Understanding Cell Cycle: Mitosis and Meiosis

1. The Cell Cycle-Mitosis and Meiosis Interphase- G1, S, G2 Mitosis or Meiosis

2. The Cell Cycle • The sequence of growth and division of a cell

4. Interphase = G1, S, G2 • Interphase is when the cell grows, and the organelles double prior to the actual splitting of the nucleus. • 93% of a cell’s life is spent in interphase. • Interphase has three parts • Growth 1 (G1) • Synthesis (S) • Growth 2 (G2)

5. G1, S, G2 • G1 is when organelles double. • Remember each new cell needs a complete set of organelles. • S when DNA is replicated. • Each cell needs a complete and identical set of DNA • G2 Proteins needed for Mitosis are produced.

6. Mitosis • The process by which the cell nucleus divides into two identical cell nuclei. • In some Human cells interphases lasts 15.3 hours, while mitosis lasts only .7 hours. • Occurs in a series of steps • Prophase • Metaphase • Anaphase • Telophase • Cytokinesis

7. Chromosomes • Must duplicate and separate during Mitosis • Structures of the tightly packaged DNA • DNA is tangled up into a substance of chromatin • The chromatin is packaged on the chromosome

8. Chromosomal structure

9. Prophase • Chromosomes now called chromatids because they doubled to form short thick rods which pair up and line up in the center of the nucleus. • A centromere connects the two halves of the doubled chromatids. • Spindle fibers begin to form. • Spindle fiber – a fibrous structure from the cytoplasm which forms to the centriole. • Centrioles move to opposite sides of the cell. • The nuclear membrane breaks down.

10. Prophase

11. Metaphase • Centromeres of the chromatid pairs line up in the middle of the cell. • Metaphase plate- location where the centromeres line up in the center of the cell. • By the end of metaphase each chromatid has attached to spindle fibers.

12. Metaphase

13. Anaphase • The spindle fibers pull the chromatids apart. • This separates each one from its duplicate. These move to opposite sides of the cell. • Now there are two identical sets of chromosomes.

14. Anaphase

15. Telophase • When the chromosomes reach opposite sides of the cell the spindle fibers break up. • The nuclear membrane begins to reform. • A furrow begins to develop between the two sets of chromosomes.

16. Telophase

17. Cytokinesis • The two identical cells completely divide and the cell membrane is completely formed.

18. Mitosis Movie 1

19. Mitosis movie 2

20. Meiosis • Diploid (2n) - A cell with two of each kind of chromosome. • One chromosome from each parent. • If two body cells were to combine nuclei, the number of chromosomes would double. • In order for sexual reproduction to occur, each cell involved must reduce its chromosome number by half. • Haploid (n)- A cell with one of each kind of chromosome.

21. Haploid cells • Haploid cells are called gametes • Gametes are either sperm or eggs • Organism diploid gamete • Human 46 23 • Pea 14 7 • Fruit fly 8 4 • Dog 78 39

22. Homologous chromosomes • Are paired chromosomes with genes for the same trait arranged in the same order. • Ex. Eye color, hair color, height, one may code for blue, blonde, tall, its homolog may code for brown, blonde, short • Homologous chromosomes may have different alleles on them • Allele- gene form for each variation of a trait of an organism.

23. Meiosis • Meiosis is the process of cell division in which gametes are formed and the number of chromosomes is halved. So that sexual reproduction and zygote formation can occur. • Zygote- Fertilized egg which has a diploid number of chromosomes.

24. Stages of Meiosis • Interphase- • Chromosomes replicate • Each chromosome consists of 2 identical sister chromatids • Prophase I • Each Pair of homologous chromosomes come together to form a tetrad. • Tetrad- 2 homologous chromosomes come together and the 4 chromatids overlap.

25. Crossing over Tetrads are so tight that non-sister chromatids from the homologous pair actually exchange genetic material. • Crossing over- The exchange of genetic material by non-sister chromatids during late prophase I of meiosis. • Results in a new combination of alleles

26. Metaphase I • Homologous chromosomes line up together in pairs. • * In mitosis homologous chromosomes line up in the middle independently of each other.

27. Anaphase I • Spindle fibers attach to the centromeres of each pair. • Homologous chromosomes separate and move to opposite ends of the cell. • Centromeres DO NOT split like they do in mitosis • Now each cell will get one chromosome from each homologous pair.

28. Telophase I • Spindle fibers break down • Chromosomes uncoil • Cytoplasm divides • Another cell division is needed because the number of chromosomes has not been reduced • After telophase I there maybe a short interphase, but not always. It is important to note that if a cell does have a second interphase, there is No replication of chromosomes.

29. Meiosis I

30. Meiosis II • Is basically just like mitosis, but remember the chromosomes did not duplicate in interphase II. • Prophase II • Chromosomes begin to line up in the middle of the cell. • Spindle fibers begin to form • Metaphase II • Chromosomes line up on the metaphase plate

31. Meiosis II • Anaphase II • Centromeres split • Sister chromatids separate and move to opposite sides of the cell • Telophase II • Nuclei reform • Spindle fibers disappear • Cytoplasm divides into two. • The number of chromosomes in each daughter cell has now been reduced by half.

32. Meiosis II

33. Meiosis Movie 1