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