Meiosis

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Meiosis

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 10

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Key Concepts

• Sexual reproduction entails meiosis, gamete
  formation, and fertilization
• Germ cells of male and female animals produce
  gametes
• Diploid cells have two of each type of
  chromosome, one from each parent
• Meiosis divides the chromosome number in half

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Key Concepts

• Gametes are haploid (n)
• The union of gametes at fertilization results in
  a diploid number (2n)
• During meiosis, chromosomes may exchange random
  segments by crossing over
• Meiosis leads to variations in traits among
  offspring

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HUMAN MALE
HUMAN FEMALE
vagina
penis
testis (where sperm originate)
ovary (where eggs develop)
FLOWERING PLANT
ovules inside ovary (where eggs originate)
anther (where sperm originate)
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Comparison of Asexual and Sexual Reproduction

• Asexual
• Offspring inherit same genes as parent (number
  and type)
• Sexual
• Meiosis
• Gamete formation
• Fertilization

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How Meiosis Halves the Chromosome Number

• Gametes
• Eggs and sperm
• Somatic cells (2n)
• Pairs of chromosomes
• Pairs are homologues
• Two divisions
• DNA replication - Meiosis I
• DNA not replicated -Meiosis II
• Result in haploid cell

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(Y)
(X)
one pair of duplicated chromosomes
Fig. 10.3, p. 163
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centromere
one chromatid
its sister chromatid
one chromosome (in duplicated state)
in-text, p. 163
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Fig. 10.5 p. 166
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Germ Cell at Interphase

• A diploid cell (2n)
• DNA duplicated
• Ready for first division of meiosis

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Meiosis I

• Duplicated chromosomes line up
• Homologues are moved apart
• Two daughter cells arise

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Meiosis I

• Prophase I
• Crossing over occurs
• Spindle forms
• Nuclear envelope disappears
• Metaphase I
• Pairs of homologous chromosomes are at spindle
  equator
• Random Arrangement

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Meiosis I

• Anaphase I
• Duplicated chromosomes pulled apart from
  homologue toward opposite spindle poles
• Telophase I
• Cell divides
• Two cells

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Meiosis II

• Sister chromatids are separated
• Result is four haploid daughter cells

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Meiosis II

• Prophase II
• New spindle formed
• chromosomes move towards equator
• Metaphase II
• Chromosomes positioned at the equator

• Anaphase II
• Sister chromatids are moved apart
• Telophase II
• Four nuclei with haploid chromosome number (n)
  resulting in four cells

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Life Cycles for Plants and Animals

• Plants
• Spores gametes
• Animals
• Spermatogenesis (males) - sperm
• Oogenesis (females) - eggs

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Mitosis and Meiosis Compared
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Meiosis and Mitosis Compared

• Mitosis
• Clones produced
• Meiosis
• Variation in traits among offspring
• Crossing-over
• Random assignment of pair of homologous
  chromosomes
• Chance fertilization

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In Conclusion

• Life cycle of sexually reproduction includes
  meiosis, gamete formation, and fertilization
• A germ cell (2n) has two of each type of
  chromosome inherited from each parent. Each pair
  is homologous.
• Chromosomes are duplicated in Interphase.
• Meiosis consists of two divisions

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In Conclusion

• In meiosis I, each chromosome moves away from
  homologous partner. In meiosis II, sister
  chromatids are separated
• Meiosis produces haploid cells
• Crossing over, random allocation of chromosomes,
  and the chance of gametes meeting at
  fertilization contribute to variation in traits
  among offspring
• developed by M. Roig