1. How Cells Reproduce
Chapter 7

2. DNA DNA serves as a storehouse of information for living things.
DNA contains a code to make proteins

3. DNA Terminology
Gene = physical unit of heredity; housed on a chromosome
Made of DNA
Human genome = 25,000 genes
Chromosome = intricately folded unbroken strand of 40% DNA and 60% protein
Humans = 23 pairs (each has 1,000-2,000 different genes! (23 is referred to as haploid number)
Penicillin fungus = 1 pair
Ferns = some > 500 pairs!

4. Human Chromosomes

5. The Cell Cycle
This cycle keeps a record of the process of a cell over time
It is made up of a repeating pattern of growth, genetic duplication and division
A complete cycle is from the origin of a new cell until that cell divides. Note: A cell does not need to complete the cycle.

6. Cell Cycle
Mitosis: cell division that occurs in non-reproductive cells (somatic cells)
Meiosis: a second process that divides DNA in cells that participate in sexual reproduction (germ cells)

8. Mitosis Preparation phase of eukaryotic cells for division 5 Phases:G1 S G2 M C
Interphase; preparation for cell division
Prophase, Metaphase, Anaphase, Telophase

9. Interphase In general, the longest part of the cell cycle
Cell increases in mass
DNA is duplicated

10. Control of the cell cycle
Figure 7.10

11. G1 Checkpoint
Key checkpoint where the cell makes the decision of whether to continue with division or not
Cell can delay division or enter a resting phase

12. The G1 Checkpoint Figure 7.11

13. G2 Checkpoint M Checkpoint Occurs right before mitosis
Occurs right before cytokinesis after mitosis has occurred

14. Meiosis Cell division where gametes are produced
Humans reproduce by sexual reproduction which allows for combinations of desirable traits and more variability
Sexual reproduction presents a problem – of combining traits were accomplished by just combining two cells life would get incredibly complicated –more DNA is added to each generation

15. Figure 7.19

16. Genetic Background of Sexual Reproduction
Somatic cells contain two of each homologous chromosome (2N, diploid number)
Gametes have one of each of the homologous chromosomes (N, haploid number)
Female gametes are the ova and are produced in the ovaries
Male gametes are sperm and are produced in the testes
The process by which a single diploid cell divides to produce haploid reproductive cells is meiosis

17. Meiosis Steps
Meiosis reduces CH number by duplication of the CH and then two divisions
Meiosis like mitosis begins with a diploid cell
The DNA (CH) is duplicated

18. Meiosis Steps: Meiosis I
Meiosis I : separation of homologous CH
Prophase I
CH become visible
Homologous CH pair together = tetrad
Crossing over occurs
Homologous CH exchange recriprocal lengths of DNA
This process takes what was called a “maternal” CH not has a portion of the “paternal” CH
Allows for increased genetic diversity

19. Crossing Over Fig. 7.20

20. Meiosis Steps: Meiosis I
Metaphase I
Tetrads line up on the equatorial plate
Alignment of one pair bears no relationship to the alignment of any other tetrad
This random alignment is Independent Assortment
Except for identical twins this process ensures that each offspring will be unique
Anaphase I
Homologous CH separate and move to poles
Telophase I
Cytokinesis creates two new haploid cells

21. Meiosis Steps: Meiosis II
Stage where sister chromatids separate; events mimic mitosis
Prophase II
CH become visible
Metaphase II
CH align at the equatorial plate
Anaphase II
Sister chromatids separate and move to poles
Telophase II
Cytokinesis occus
A total of four haploid cells each containing chromatin are formed

22. Prophase I Each duplicated chromosome pairs with homologue
Homologues swap segments
Each chromosome becomes attached to spindle

23. Metaphase I Chromosomes are moved to middle of cell
Spindle is fully formed

24. Anaphase I Homologous chromosomes separate
Sister chromatids remain attached

25. Telophase I Chromosomes arrive at opposite poles
Usually followed by cytoplasmic division

26. Prophase II
Microtubules attach to duplicated chromosomes

27. Metaphase II
Duplicated chromosomes line up midway between spindle poles

28. Anaphase II
Sister chromatids separate to become independent chromosomes

29. Telophase II Chromosomes arrive at opposite ends of cell
Nuclear envelopes form around chromosome sets
Four haploid cells

30. What is the Significance of Meiosis?
The process generates genetic diversity by ensuring that the gametes formed are different from one another
Crossing Over
Independent Assortment

31. Genetic Diversity
The above indicates that even though the two homologous chromosomes contain the same genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each of the 4 gametes produced. With 23 chromosomes in a human gamete, their are 223 combinations ( combinations)

32. What happens when Meiosis runs amuck?
Genetic abnormalities
Aneuploidy
A condition in which an organism have more or fewer CH that normally exist in its species full set
Responsible for large amount of human miscarriages
In humans, live births will only result with one extra or fewer CH (2N +/- 1)
Monosomy: 2N – 1
Trisomy: 2N + 1

33. What happens when Meiosis runs amuck? Aneuploidy
The cause of aneuploidy is nondisjunction
Homologous CH or sister chromatids fail to separate correctly in meiosis
Eggs or sperm will then have an extra CH(s) or will be missing some
Human aneuploid conditions:
Down’s Syndrome: Trisomy 21
Turners: XO
Metafemale: Trisomy X
Klinefelter’s: XXY
XYY males