. Martino

I. Introduction

• Germ Cells: develop into sex cells (sperm and egg)

• Gametes: mature sex cells – sperm and egg

III. Sexual and Asexual Reproduction

• Sexual Reproduction: reproductive process that includes the union of sperm and egg– Two parents – each

donate a complete set of genes

• Asexual Reproduction: production of offspring without the participation of sperm and egg– Single parent

• Allele: unique form of the same gene (a gene)

IV. Meiosis Halves Chromosome Number

• Meiosis: nuclear division that divides chromosomes twice

• Diploid number: (2n) pair of each type of chromosome– Homologous

chromosomes: both chromosomes of a pair• Same length and shape• Genes are for same traits

• Haploid number: (n) results from the second division of meiosis (sex cells)

• Sister chromatids: two attached chromosomes– Once they pull apart,

they are considered chromosomes

• Meiosis consists of 2 divisions:– Meiosis I: DNA is

replicated and divides– Meiosis II: another cell

division without more chromosome doubling

V. Stages of Meiosis• Meiosis: the process

that produces haploid gametes (sex cells)– 2 consecutive

divisions: meiosis I and meiosis II• Meiosis II is

essentially the same as mitosis

– 4 daughter cells produced

– Only 1 duplication of chromosomes, second division halves chromosome number

Interphase I

• Like interphase in mitosis

• Each pair replicates forming a 4 chromatid structure

Prophase I• Most complex phase of

meiosis • Lasts 90% of meiosis• homologous

chromosomes form tetrads (4 chromatids)

Structure of a Tetrad

• Tetrad = structure consisting of 2 pairs of chromosomes– 2 chromosomal

pairs = 4 sister chromatids

– Visible during Prophase I only

Metaphase I

• Tetrads align on metaphase plate

Anaphase I• Begins as homologous

chromosomes (still attached at centromere) separate and move towards opposite poles

• Only tetrads are separated

Telophase I and Cytokinesis

• Chromosomes arrive at opposite poles

• Cytokinesis results in 2 diploid cells

• No DNA replication occurs between meiosis I and meiosis II

Prophase II

• Chromosomes must condense again since there was a pause prior to meiosis II

Metaphase II

• Chromosomes are aligned at metaphase plate

Anaphase II

• Begins when centromeres of sisters chromatids separate

Telophase II and Cytokinesis

• Nuclei form at opposite poles

• Cytokinesis occurs

• Forms 4 haploid daughter cells

VI. From Gametes to Offspring• Spermatogenesis:

sperm formation– results in 4 viable

sperm• Oogenesis: egg

formation– First meiotic division

results in a polar body and a secondary oocyte

– Second division results in 3 polar bodies and 1 egg

– Polar bodies are reabsorbed – egg is viable

VII. Meiosis and Mitosis Compared

• Chromosomes of both replicate only once

• Mitosis consists of one cell division and two diploid cells

• Meiosis entails two cell divisions and four haploid cells

• Mitosis and meiosis II are virtually identical

9.6 Meiosis Increases Genetic variation among offspring

• Genetic variation – differences among the gene combinations in the offspring due to sexual reproduction

• Assortment of Chromosomes – the aligning of chromosomes during metaphase I is random; which results in many possible chromosome combinations

• Crossing Over – the exchange of genetic material in between homologous chromosomes; during prophase I when tetrads are present