PowerPoint Presentation Homologous Chromosomes

1
Meiosis
Principles of Biology Chapter 10
2
Homologous Chromosomes
3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
Meiosis

• Most cells have two sets of chr
• This is called diploid (2 sets of 23)
• 2n diploid , humans 2n 46
• Gametes have one set of chr
• This is called haploid cell (1 set of 23)
• n haploid , humans n 23
• Sperm, egg, pollen, ovule are examples
• How chr separate to make haploid cells is meiosis
• Haploid haploid diploid /fertilization

8
Two divisions of meiosis
9
(No Transcript)
10
Meiosis

• Meiosis has two divisions
• First division separates the pairs
• Each new cell has 1 set of chr. - haploid
• 2nd division separates the chromatids
• Process is like mitosis
• Except for the number of chromosomes
• Each division has same 4 steps
• Prophase, metaphase, anaphase, telophase

11
(No Transcript)
12
Meiosis I

• Prophase I
• Chr condense
• Homologous chr pair up - aka tetrad
• Tetrad - 4 chromatids
• Gene for gene alignment of chr
• Crossing over occurs
• Non sister chromatids exchange pieces
• Nuclear membrane breaks down
• Spindles form
• Capture pairs
• Each homologue attached to opposite pole

13
Meiosis I continued

• Metaphase I
• Homologous pairs line up at equator
• Pushed/pulled into place by microtubules
• Anaphase I
• Homologous pairs separate
• Move toward opposite poles
• Sister chromatids still together
• Telophase I
• Chr arrive at poles
• New nuclear membrane forms
• Spindles break down
• Cytokinesis - division of cytoplasm/organelles

14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
Meiosis II

• Prophase II
• Chr condense
• Spindles form
• Nuclear membrane breaks down
• Microtubules capture chr at centromere
• Metaphase II
• Chr line up at equator
• Anaphase II
• Sister chromatids separate
• Move toward opposite poles

18
(No Transcript)
19
Meoisis II continued

• Telophase II
• Chr arrive at poles
• New nuclear membrane forms
• Spindles break down
• Cytokinesis
• Cytoplasm divides
• Includes cytosol, and organelles

20
(No Transcript)
21
Specific cases of meiosis

• Spermatogenesis
• Sperm formation in male animals
• One diploid parent cell to 4 haploid cells
• Straight forward meiosis
• Oogenesis
• Egg formation in female animals
• One diploid parent cell to haploid cells
• Cytokinesis is unequal
• 1st division produces one large egg cell and a
  polar body
• 2nd division of egg cell also unequal
• End product is one viable egg cell and up to 3
  polar bodies

22
Homologous chromosomes

• Chr come in pairs homologous chr
• Humans have 23 pairs in somatic cells
• One set from each parent
• Each pair has same genes at same location
• Must separate pairs to make gametes
• Each gamete (sex cell) gets one set of chr
• Process called meiosis
• Chr are numbered according to size
• 1 - largest, 22nd - smallest autosomes
• 23rd pair - sex chromosomes

23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
Mitosis vs Meiosis

• Mitosis
• 1 division
• 2 daughter cells
• Exact copies of parent cells
• Diploid to diploid
• Purpose
• Growth
• Repair
• Asexual reproduction

• Meiosis
• 2 divisions
• 1st separates pairs
• 4 daughter cells
• Each unique
• Diploid to haploid
• Purpose
• Make gametes/ sex cells
• Leads to genetic variation

28
Genetic Variation
29
(No Transcript)
30
Genetic variation

• Sexual reproduction results in variation
• Natural selection acts on variation
• Meiosis promotes variation
• 1. Independent assortment
• Only one chr of each homologous pair goes into
  gametes
• Which one goes into gametes is random
• Could be one from your father or from mother
• Depends on how they line up at metaphase I

31
Genetic variation cont.

• Two pairs can produce 4 different gametes
• Possible combinations 2n
• n 23 in humans
• 223 over 8.3 million possible combinations!
• 2. Random fertilization
• Which sperm fertilizes which egg is random
• One man one woman have over 64 trillion
  possible offspring!

32
Crossing Over Genetic Variation
33
Genetic variaiton cont.

• 3. Crossing over
• Prophase I of meiosis
• Homologous pairs synapse
• Gene for gene alignment
• Non sister chromatids exchange parts
• Recombines chr from your mom and dad
• Recombinant chr contains some genes from mom and
  some from dad
• Its amazing that siblings look as much alike as
  they do

34
(No Transcript)
35
(No Transcript)
36
(No Transcript)