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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.

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chapter 9 5 meiosis

Chapter 9.5 - Meiosis

College Prep Biology

Mr. Martino

i introduction
I. Introduction
  • Germ Cells: develop into sex cells (sperm and egg)
  • Gametes: mature sex cells – sperm and egg
iii sexual and asexual reproduction
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
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
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
Interphase I
  • Like interphase in mitosis
  • Each pair replicates forming a 4 chromatid structure
prophase i
Prophase I
  • Most complex phase of meiosis
  • Lasts 90% of meiosis
  • homologous chromosomes form tetrads (4 chromatids)
structure of a tetrad
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
Metaphase I
  • Tetrads align on metaphase plate
anaphase i
Anaphase I
  • Begins as homologous chromosomes (still attached at centromere) separate and move towards opposite poles
  • Only tetrads are separated
telophase i and cytokinesis
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
Prophase II
  • Chromosomes must condense again since there was a pause prior to meiosis II
metaphase ii
Metaphase II
  • Chromosomes are aligned at metaphase plate
anaphase ii
Anaphase II
  • Begins when centromeres of sisters chromatids separate
telophase ii and cytokinesis
Telophase II and Cytokinesis
  • Nuclei form at opposite poles
  • Cytokinesis occurs
  • Forms 4 haploid daughter cells
vi from gametes to offspring
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
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
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