Meiosis
Reproduction
• Mitosis produces somatic cells (body cells)
• Somatic cells have 46 chromosomes (23 pairs)
• What if somatic cells were involved in reproduction?
Oocyte
46
How do
we solve
92
this problem?
Spermatocte
I’ve got too many
chromosomes!!!
Zygote
Reproduction
• Mitosis produces somatic cells (body cells)
• Somatic cells have 46 chromosomes (23 pairs)
• What if somatic cells were involved in reproduction?
23
Haploid
Much
better
46
Diploid
Gametes
• Haploid gametes keeps the chromosome number from
doubling in each succeeding generation
• Haploid gametes are produced by a special sort of cell
division called meiosis
• Meiosis
– occurs only in reproductive organs, ovaries and testes
– Purpose of meiosis is to produce sperm and egg
• Spermatogenesis – Sperm production
• Oogenesis – Egg cell production
Summary of Meiosis
• Meiosis involves 2 cell divisions
• Meiosis produces 4 cells from 1 parental cell
• Each of the 4 daughter cells has 23 individual chromosomes
rather than 23 pairs of chromosomes
• Meiosis reduces the chromosome number from diploid to
haploid
• Meiosis, like mitosis, is preceded by chromosome duplication
– However, in meiosis the cell divides twice to form four
daughter cells
Meiosis I
Meiosis II
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with
centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Microtubules
attached to
Spindle kinetochore
Sites of crossing over
Chromatin
Sister
chromatids
Tetrad
Metaphase
plate
Centromere
(with kinetochore)
ANAPHASE I
Sister chromatids
remain attached
Homologous
chromosomes separate
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Haploid
daughter cells
forming
Genetic Variation in Meiosis
• Meiosis provides for a vast amount of genetic
variation
– Crossing over – in prophase I, chromosomes
exchange DNA
– Independent Assortment – in anaphase I
homologs independently separate, in anaphase II
sister chromatids independently separate
• Additional genetic variation is achieved
through mating of non-related individuals
Crossing Over
Independent Assortment
POSSIBILITY 1
POSSIBILITY 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Independent Assortment
• The number of genetically distinct gametes
that can be produced from a diploid cell is 2n
• n = the number of chromosome pairs
I wonder how many distinct
23
gametes can be produced
from independent
assortment in humans?
2 = 8,388,608
combinations
MITOSIS
MEIOSIS
PARENT CELL
(before chromosome replication)
Site of
crossing over
MEIOSIS I
PROPHASE I
Crossing over
occurs
PROPHASE
Duplicated
chromosome
(two sister chromatids)
METAPHASE
ANAPHASE
TELOPHASE
2n
Daughter cells
of mitosis
Chromosome
replication
Chromosome
replication
2n = 4
Chromosomes
align at the
metaphase plate
Homologous pairs
line up at the
equator
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate
during
anaphase I;
sister
chromatids
remain together
2n
No further
chromosomal
replication; sister
chromatids
separate during
anaphase II
METAPHASE I
ANAPHASE I
TELOPHASE I
Haploid
n=2
Daughter
cells of
meiosis I
n
n
MEIOSIS II
n
n
Daughter cells of meiosis II
Karyotype
• A karyotype is a display of a persons
chromosomes
What does
this karyotype
tell you about
the person?
Errors in Meiosis
• Nondisjunction occurs when either:
– homologues fail to separate during anaphase I of
meiosis, or
– sister chromatids fail to separate during anaphase II.
• Trisomy - one extra chromosome (2n +1)
• Monosomy - one missing chromosome (2n - 1)
Down syndrome (trisomy 21): The result of an extra copy of chromosome 21. People with Down syndrome are
47, 21+. Down syndrome affects 1:700 children and alters the child's phenotype either moderately or severely:
- characteristic facial features, short stature; heart defects
- susceptibility to respiratory disease, shorter lifespan
- prone to developing early Alzheimer's and leukemia
- often sexually underdeveloped and sterile, usually some degree of mental retardation.
- Down Syndrome is correlated with age of mother but can also be the result of nondisjunction of the father's
chromosome 21.
Patau syndrome (trisomy 13): serious eye, brain, circulatory defects as well as cleft
palate. 1:5000 live births. Children rarely live more than a few months.
Edward's syndrome (trisomy 18): almost every organ system affected 1:10,000 live
births. Children with full Trisomy 18 generally do not live more than a few months.
Klinefelter syndrome: 47, XXY males. Male sex organs; unusually small testes, sterile.
Breast enlargement and other feminine body characteristics. Normal intelligence.
47, XYY males: Individuals are somewhat taller than average and often have below
normal intelligence. At one time (~1970s), it was thought that these men were likely to
be criminally aggressive, but this hypothesis has been disproven over time.
Trisomy X: 47, XXX females. 1:1000 live births - healthy and fertile - usually cannot be
distinguished from normal female except by karyotype
Monosomy X (Turner's syndrome): 1:5000 live births; the only viable monosomy in
humans - women with Turner's have only 45 chromosomes!!! XO individuals are
genetically female, however, they do not mature sexually during puberty and are sterile.
Short stature and normal intelligence. (98% of these fetuses die before birth)