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Presentation on theme: "Meiosis."— Presentation transcript:

1 Meiosis

2 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

3 Mitosis allows asexual reproduction


5 Meiosis allows sexual reproduction.
Parents are diploid Homologous chromosomes are separated into single sets.

6 Humans have 23 pairs of chromosomes.

7 One pair (the sex chromosomes) are not identical in males.
FEMALE One pair (the sex chromosomes) are not identical in males. MALE Sex ratios: 106 boys: 100 girls (at birth)

8 Karyotypes are pictures of chromosomes.







15 Some (More) Vocabulary

16 Alleles

17 Eye Color

18 Eye Color Brown, green

19 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.

20 MEIOSIS I Interphase I Interphase is very similar to interphase in mitosis

21 MEIOSIS I Prophase I Double stranded chromosomes and spindle fibers appear; Nuclear membrane and nucleolus fade (synapsis aligns homologous chromosomes)

22 MEIOSIS I Metaphase I Chromosome pairs (chromatids) line up
Spindle fibers attach to centromeres and centrioles

23 MEIOSIS I Anaphase I Chromatids separate from matching pair (independent assortment occurs here)

24 MEIOSIS I Telophase I Cytoplasm divides and 2 cells form
Each cell still has a diploid (2n) number

25 Telophase/cytokinesis II
MEIOSIS II Prophase II Metaphase II Anaphase II Telophase/cytokinesis II

26 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) SUMMARY OF MEIOSIS II

27 Genetic Variation Independent Assortment: Homologous pairs can separate in different parental combinations. 2n possible gametes created

28 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) Possible Gametes

29 Genetic Variation Crossing Over: During synapsis (Prophase I) portions of non-sister chromatids are exchanged.

30 64 trillion possible zygotes
Genetic Variation Random Fertilization: Each sperm will fertilize and egg randomly 8 million possible gametes for each parent: 8,000,0002 = 64 trillion possible zygotes Without Crossing over. With it, there are even more!


32 These are a pair of homologous chromosomes with genes A - E.
CROSSING OVER These are a pair of homologous chromosomes with genes A - E.

33 These are called TETRADS
CROSSING OVER These homologous chromosomes are made of 2 sister chromatids each. These are called TETRADS

34 The tetrads line up side by side (synapsis) in Prophase I
Synaptonemal Complex! CROSSING OVER The tetrads line up side by side (synapsis) in Prophase I Segments cross over (form chiasma) and the pieces are traded.

35 The sister chromatids end up with different copies of the genes.
CROSSING OVER The sister chromatids end up with different copies of the genes.



38 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.

39 Frequency of Crossing Over
Measuring the frequency of crossing over tells you the distance between two genes (in map units)


41 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? What do you think?

42 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 Errors in Meiosis

43 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 Types of errors

44 Inversion

45 Non-disjunction Failure of chromosomes to separate properly in meiosis
Meiosis I: homologues don’t separate Meiosis II: Sister chromatids don’t separate

46 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 Types of errors

47 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

48 Turner syndrome

49 Trisomies When one parent gives two copies of a chromosome
The child has 3 total of that chromosome (47 instead of 46 total chromosomes)

50 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 21


52 Trisomy 18 Edwards syndrome 95% miscarry
May be a partial trisomy (less severe) Edwards syndrome 2nd 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

53 Trisomy 13 Patau syndrome May be a partial trisomy (less severe)
Mental impairment Polydactyly (extra fingers or toes) Microcephaly (small brain) 85% do not survive 1 year

54 Trisomies Trisomy 16 Trisomy 9
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 Trisomies

55 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 Mosaicism

56 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

57 Trisomies in sex chromosomes
Many other possibilities, many don’t have any symptoms Trisomies in sex chromosomes

58 Meiosis Interphase I

59 Meiosis Early prophase I

60 Meiosis Mid-prophase I

61 Meiosis Late prophase I

62 Meiosis Metaphase I

63 Meiosis Anaphase I

64 Meiosis Telophase I

65 Meiosis Cytokinesis complete

66 Meiosis I complete; cytokinesis by cell plate formation.

67 Meiosis Prophase II

68 Meosis Metaphase II

69 Meiosis Anaphase II

70 Meiosis Telophase II and cytokinesis beginning.

71 Meiosis Cytokinesis complete.





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