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Cell Division
•
Chapter 5
Cellular Functions
Lecture 4: Meiosis and Gametogenesis
Remember the goals of cell division…
•
Growth
•
Replacement
•
Reproduction
•
Repair
•
For some species, mitosis is sufficient for all 4
•
Not the case for species that use sexual
reproduction
Asexual Reproduction
Sexual Reproduction
•
Offspring genetically identical to parent
•
Two individuals combine genetic material
•
Some advantages…
•
Gametes fuse to produce offspring
•
In animals…
Examples of Asexual Reproduction
Organism
Bacteria
Strawberry plant
Sea anemone
(also yeast)
•
Benefit(s) of Asexual Reproduction
Binary fission - rapid reproduction if
conditions are right
Spread via runners (no pollination needed) rapid reproduction
Budding (don't wait for a mate) - rapid
reproduction
Male gamete = spermatozoon or sperm
•
Female gamete = ovum or egg
•
Mitosis will not work to produce the gametes
•
Let’s see why…
Major disadvantage: no opportunity to recombine
genetic material
Sexual Reproduction
•
•
Generation 1: Parents each have 46 chromosomes
Sexual Reproduction
•
Generation 2: Parents each have 92 chromosomes
Ovum
Sperm
Ovum
Sperm
46
46
92
92
Offspring
Offspring
92
184
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Sexual Reproduction
•
Generation 3: Parents each have 184 chromosomes
Ovum
Sperm
184
184
Meiosis
•
Solution: produce gametes with half the number
of chromosomes = meiosis
•
Human somatic cells = 46 chromosomes
•
Human gametes = 23 chomosomes
•
•
Offspring
368
2 copies of each (diploid, 2N)
1 copy of each (haploid, N)
Ovum
Sperm
23
23
Offspring
46
Meiosis
•
Meiosis
Can’t simply take 46 chromosomes and randomly
put 23 in each daughter cell
•
Begins with DNA replication
•
Same stages as mitosis (PMAT), but each occurs
twice
•
Each chromosome must have a copy in each
daughter cell
•
Results in 4 haploid daughter cells
•
Somatic cells are diploid
•
Let’s sketch this out…
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Two versions of same chromosome are
homologous
•
Homologous chromosomes must be separated
when cells divide
Meiosis
Meiosis
One copy of each
chromosome
from each parent
Cell nucleus
Chromosomes
from each parent
are copied; sister
chromatids are
attached at
centromere
Chromosomes
replicate
Homologous
chromosomes
pair up
This creates a
new mix of
genetic material
in the offspring
Chromosomes
swap sections
of DNA
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Meiosis
Chromosome
pairs divide;
daughter cells
have 2 copies of
each chromosome
Meiosis
Cell divides
4 haploid
daughter
cells are
produced
Daughter
cells divide
again
Meiosis
Meiosis
•
In humans used exclusively to produce gametes
•
Called gametogenesis
•
Process is almost the same…
Spermatogenesis
Spermatogenesis
46
Oogenesis
46
Oogenesis
1.
Starts at puberty
1.
2.
Produces 4 viable sperm
cells
Starts before birth
(~400,000!!)
2.
3.
Potentially unlimited
Continues at puberty but
arrests at M2
3.
Completed only if
fertilized
4.
Produces 1 viable egg and
3 polar bodies
5.
Stops at menopause
92
92
46
23
46
23
23
46
23
23
Chromosome NonNon-Disjunction
•
Chromosomes do not properly separate into
daughter cells
•
Can happen in mitosis; much more likely in
meiosis
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Chromosome NonNon-Disjunction
•
Abnormal gamete can participate in fertilization
with a normal gamete
•
Offspring with either…
•
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Trisomy
•
Trisomy 13 – Patau Syndrome
•
Trisomy 18 – Edward’s Syndrome
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Trisomy 21 – Down Syndrome
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Have one copy of a chromosome
•
Monosomy
•
Nearly always fatal
Have 3 copies of a chromosome
•
Trisomy
•
Usually fatal, some exceptions
•
•
•
80% fatal in first year
50% fatal in first year
Survival to adulthood common
Also occurs in sex chromosomes
•
XXY or XYY
Is This Karyotype Normal?
4