The 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

3
(No Transcript)
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 cells 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