Meiosis / Why Sex?
Biol 1011 - General Biology
October 31, 2011
Both mitosis and meiosis result in divided nuclei and cells, but
they differ in their fundamental results.
Mitosis: duplication of chromosomes followed by cell division;
- each daughter cell has exact copy of chromosomes from
the parent cell.
Meiosis: duplication of chromosomes followed by two cell divisions;
- four daughter cells each contain half the chromosome
number of the parent cell
- crossing over and independent assortment result in variation
in chromosomes of each daughter cell
- daughter cells become gametes (sperm and eggs)
MITOSIS
MEIOSIS
Diploid parent cell
Chromosome
replication
Prophase
Metaphase
Anaphase
and
Telophase
Two diploid daughter
cells of mitosis
Prophase I
- tetrads form
- crossing over
Metaphase I
Anaphase I
and
Telophase I
Meiosis II
Four haploid daughter cells of meiosis
Each species has a specific number of different chromosomes;
a karyotype is a display of the number and type of chromosomes.
Nucleus
Karyotype
(human)
Chromosomes
(condensed)
Ploidy refers to the number of chromosome sets a cell contains.
e.g. chromosome 5
one from one from
Gene for eye formation
(allele for rounder eyes)
Gene for eye formation
(allele for narrower eyes)
Diploid (2n) – two sets
Haploid (n) – one set
Homologous
chromosomes
(homologs)
etc:
Triploid (3)
Tetraploid (4)
Polyploid
MEIOSIS
Diploid (2n)
Haploid (n)
Amount of hereditary
material reduced by half
Haploid
gametes (n)
Diploid
adult
(2n) Normal amount
of hereditary
material restored
MITOSIS
Zygote
(2n)
Sperm
Egg
FERTILIZATION
Meiosis consists of two cell divisions, meiosis I and meiosis II.
First each chromosome in the diploid (2n) parent cell is duplicated.
Meiosis overview 1
Maternal
chromosome
Paternal
chromosome
Nuclear envelope
MEIOSIS I
Replication
Replicated
chromosome
Centromere
Sister chromatids
Homologous chromosomes
Parent cell
contains
homologous
pair of
chromosomes
(2n)
Homologs
separate
Sister
chromatids
separate
Daughter
cells
contain
just one
homolog
(n)
Four daughter cells containing one chromosome each (n).
In animals, these cells become gametes.
MEIOSIS II
MEIOSIS I
Meiosis overview 2
The results of meiosis are haploid cells, which will form the
gametes which recombine at fertilization to form a diploid zygote.
Meiosis overview 3
Female
gamete
(egg)
(n)
Fertilization
Male
gamete
(sperm)
(n)
Diploid offspring
contains homologous
pair of chromosomes
Zygote (2n)
Details of Meiosis
• Meiosis I
– Homologous chromosomes separate
• Meiosis II
– Sister chromatids separate
Meiosis details.
Tetrad
1. Interphase
2. Early
Prophase I
- synapsis
3. Late
4. Metaphase I 5. Anaphase I
Prophase I
- tetrads form
- metaphase
- migration
- crossing over
plate
Meiosis details.
6. Telophase I & 7. Prophase II 8. Metaphase II 9. Anaphase II 10. Telophase II &
Cytokinesis
- sister
Cytokinesis
- metaphase
- spindle
chromatids
plate
formation
- migration
- migration
separate
Crossing over is very common; it usually occurs at least once in
each non-sister chromatid.
Process: Chromosome replication, synapsis, and crossing over
Sister chromatids
Centromere
1. Replication
(interphase)
Chromosomes
Homolog one
Synaptonemal
complex
Tetrad
2. Synapsis
(prophase I)
Homolog two
Non-sister
chromatids
Protein complex
Crossing over in
two chromatids
3. Crossing over
(prophase I)
Q. Why should meiosis (and sexual reproduction) be such a
widespread strategy for many types of organisms?
Binary fission - bacteria
Budding - Hydra
Vegetative reproduction - plants
Budding - yeast
UC Berkeley
Q. Why should meiosis (and sexual reproduction) be such a
widespread strategy for many types of organisms?
A. Sexual reproduction introduces genetic variation into organisms.
Mechanisms for creating genetic variation:
1. Separation and random distribution of homologous
chromosomes during meiosis I
- independent assortment (random distribution of
chromosomes from maternal and paternal sets)
2. Crossing over (meiosis I)
3. Fertilization
Independent assortment: random distribution of homologous
chromosomes from paternal and maternal sets during meiosis I.
e.g.
Maternal
chromosome
Maternal
chromosome
Paternal
chromosome
Allele that
contributes
to black hair
Allele that
contributes
to brown eyes
Allele that
contributes
to green eyes
An eye-color gene
Paternal
chromosome
Allele that
contributes
to red hair
A hair-color gene
Independent assortment: random distribution of homologous
chromosomes from paternal and maternal sets during meiosis I.
During meiosis I, tetrads can line up two different
ways before the homologs separate.
OR
Brown eyes
Black hair
Green eyes
Red hair
Brown eyes
Red hair
Green eyes
Black hair
Crossing over, independent assortment and fertilization produce
tremendous potential for genetic variation in gametes.
Independent assortment:
How many combinations of maternal and paternal chromosome homologs
could result from meiosis?
2n, where n = haploid number
e.g. organism X; n = 2
Humans; n = 23
2n = 4
2n = 223 ≈ 8.4 million combinations
Fertilization:
(8.4 X 106) X (8.4 X 106) = 7.06 X 1013 = 70.6 trillion combinations
Crossing over:
~ 1/chromosome/ meiosis
What is the evolutionary advantage of having all of this potential
variation in the genes in gametes and offspring?
Meiosis and sexual reproduction may be evolutionary adaptations
to an ever-changing environment.
What changes?
e.g. Weather/climate
Moisture
Food supplies
Predators
Diseases
- bacteria
- viruses
- parasites
Pathogens (disease-causing organisms)
tend to evolve rapidly
- this means that our mechanisms for
fighting disease (e.g. immune system)
also has to evolve quickly
Meiosis and sexual reproduction may be evolutionary adaptations
to an ever-changing environment.
What changes?
e.g. Weather/climate
Moisture
Food supplies
Predators
Diseases
- bacteria
- viruses
- parasites
Trypanosoma - causes sleeping sickness
- evolves extremely rapidly to evade
the immune system
Occasionally homologous chromosomes fail to separate
during meiosis I. This is termed nondisjuction.
n+1
n+1
n-1
2n = 4
n=2
n-1
1. Meiosis I
2. Nondisjunction. 3. Meiosis II
4. Aneuploidy
starts normally.
occurs normally.
results.
Nondisjuction syndromes occur fairly commonly in human
populations.
e.g.
Down syndrome - Trisomy 21
Turner syndrome - X0
Klinefelter syndrome - XXY
Jacob syndrome - XYY
Incidence of Down syndrome
per number of births
Maternal age has a striking effect on the prevalence of
Down syndrome.
1
12
1
16
1
20
1
36
1
1000
1
300
1
180
1
100
1
60
Age of mother (years)