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MEIOSIS makes gametes. Meiosis produces gametes. Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887 60 years before we knew that DNA contained the genetic material Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells

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meiosis produces gametes
Meiosis produces gametes
  • Meiosis was first observed by the Belgian cytologist Pierre-Joseph van Beneden in 1887
  • 60 years before we knew that DNA contained the genetic material
  • Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells
  • The fusion of gametes is called fertilization
    • It creates the zygote, which contains two copies of each chromosome
  • Meiosis only occurs in the gonads (ovaries and testes).
  • All other cell divisions are mitotic
the sexual life cycle
The Sexual Life Cycle
  • The life cycles of all sexually-reproducing organisms follows the same basic pattern
    • Haploid cells or organisms alternate with diploid cells or organisms

Figure 9.4

the stages of meiosis
The Stages of Meiosis
  • Meiosis consists of two successive divisions, but only one DNA replication
    • Meiosis I
      • Separates the two homologs
    • Meiosis II
      • Separates the two sister chromatids
  • Meiosis halves the number of chromosomes

see Figure 9.5

prophase i
Prophase I
  • Chromosomes condense (coil)
  • Nuclear membrane breaks down
  • Homologous chromosomes undergo synapsis (Pair up )
  • Cross over occurs (Recombination) and chromosomes exchange segments

newly forming microtubules

metaphase i
Metaphase I

spindle equator

one pair of

homologous

chromosomes

  • Homologous chromosome pairsline up at random at the equatorial midline
anaphase i
Anaphase I
  • Centromeres do not divide
  • Homologs

separate and move to opposite poles

slide8
Telophase I
  • Nuclear membrane reforms
  • Chromosomes uncoil

After Cytokinesis

  • Two haploid cells

are produced

slide9
Meiosis II
    • After meiosis I there is a brief interphase
      • No DNA synthesis occurs
    • Meiosis II is similar to mitosis,

but with two main differences

      • 1.Haploid set of chromosomes
      • 2. Sister chromatids are not identical

because of cross over

prophase ii begins with haploid cells
Prophase IIbegins with haploid cells
  • Chromosomes coil
  • Spindle forms
  • Nuclear membrane breaks down
  • Each chromosome is composed of two sister chromatids attached at the centromere
slide11
Metaphase II
  • Chromosomes line up on the midline and attach to spindle fibers
slide12

Anaphase II

  • Centromeres divide
  • Sister chromatids move to opposite poles
slide13
Telophase II
  • Nuclear membrane reforms
  • Chromosomes uncoil

After Cytokinesis

  • Four unique haploid cells are produced
meiosis i has two unique features
Meiosis I has two unique features

1. Synapsis

  • Homologous chromosomes pair all

along their lengths in prophase I

  • While paired homologs cross over

2. Reduction division

  • Homologs separate in Anaphase I

reducing the chromosome

number in 1/2 (2n to 1n)

  • This produces haploid gametes
evolutionary consequences of sex
Sexual reproduction increases genetic diversity through three key mechanisms

1. Crossing over

2. Independent assortment

3. Random fertilization

Evolutionary Consequences of Sex
crossing over
Crossing over
  • DNA exchanges between maternal and paternal chromatid pairs

see Figure 9.7

independent assortment
Independent Assortment

Label these SEE process!

independent assortment1
Independent assortment

Three chromosome pairs

  • In humans, a gamete receives one homolog of each of the 23 chromosomes
    • Humans have 23 pairs of chromosomes
      • Independent assortment gives 223 combinations in an egg or sperm
        • 8,388,608 possible kinds of gametes
      • Random fertilization of two independently-produced gametes
        • Therefore, the possible combinations in an offspring
        • 8,388,608 X 8,388,608 =70,368,744,177,664
        • More than 70 trillion!
      • And this number does not count crossing-over

23 combinations