Meiosis

1
Meiosis Sexual Reproduction

• Chapter 13

2
Human Life Cycle

• Human Life Cycle
• 1) 23 pairs of homologous
• chromosomes
• 2) 1 pair of sex chromsomes
• 23
• 3) 22 pairs of autosomes
• 1-22
• 4) Karyotype picture of
• chromosomes
• 5) Gametes are haploid (n)
• 6) All other cells are diploid (2n)
• 7) Fertilization (syngamy)
• results in a zygote
• Meiosis cell division to produce
• haploid (n) gametes

3
Alternative Life Cycles

• Fungi/Some Algae
• Meiosis produces n
• cells that divide by
• mitosis to make n
• adults (gametes by
• mitosis)
• Plants/Some Algae
• Alternation of Generations
• 2n sporophyte, by meiosis,
• makes n spores spore
• divides by mitosis to generate
• a n gametophyte gametes are
• formed by mitosis, which then
• fertilize into 2n sporophyte

4
Meiosis

• Preceded by
• DNA/chromosome
• replication, but is
• followed by 2 cell
• divisions (Meiosis I
• Meiosis II)
• Produces 4 daughter
• cells with ½ the number of
• chromosomes
• (2n ? n)
• Creates variation

5
Animation of Meiosis

• Meiosis Animation
• Homologous chromosomes separate in Meiosis I
• Chromosomes separate in Meiosis II
• Reduction in chromosome occurs in Meiosis I
• End Result 4 cells ? genetically different from
  each other and from the parent cell with half the
  number of chromosomes as the parent cell
• Crossing over in Prophase I creates variation

6
Mitosis vs. Meiosis

• Synapsis/tetrad/chiasmata
• (Prophase I) ? Xing over
• Homologous vs.
• individual chromosomes
• (Metaphase I)
• Sister chromatids do not
• separate (Anaphase I)
• Meiosis I separates
• homologous pairs of
• chromosomes, not sister
• chromatids of individual
• chromosomes

7
Genetic Variation Part I

• Independent Assortment
• Homologous pairs of
• chromosomes position orient randomly
• (metaphase I)
• Combinations Possible ?
• 2n with n being the haploid of the
  organism

8
Genetic Variation Part II

• Crossing Over
• The reciprocal exchange
• of genetic material b/w
• non-sister chromatids
• during synapsis of
• Meiosis I (Prophase I)
• Maternal paternal chromosomes get mixed up

9
Genetic Variation Part III

• Random Fertilization
• 1 sperm (of 8 million) X 1 egg (of 8
  million)
• 64 trillion diploid combinations

10
Summary of Meiosis

• Makes sex cells or gametes
• Reduces the chromosome from 2n ? n
• DNA Replication occurs before meiosis
• Two cell divisions
• Synapsis of homologous chromosomes ? crossing
  over (Prophase I)
• Makes 4 genetically different cells
• Introduces genetic variation
• Associated with sexual reproduction
• Unique Features of Meiosis

11
Spermatogenesis

• Occurs in the
• seminiferous tubules
• of the testes
• Makes 4 sperm per
• cell undergoing
• meiosis
• Continuous process!

12
Oogenesis

• Occurs in the
• ovary about
• every 28 days
• Makes 1 egg/ovum
• per cell undergoing
• meiosis
• Through prophase I ?
• occurs in female embryo
• At puberty ? completes
• Meiosis I
• Ovulation ? Meiosis II
• occurs if egg is fertilized!

13
Spermatogenesis vs. Oogenesis

• Spermatogenesis
• Cytokinesis is equal
• Occurs thorughout a males entire life
• Uninterrupted sequence of meiosis

• Oogenesis
• Cytokinesis is unequal
• Occurs through menopause and stops. At birth, an
  ovary has all the primary oocytes it will ever
  have.
• Long periods of rest interrupted sequence of
  meiosis

14
Abnormal of Chromosomes in Gametes

• Nondisjunction chromosomes do not separately
  correctly in meiosis ? during Meiosis I
  (homologous chromosomes) or Meiosis II (sister
  chromatids)
• Result may be gametes with 22 or 24 chromosomes

15
Picture of Nondisjunction
16
Nondisjunction

• TRISOMY
• Three copies of 1 chromosome
• Total of 47 chromosomes
• Down Syndrome
• 21 XXX
• Klinefelter Syndrome
• 23 XXY

• MONOSOMY
• One copy of a chromosome
• Total of 45 chromosomes
• Turner Syndrome
• 23 X0

17
Nondisjunction

• High frequency in humans
• Most embryos are spontaneously aborted
• Alterations in chromosome are too detrimental
• Developmental problems may result from imbalance
  of chromosomes

18
Down Syndrome

• Trisomy 21
• 1 in 700 children born in U.S.
• Frequency increases as mothers age increases

19
Down Syndrome Karyotype
20
Klinefelter Syndrome

• XXY Male
• 1 in every 2000 live births
• Male sex organs present, but are sterile
• May have feminine characteristics
• Normal intelligence
• Tall

21
Klinefelter Syndrome Karyotype
22
Turner Syndrome

• X0 Female
• Monosomy of 23
• 1 in every 5000 live births
• Webbed neck
• Short stature
• Immature sterile females

23
Turner Syndrome Karyotype
24
Jacobs Syndrome Male

• XYY Male
• 1 in 1000 live male births
• Extra Y chromosome
• More active
• Slight learning disabilities
• Delayed emotional immaturity
• Normal intelligence
• Normal sexual development
• Also called criminal karyotype