Chromosomes and Cell Division

1. Chromosomes and Cell Division

2. Chromosomes • Visible only during cell division. • Most condensed form of DNA. • DNA double helix tightly coiled around proteins.

3. After DNA replication Prior to DNA replication Chromatin = loosely coiled form of DNA An interphase cell has no visible chromosomes. Although not visible, every chromosome in a non-dividing cell is made up of only 1 chromatid. Only if a cell is preparing to divide, does a chromosome become made up of 2 chromatids.

4. Human Chromosomes 46 Chromosomes 23 pairs

5. Diploid Cells • Diploid = 2n • n = # of types (sizes) of chromosomes • n = 23 for humans, so the diploid # for humans is 46 (2 x 23) • Diploid cells have pairs of chromosomes • Homologous chromosomes: chromosomes of the same shape and size, carry genes for the same traits. • Each species has a unique diploid number.

6. Diploid numbers of some commonly studied organisms

7. Homologous Chromosomes e.g.: eye color From “Dad” From “Mom”

8. Fertilization produces diploid cells • Example: Humans • The mother’s egg cell has 23 chromosomes • The father’s sperm cell has 23 chromosomes • When these cells fuse, the fertilized egg has 46 chromosomes (23 pairs); it is diploid. • Mitosis produces trillions of body cells that are all diploid. • Each of the 23 pairs of chromosomes is called a “homologous pair”.

9. Haploid cells • Gametes (reproductive cells) • Egg cells (ova) and sperm cells • Produced in the reproductive organs, ovaries or testes • Haploid = 1n • n = 23 for humans so human haploid cells contain 23 chromosomes. • Haploid cellshave one of each chromosome; they do NOT have pairs of chromosomes.

10. Diploid vs Haploid Cells Only 1 of each type of chromosome is present in the cell. 2 of each type of chromosome is present in the cell. Note: the chromosomes in both pictures have 1 chromatid each. The # of chromatids does NOT relate to diploid/haploid.

11. Cells must be haploid to maintain the chromosome # of a species during reproduction • If reproductive cells were diploid, then after fertilization a human zygote (fertilized egg) would have 96 chromosomes. • To have the 46 chromosomes of a typical human cell, each reproductive cell should only have 23 chromosomes.

12. Meiosis produces Haploid cells • Involves 2 cell divisions: Meiosis I and Meiosis II. • Each cell division consists of PMAT. • Results in 4 cells.

13. For cell division, what needs to occur within a cell? • 2 copies of every DNA molecule • DNA replication during the S phase of interphase • DNA packaged into easily moveable units • Prophase: DNA “condenses” (becomes more tightly coiled around proteins) • Dissolving of the nuclear membrane to allow for distribution of genetic material (prophase) • Structures to attach to, and physically move, the chromosomes • Spindle fibers (microtubules) • Made by centrosomes

14. An organized way of distributing the genetic material equally between two cells. • Metaphase and Anaphase • Separation of the material into distinct cells. • Telophase: formation of new nuclear membranes, cytokinesis (division of cytoplasm)

15. Meiosis I • Prior to the beginning of cell division, DNA is copied (replicated). • Prophase I: • Similar to mitosis: dissolving of nuclear membrane, condensing of chromosomes, formation of spindle fibers. What is unique is: • Synapsis: pairing of homologous chromosomes (formation of tetrads). • Crossing-over: chromatids of homologous chromosomes exchange genetic material. • Results in genetic recombination: a new, unique combination of genes.

16. Prophase I • Letter T = tetrads • Each red box surrounds a tetrad. • Letter C = centrosomes • Produce spindle fibers. • Crossing-over is occurring in the lower tetrad.

17. Crossing-over

18. Metaphase I • Lining up of tetrads (homologous pairs) along the “equator” of the cell. • The arrangement of the chromosomes is random. • This allows for “independent assortment”. • The random distribution of genes from different chromosomes to different gametes. • Genetic recombination: new, unique combination of genes will be present in the daughter cells.

19. Metaphase I • Chromosomes arranging like in “A” is just as likely as chromosomes arranging like in “B”. • Each time cells in the testes/ovaries go through meiosis, unique cells are produced.

20. How many arrangements are possible? • For our simulation? • For humans?

21. How many arrangements are possible? • For our simulation: • 4 possibilities • 2n = 23 = 8 • For humans: • 2n = 223 = 8,388,608 • Wow! • And this doesn’t even include recombination resulting from crossing-over!

22. Anaphase I • Separation of homologous chromosomes to opposite sides of the cell. • Sister chromatids do NOT separate. • The direction they are pulled depends on their alignment during metaphase I. • Chromosomes are “independently assorted”.

23. Anaphase I

24. Telophase I: • Chromosomes are segregated at opposite poles, one group on each side of the cell. Cytokinesis begins, forming two daughter cells. Each chromosome consists of two chromatids.

25. Telophase I: • Are the new cells diploid or haploid? • HAPLOID • Only one of each chromosome

26. Meiosis II • After PMAT I the daughter cells are haploid. So why round two of PMAT? • The chromosomes are still in a “duplicated” state, two DNA molecules per chromosome. • The normal state of genetic material in a cell is one DNA molecule per chromosome.

28. Prophase II Nucleus dissolves and spindle fibers form again

29. Metaphase II Chromosomes line up “single-file” along the midline of the cell.

30. Anaphase II Sister chromatids of each chromosome detach from each other and move to opposite sides of the cell.

31. Telophase II A nuclear membrane forms around each set of chromosomes and cytokinesis begins, forming 4 daughter cells.

32. After Telophase II and Cytokinesis • A new nuclear membrane should be shown. • Cells are in Interphase.

33. Review: Separation of chromosomes during meiosis.