Cell Division and Mitosis

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DMA

• What is the difference between mitosis and
  meiosis?

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Meiosis

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

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Review

• What are the four reasons why cells divide?
• Growth and development
• Healing
• Increase the surface area to volume ration
• To create reproductive cells
• Meiosis and cytokinesis are the processes that
  create sex cells (a.k.a. gametes, germ cells)
• All sexually reproductive organisms engage in
  these processes.

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Review

• Chromosomes made of condensed chromatin (DNA and
  proteins)
• DNA contains genes
• Our somatic or body cells are diploid
• Our gametes or sex cells are haploid

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(Y)
(X)
Remember you inherit one chromosome of a pair
from Mom and one from Dad!
one pair of duplicated chromosomes they are
homologous
Fig. 10.3, p. 163
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Meiosis Key Concepts

• Meiosis is a form of nuclear division.
• Gametes are haploid (n) and are the result of
  this process.
• During meiosis, chromosomes may exchange random
  segments by crossing over
• Meiosis leads to variations in traits among
  offspring.

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How Meiosis Halves theChromosome Number

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

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Germ Cell at Interphase

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

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Prophase I

• Prophase I
• Condensing of duplicated chromosomes
• Nuclear envelope disappears
• Crossing over occurs at synapsis
• Spindle forms

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Prophase I

• Crossing over and synapsis

Chiasma the x-shaped region representing
homologous chromsomes that have crossed over
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Metaphase I

• Metaphase I
• Pairs of homologous chromosomes lined up randomly
  at equator

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Meiosis I

• Anaphase I
• Duplicated chromosomes pulled apart from
  homologue toward opposite spindle poles
• Telophase I
• Very brief
• Cytokinesis I

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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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Gametogenesis

• Gametogenesis the production of reproductive
  cells by reproductive tissues and organs
• NOTE many plants produce spores (haploid cells).
  Animals produce sperm and egg.
• Three opportunities for genetic variation
• Prophase I crossing over at synapsis
• Metaphase I random alignment of chromosomes
• Random fertilization

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Spermatogenesis
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Unequal division of cytoplasm
Polar bodies die.
Oogenesis