College Prep Biology Mr. Martino Chapter 9.5 - Meiosis College Prep Biology Mr. 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