1. Mitosis versus Meiosis
- the division of 1 cell into 4 daughter cells (egg or sperm) containing ½ the genetic material
- haploid (n) = contains 1 copy of the genome
Mitosis
- the division of 1 cell to produce 2 genetically identical daughter cells
- diploid (2n) = contains 2 copies of the genome 2n
46 chrom 2n
46 chrom n
(23) n
(23) 2n
(46) 2n
(46) n 23 n 23 n 23 n 23

3. Development of male and female gametes
The Formation of sex cells during meiosis is referred to as gametogenesis
Sperm and egg production are different:
In males 4 viable sperm are produced
 Spermatogenesis
In females 3 of the cells produce are known as polar bodies and do not survive. Only one egg is formed
 Oogenesis

4. Differences
The cyctoplasm of the female oocyte does not divide equally; one of the daughter cells called an ootid, receives most of the cyctoplasm while the other cells called polar bodies (“nurse cells”) die and are reabsorbed to provide nutrients
Sperms show equal division of cytoplasm; all 4 daughter cells become viable sperm

5. Spermatogenesis
Occurs in the seminiferous tubules where diploid spermatogonia, stem cells that are the precursors of sperm.
Spermatogonia
divide by mitosis to produce more spermatogonia or
differentiate into spermatocytes

6. Spermatogenesis
Meiosis of each spermatocyte produces 4 haploid spermatids. This process takes over three weeks to complete
Then the spermatids differentiate into sperm, losing most of their cytoplasm in the process.

7. Sketch Spermatogeneis showing meiotic divisions46
Spermatogonium
2. 1° Spermatocyte
3. 2° Spermatocyte
4. Spermatid
5. Sperm
1st Meiotic division 46 23 23
2nd Meiotic division 23 23 23 23

8. Oogenesis Egg formation takes place in the ovaries.
the initial steps in egg production occur prior to birth.
Diploid stem cells called oogonia divide by mitosis to produce more oogonia and primary oocytes

9. Sketch Oogeneis showing meiotic divisions46
Oogonium
2. Oocyte
a) 2° Oocyte
b) 1st polar body
a) Ootid
b) polar bodies
5. Ovum 46
1st Meiotic division 23 23
2nd Meiotic division

10. By the time the fetus is 20 weeks old, the process reaches its peak and all the oocytes that she will ever possess (~4 million of them) have been formed.
By the time she is born, 1–2 million of these remain. Each has begun the first steps of the first meiotic division (meiosis I) and then stopped.

11. No further development occurs until years later when the female becomes sexually mature.
Then the primary oocytes recommence their development, usually one at a time and once a month.
Unfertilized oocyte

12. The primary oocyte grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance.

13. Meiosis: Male and Female Differences

14. In humans, 22 of the 23 are homologous; these are the autosomes
Thomas Hunt Morgan discovered that having 2 rod shaped (X chromosome) indicated female and 1 rod with 1 hooked shaped chromosome (Y chromosome) identified males
The X and Y are not homologous, they are the sex chromosomes

15. Meiosis and Variation
Unlike mitosis, meiosis does not produce identical cells
The cells produced only have half the number but the chromosomes therefore only ½ the genetic information
What chromosomes end up in what cell all depend upon how the chromosomes line up in Metaphase I

16. Meiosis and Variation
If the two blue chromosomes line up on the same side and the two red chromosomes line up on the same side
Then the daughter cells will have either the genetic information from the red or blue chromosome

17. Meiosis and Variation If 1 red and 1 blue line up on the same side
The daughter cells will have genetic information from both red and blue chromosomes

18. MITOSIS MEIOSIS PARENT CELL (before chromosome replication)
Site of crossing over
MEIOSIS I
PROPHASE
PROPHASE I
Tetrad formed by synapsis of
homologous chromosomes
Duplicated chromosome (two sister chromatids)
Chromosome replication
Chromosome replication
2n = 4
Chromosomes align at the metaphase plate
Tetrads align at the metaphase plate
METAPHASE
METAPHASE I
ANAPHASE TELOPHASE
Sister chromatids separate during anaphase
Homologous chromosomes separate during anaphase I; sister chromatids remain together
ANAPHASE I
TELOPHASE I
Haploid n = 2
Daughter cells of meiosis I
No further chromosomal replication; sister chromatids separate during anaphase II
MEIOSIS II 2n 2n
Daughter cells of mitosis n n n n
Daughter cells of meiosis II

20. Abnormalities during Meiosis
The movement of the chromosomes in a dividing cell is so precise that only 1 in every 100,000 divisions will contain an error
Non-disjunction
= an error during meiosis where sister chromatids fail to come apart resulting in gametes that are missing or have extra chromosomes

21. Non-disjunction
One daughter cell will be missing one of the chromosomes (22)
Other daughter cell will contain an extra chromosome (24)
In humans, non-disjunction produces gametes with 22 or 24 chromosomes

22. Non-disjunctions
When gamete with 24 chromosomes joins a normal gamete with 23 chromosomes, the zygote will contain 47 (instead of 46). This zygote will have 3 chromosomes in place of the normal pair = trisomy
When the gamete with 22 chromosomes joins a normal gamete with 23 chromosomes, the zygote has 45; this zygote will have 1 chromosome in place of the normal pair = monosomy

23. KARYOTYPES
The best way to study non-disjunctions is by looking at karyotypes
Karyotypes are an inventory of an individuals chromosomes
A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes

24. Preparation of a karyotype
Packed red
And white
blood cells
Hypotonic solution
Fixative
Blood culture
Stain
White
Blood cells
Centrifuge 3 2 1
Fluid
Centromere
Sister chromatids
Pair of homologous chromosomes 4 5
Figure 8.19

25. Non-disjunctions A number of disorders are caused by non-disjunctions
Down’s Syndrome
Edward’s Syndrome
Patau’s Syndrome
Turner’s Syndrome
Kleinfelter’s Syndrome
Other severe abnormalities

26. Non-disjunctions
The risk of chromosomal abnormalities increases with maternal age because the egg cells are older
Women over the age of 35, who have children increase their chance exponentially

27. Karyotype for Down’s Syndrome Trisomy 21

28. Down’s Syndrome Trisomy 21
Cause:
Non-disjunction of chromosome #21
Individual has 47 chromosomes
Karyotype:
- has 3 chromosome #21
Symptoms:
- mentally & physically delayed, large forehead, large space between eyes

29. What does this karyotype tell us?

30. Patau’s Syndrome (trisomy)
Cause:
- non-disjunction of chromosome # 13
Karyotype:
- has 3 chromosome #13
Symptoms:
- child with multiple and severe abnormalities, and severe mental retardation
-  head very small, eyes absent or very small
- hairlip, cleft palate, usually malformations of internal organs
- most cases child dies soon after birth

31. What does this karyotype tell us?

32. Edward’s Syndrome: Trisomy 18
Cause:
- Non-disjunction of chromosome #18
Karyotype:
-has 3 of chromosome #18
Symptoms:
-very small and weak, head flattened, hands short with very little development
- severe mental handicap with life expectancy of less than one year

33. What does this Karyotype tell us?

34. Klinefelter’s Syndrome (Trisomy 23)
Cause:
Non-disjunction sex chromosomes
Karyotype:
- 2 X chromosomes and a Y chromosome (XXY)
Symptoms:
Appears male at birth but upon puberty releases high levels of female hormones
Sterile males, breast development, slightly feminized physique
Affects 1/1000 male births

35. What does this karyotype tell us?

36. Turner Syndrome (Monosomy)
Cause:
Non-disjunction of X chromosome in females
Karyotype:
- females have a single X chromosome (XO)
Symptoms:
sterile females, short stature, underdeveloped gonadal structures
Affects 1/5000 female births

37. Non-disjunction in MalesXX XY X X O XY
XXY XO
Klinefelter Turner

38. Non-disjunction in femalesXX XY O XX X Y
XXY XO
Klinefelter Turner

39. Other possible abnormalites caused by non-disjunction
these infants do not live very long – a few days at the most; many are premature and stillborns
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