Download meiosis

MEIOSIS
MEIOSIS
• Genes:
– Lined up on chromosomes
– Chromosome = >1,000 genes
MEIOSIS
• Diploid and Haploid Cells
– Chromosomes occur in pairs, 1 from male and
one from female
– Diploid = 2 of each kind of chromosome (2n)
• Zygote
– Haploid = 1 of each kind of chromosome (n)
• Sperm or Egg (Gamete)
MEIOSIS
MEIOSIS
• Homologous Chromosomes
– Chromosomes matched in homologous pairs
– Humans have 46 chromosomes (23 from egg,
23 from sperm)
– 23 SETS of matching pairs of chromosomes
MEIOSIS
• Homologous Chromosomes
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2 chromosomes make matched pair
Both carry same genes
Genes exist at same location
Similar length
Centromere in same location
Not always identical
Can carry different alleles for same gene
• Hair color location same, color different
MEIOSIS
MEIOSIS
• Let’s review mitosis first!
– What are the 4 phases? What happens in each?
– Cell division produces new cells exactly like
original cell
MEIOSIS
• Why can’t Mitosis be only means of cell
division?? (DON’T WRITE!)
– If pea plant, for example, produces 14
chromosomes
– Gametes would each contain 14 chromosomes
– Each offspring would have 28
– Offspring of next generation would have 56
– DOES NOT WORK!!!
MEIOSIS
• Meiosis:
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Sexual reproduction
Production of gametes
Reduction of chromosomes
2 Phases
MEIOSIS
• Sexual Reproduction:
– Sperm (male gamete) and Egg (female gamete)
fuse
– Each gives 23 chromosomes to zygote = 46 total
chromosomes in offspring
MEIOSIS
• Production of gametes:
– BOTTOM LINE: Meiosis occurs in
specialized body cells of each parent to
produce gametes for sexual reproduction.
MEIOSIS
• Reduction of Chromosomes:
– Allows for offspring to have same number of
chromosomes as parents (not double)
• Gametes (egg or sperm) with ½ number of
chromosomes as parent’s body cells
MEIOSIS
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2 Phases:
1. MEIOSIS I: Genetic Variation
2. MEIOSIS II: “Mitosis-Like”
MEIOSIS
• MEIOSIS I: INTERPHASE
– Cell replicates chromosomes
– Result: 2 identical sister
chromatids held together by
centromeres
MEIOSIS
• MEIOSIS I: PROPHASE I
– Similar to Mitosis
– DNA coils to form
Chromosomes
– Homologous chromosomes
line up gene by gene along
length
• Tetrad: 2 homologous
chromosomes w/ 2 sister
chromatids each
MEIOSIS
• MEIOSIS I: PROPHASE I
– TETRAD: 4 chromatids
• Chromatids pair tightly
• Crossing over: non-sister
chromatids can break and
exchange genetic information
• Estimation: 2 or 3 crossovers
• NEW GENETIC
COMBINATION!
MEIOSIS
• MEIOSIS I: Metaphase I
– Centromere attaches to spindle
– Spindles pull tetrads into middle
• Unique to meiosis
• Tetrad = homologous pair of chromosomes
MEIOSIS
• MEIOSIS: Anaphase I
– Begins as homologous chromosomes
separate and move to opposite ends
of cells
• Each has two chromatids
– Centromeres do not split
• Why is this important?
– Ensures that each new cell receives
only 1 chromosome from each
homologous pair
MEIOSIS
• MEIOSIS I: Telophase I
– Spindle is broken down
– Chromosomes uncoil
– Cytoplasm divides  2 new
cells
– One of each chromosome per
new cell
• ANOTHER DIVISION IS
NEEDED! WHY?
MEIOSIS
• MEIOSIS II:
– Like Mitotic division
– Phases: Prophase II, Metaphase II, Anaphase II,
Telophase II
– The only difference from mitotic division is that
chromosomes do not replicate before they
divide at centromeres.
MEIOSIS
MEIOSIS
• Meiosis provides for genetic variation
– Crossing over provides rearrangement of genes
– Can happen anywhere at random on
chromosome
– Endless possibilities of different chromosomes
• Genetic Recombination: reassortement of
chromosomes and genetic information they
carry by crossing over
MEIOSIS
MEIOSIS
• Nondisjunction:
– Caused when chromosomes do not separate
properly in meiosis I
– Chromosomes in tetrad move to one side of cell
rather than separate
– Two gametes are result:
• 1 has extra chromosome
• Other has missing chromosome
MEIOSIS
• Nondisjunction:
– When zygote formed with an extra
chromosome, condition called trisomy
• Ex: In humans, having an extra Chromosome 21,
zygote has 47 chromosomes instead of 46. This
zygote develops into a baby with Down syndrome.
– When zygote formed with a missing
chromosome, condition called monosomy
• Ex: In humans, a non-lethal monosomy zygote
develops Turner syndrome. This results in a female
that has only one x chromosome instead of two.
MEIOSIS
• Polyploidy:
– Organisms with more than the usual number of
chromosome sets
– Rare in animals—causes death
– Frequent in plants
• Plants appear larger and healthier
• Ex: Apples = triploid (3n)
Wheat = hexaploid (6n)
MEIOSIS