Presentation on theme: "Meiosis Meiosis. Vocabulary SOMATIC: Any cell except a sperm or egg AUTOSOME Any chromosome other than a sex chromosome (X or Y) GAMETE An egg or sperm."— Presentation transcript:
Vocabulary SOMATIC: Any cell except a sperm or egg AUTOSOME Any 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
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. 2 n possible gametes created
Possible Gametes In humans: 2 23 = 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: 8,000,000 2 = 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!
The sister chromatids end up with different copies of the genes.
Notice 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 Over Measuring 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
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 chromosomes The only monosomy that we see in humans (that is nonfatal) is Turner syndrome
Turner syndrome Females inherit only one X chromosome (XO), instead of XX 1/2500 girls 98% of fetuses with Turner Syndrome miscarry Short stature, broad chest, webbed neck
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 syndrome 3 copies of chromosome 21 Impairment of cognitive ability (avg. IQ 50, vs. 100 in rest of population) Almond-shaped eyes Health problems: heart failure, GERD, ear infections, sleep apnea 1/733 births ◦More common with older mothers, but 80% are born to women under 35
Trisomy 18 Edwards syndrome ◦2 nd most common trisomy 95% miscarry Of 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 miscarriage Trisomy 9 ◦Mental retardation, deformities of skull, nervous system Trisomies may be partial (due to translocation) or mosaic
Mosaicism n 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 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.