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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