Meiosis
Meiosis
Vocabulary
SOMATIC:Any cell except a sperm or egg
AUTOSOMEAny chromosome other than a sex chromosome (X or Y)
GAMETE An egg or sperm
Mitosis allows asexual reproduction
Meiosis allows sexual reproduction.
Parents are diploid
Homologous chromosomes are separated into single sets.
Humans have 23 pairs of chromosomes.
One pair (the sex chromosomes) are not identical in males.
MALE
FEMALE
Sex ratios: 106 boys: 100 girls (at birth)
Karyotypes are pictures of chromosomes.
Some (More) Vocabulary
Alleles
Eye Color
Eye Color
Brown, green
Meiosis separates chromosome pairs to form
haploid gametes.
Meiosis I: Homologous pairs are separated (as sister chromatids) into 2
cells
Meiosis II: Similar to mitosis: sister chromatids are separated = 4 haploid
cells.
MEIOSIS I
Interphase I• Interphase is very
similar to interphase in mitosis
MEIOSIS I
Prophase I• Double stranded
chromosomes and spindle fibers appear;
• Nuclear membrane and nucleolus fade (synapsis aligns homologous chromosomes)
MEIOSIS I
Metaphase I• Chromosome pairs
(chromatids) line up• Spindle fibers attach to
centromeres and centrioles
MEIOSIS I
Anaphase I
• Chromatids separate from matching pair (independent assortment occurs here)
MEIOSIS I
Telophase I• Cytoplasm divides
and 2 cells form• Each cell still has a
diploid (2n) number
MEIOSIS II
Prophase II
Metaphase II
Anaphase II
Telophase/cytokinesis
II
SUMMARY OF MEIOSIS II
Meiosis II follows the same steps as mitosis
Key differences:◦In interphase II, there is no replication of genetic material
◦When the sister chromatids separate during anaphase II, they are now haploid (1n)
◦After telophase II, you have 4 haploid (1n) cells with genetic variation (they are no longer identical)
Genetic Variation
Independent Assortment: Homologous pairs can separate in different parental combinations.
2n possible gametes created
Possible Gametes
In humans:223 = 8 million possible gametes
This is without crossing over, which increases the number of distinct chromatids (exact number of chiasma determines how many different chromatids will exist)
Crossing Over: During synapsis (Prophase I) portions of non-sister chromatids are exchanged.
Genetic Variation
Random Fertilization: Each sperm will fertilize and egg randomly
Genetic Variation
8 million possible gametes for each parent:
8,000,0002 =
64 trillion possible zygotes
Without Crossing over. With it, there are even more!
These are a pair of homologous chromosomes with genes A - E.
These homologous chromosomes are made of 2 sister chromatids each.
These are called TETRADS
The tetrads line up side by side (synapsis) in Prophase I
Segments cross over (form chiasma) and the pieces are traded.
Synaptonemal Complex! CROSSING OVER
The sister chromatids end up with different copies of the genes.
Notice that genes that are located close together will not separate during crossing over as much as a pair of genes that are further apart.
Frequency of Crossing OverMeasuring the
frequency of crossing over tells you the distance between two genes (in map units)
What do you think?
With all of the variation produced in meiosis, how can parents have IDENTICAL TWINS?
How does the production of sperm differ from production of egg?
Errors in Meiosis
Errors in meiosis are passed down to offspring (because they occur in sperm or eggs)
These errors can create sperm and eggs that have the wrong number or types of chromosomes
Types of errors
Deletion: part of a chromosome is deleted (removed)
Duplication: part of a chromosome is repeated
Translocation: part of a chromosome is moved to another chromosome
Inversion: part of a chromosome is removed, then flipped around and reattached
Inversion
Non-disjunction
Failure of chromosomes to separate properly in meiosis
Meiosis I: homologues don’t separate
Meiosis II: Sister chromatids don’t separate
Types of errors
Monosomy: one parent does not give a copy of one chromosome◦There is only 1 total in the child
45 instead of 46 total chromosomesThe only monosomy that we see in
humans (that is nonfatal) is Turner syndrome
Turner syndromeFemales inherit only one X chromosome
(XO), instead of XX
1/2500 girls98% of fetuses with Turner Syndrome
miscarry
Short stature, broad chest, webbed neck
Turner syndrome
Trisomies
When one parent gives two copies of a chromosome◦The child has 3
total of that chromosome (47 instead of 46 total chromosomes)
Trisomy 21
Down’s syndrome3 copies of chromosome 21Impairment of cognitive ability (avg. IQ 50,
vs. 100 in rest of population)Almond-shaped eyesHealth problems: heart failure, GERD, ear
infections, sleep apnea1/733 births
◦More common with older mothers, but 80% are born to women under 35
Trisomy 18 Edwards
syndrome◦2nd most common
trisomy95% miscarryOf those born,
50% die within 2 months; 5-10% survive first year
Apnea, heart and kidney defects
1/3000 live births
May be a partial trisomy (less severe)
Trisomy 13
Patau syndrome◦Mental
impairment◦Polydactyly
(extra fingers or toes)
◦Microcephaly (small brain)
◦85% do not survive 1 year
May be a partial trisomy (less severe)
Trisomies
Trisomy 16◦Most common trisomy, but results in
miscarriageTrisomy 9
◦Mental retardation, deformities of skull, nervous system
Trisomies may be partial (due to translocation) or mosaic
Mosaicism
Mosaicism: During very early development, non-disjunction occurs in mitosis and as a result some percentage of an individual’s cells (but not all) have the trisomy/monosomy
46
4646
46
47
47 47
Trisomies in sex chromosomes
XXX: No difference between this and a normal woman (XX)◦(X inactivation -> Barr bodies)
XXY: Klinefelter◦Small testicles, reduced fertility◦Most common sex trisomy (1/500 males)
XYY: Not named because normally asymptomatic (normal phenotype)
Trisomies in sex chromosomes
Many other possibilities, many don’t have any symptoms
Meiosis Interphase I
Meiosis Early prophase I
Meiosis Mid-prophase I
Meiosis Late prophase I
Meiosis Metaphase I
Meiosis Anaphase I
Meiosis Telophase I
Meiosis Cytokinesis complete
Meiosis I complete; cytokinesis by cell plate formation.
Meiosis Prophase II
Meosis Metaphase II
Meiosis Anaphase II
Meiosis Telophase II and cytokinesis beginning.
Meiosis Cytokinesis complete.