1. Meiosis: Sex Cell Formation
Bio 100
Tri-County Technical College
Pendleton, S. C.

2. Mitosis vs. Meiosis Mitosis Meiosis
resulting cells have same chromosome number
one division cycle
resulting cells are same as parent cells
Meiosis
resulting cells -- 1/2 the chromosome number
two division cycles
resulting cells are gametes

3. Some key terms Somatic cells are all the body’s cell except sex cells
Sex cells produce gametes
Karotype is photo taken of chromosomes during late prophase, enlarged, cut out and homologous chromosomes are paired for comparison

4. One long, light chromosome; one long, dark chromosome; one short, light chromosome, one short, dark chromosome. The two long chromosomes are similar and the two short ones are similar.

5. Prophase of mitosis and prophase I of meiosis look the same.

6. In metaphase of mitosis, the chromosomes line up single file
In metaphase of mitosis, the chromosomes line up single file. In meiosis, the similar chromosomes line up opposite each other.

7. Anaphase of mitosis involves separating the chromatids each of which becomes a chromosome. In Anaphase I of meiosis similar chromosomes are separated but the chromatids remain attached.

8. The cells of late telophase of mitosis each have 4 chromosomes
The cells of late telophase of mitosis each have 4 chromosomes. Mitosis stops here. Meiosis goes into a second division. The cells in Prophase II have 2 chromosomes composed each of two chromatids.

9. During metaphase II, the chromosomes line up single file like they line up during mitosis.

10. During anaphase II, the chromatids are separated just like they are in the metaphase of mitosis.

11. The result of meiosis is 4 cells that have 1/2 the original chromosome number.

12. Plain and Simple
Primary purpose of meiosis I is separation of the homologous chromosomes
Primary purpose of meiosis II is separation of sister chromatids
Diploid (2N) cell contains ALL of the homologous pairs of chromosomes
Haploid (N) cell contains only ONE of each of the homologous pairs of chromosomes

13. A Few General Points Reduction division actually occurs in Anaphase I.
The cells resulting from meiosis are sperm cells in the male and an ovum in the female.
The way the chromosomes line up at metaphase I is related to Independent Assortment.

14. Crossing Over
During metaphase I while similar chromosomes (homologous chromosomes) are lined up, they may exchange pieces.
The resulting gametes contain new combinations of genetic information.
Crossing over is a source of variation.

15. Segregation A concept of genetics.
Genes separate from one another during the formation of gametes.
Occurs because of the Anaphase I and II divisions of Meiosis.

16. Independent Assortment
A concept of genetics.
Genes end up in gametes independent of each other during gamete formation.
In metaphase I of meiosis we show the long light on the left, the long dark on the right, the short dark on the left, the short light on the right.

17. Let me answer this one, Doc
Sexual reproduction increases genetic variability in the offspring
Crossing over in Prophase I
Independent Assortment at Metaphase I
Segregation during Anaphase II
Random fertilization (300+ million and just matter of chance which one (if any) will fertilize egg
Mate choice? (Darwin was really right!!)

18. Nondisjunction Rare event
Chromosomes don’t separate correctly during Anaphase 1
One of the cells at the end of Meiosis I has an extra chromosome and the other has one less

20. Trisomy
Results when one of the gametes with an extra chromosome participates in fertilization.
Three of one of the kinds of chromosomes instead of two which is normal.

21. Down’s Syndrome Nondisjunction of the 21st chromosome
Trisomy of the 21st chromosome
Symptoms of Down’s Syndrome
mongoloid facial features
reduced mental ability
susceptibility to infections

22. Frequency of Nondisjunction
Increases with age of the mother
Increases very rapidly after age 37
Fetal cells can be checked for chromosome number
amniocentesis
Genetic Center in Greenwood, S. C.

23. Inheritance of Sex There are two chromosomes that determine sex.
Autosomes
44 chromosomes that do not determine sex.
Sex chromosomes
males - XY
females - XX

24. Who determines the sex of a baby?
Males do that!
Females always produce eggs with an X-chromosome
50% of all sperm have the X-chromosome and 50% have the Y-chromosome
If a X-containing sperm fertilizes the egg the offspring will be female
If a Y-containing sperm fertilizes the egg the offspring will be male.

25. Once more..with feeling
Several other situations affect the genetic material (karotype helpful sometimes)
Deletion = some genetic material missing
Duplication = missing from one homolog and attached to the other homolog
Translocation = missing from one chromosome and attached to a chromosome other than its homolog
Inversion = detached and then inserted back but in reverse order

26. More key terms and concepts
Gametes are sex cells—we are familiar with sperm and egg but could also be + cell or a – cell
Fertilization is term applied to union of sperm and egg (or + and – cell)
Zygote is single cell produced by the union of sperm and egg (union of two sex cells)

27. Other nondisjunction errors
Turner syndrome (X0)
Metafemale (XXX)
Klinefelter syndrome (XXY)
XYY