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Ch.11.4 - Meiosis

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Ch.11.4 - Meiosis Formation of Gametes (egg & sperm) – PowerPoint PPT presentation

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Title: Ch.11.4 - Meiosis


1
Ch.11.4 - Meiosis
Formation of Gametes (egg sperm)
2
Our Chromosomes
  • 46 Chromosomes (23 pairs)
  • 1-22 Pairs are Autosomes (body cell
    chromosomes)
  • Determine all traits except gender
  • 23 pair are Sex chromosomes
  • referred to as X Y
  • male (XY)
  • female (XX)

X - Chromosome
Y-Chromosome
3
Chromosome Numbers
Vary in organisms
4
A closer look at Chromosome Pairs
5
Karyotype
  • Picture of chromosomes arranged by size
  • Pairs 1-22 autosomes
  • Pair 23 sex chromo
  • Detects abnormalities
  • XX female or XY male

6
Homologous Chromosomes (Buddy-Buddy)
  • Matching pair homologous chromosomes
  • Homologous chromosomes 2 chromosomes (one from
    mom and one from dad) that are alike in
  • size,
  • location of centromere,
  • dark/light banding pattern of genesRemember
    only non matching pair are sex chromosomes

7
Homologous Chromosome Pairs
Moms Blue eye gene
Dads Brown eye gene
Allele different versions of the same gene
(traits)
8
  • If a cell has all 46 chromo (23 pairs), its
    called a diploid cell
  • Shorthand 2N

Homologous Pair ?
9
Somatic Cell Gamete
  • Body Cell
  • Skin cell, etc.
  • Mitosis
  • 2 sets of DNA
  • 46 total chromo
  • 1 set from each parent
  • DIPLOID (2N)
  • Sex Cell
  • Egg/sperm
  • Meiosis
  • 1 set of DNA
  • 23 total chromo
  • ½ set from each parent
  • HAPLOID (N)

10
Important Vocab
  • Somatic Cell vs. Gamete
  • Body cell vs. Sex cell
  • Diploid (2N) vs. Haploid (1N)
  • 2 sets of DNA vs. 1 set of DNA
  • Homologous Chromosomes
  • Matching pairs of chromo in 2N cell
  • Alleles
  • Different version of the same trait
  • Fertilization
  • When sperm meets egg and combines DNA
  • Zygote
  • Cell in growth state following fertilization
  • Tetrad
  • Structure containing 4 chromatids

11
Meiosis Info
  • Similar but different from Mitosis
  • Sex cell division only
  • Involves 2 cell divisions
  • Results in 4 cells with half the normal genetic
    info
  • Produces gametes (egg/sperm)
  • Male Testes (spermatogenesis)
  • Female Ovaries (oogenesis)

12
Why Do we Need Meiosis?
  • Basis of sexual reproduction
  • Accounts for individual genetic diversity
  • You are unique! You look a little bit like your
    mom and a little like your dad!
  • Two haploid (1n) gametes are brought together
    through fertilization to form a diploid (2n)
    zygote (fertilized egg)

13
Heres the key to your uniqueness
Gene X
SisterChromatids (same genes, same alleles)
Homologous pairs separate in meiosis and
therefore different alleles (versions of traits)
separate. So many combos of traits are possible!
14
Meiosis Forms Haploid Gametes
  • Meiosis must reduce the chromosome by half
  • Fertilization then restores the 2n number
  • 23 chromo from egg 23 chromo from sperm you
    46!

EGG
SPERM
YOU
The right number!
YOU
EGG
SPERM
15
Meiosis 2-Part Cell Division
Sister chromatids separate
Homologous Pair separate
MeiosisII
Interphase
Diploid (2x46 92)
Diploid (46)
Haploid (23)
1 replication of chromosomes is followed by 2
cell divisions (aka Interphase only happens
once!)
16
Meiosis Reduction Division
  • 2 part cell division
  • Meiosis I
  • Interphase I
  • PMAT I
  • Meiosis II
  • PMAT II
  • End result 4 genetically different haploid
    cells
  • 4 sperm or 1 egg

Interphase
Meiosis I
Meiosis II
17
Meiosis I Reduction Division
Chromo pair up
Early Prophase I (Chromosome number doubled)
Late Prophase I
Metaphase I
Anaphase I
Telophase I (diploid)
18
Meiosis II Reducing Chromo
Prophase II
Metaphase II
Telophase II
Anaphase II
4 genetically different haploid cells
19
Interphase I
  • SAME as MITOSIS
  • Chromosomes will double
  • G1, S, G2

20
Prophase I
  • Homologs pair up and form tetrad (a pair of
    homologous chromosomes
  • Chromosomes condense.
  • Spindle forms.
  • Nuclear envelope disappears.
  • Crossing over occurs

21
Tetrads Form in Prophase I
Homologous chromosomes Pair up(each with sister
chromatids)  
Join to form a TETRAD
Called Synapsis
22
Crossing-Over occurs in Prophase I
  • Tetrad Forms
  • Definition Pieces of chromosomes or genes are
    exchanged
  • Advantage of sexual reproduction genetic
    variation!

23
Crossing-Over
Its hard to predict what traits youll get from
mom and dad because there is so many possible
combinations! THIS IS ONE SOURCE OF GENETIC
VARIABILITY!
24
Genetic Variability is due to
  • Crossing over
  • Independent Assortment
  • Random Segregation
  • Random Fertilization
  • Meaning You are unique for these 4 reasons!

25
  • Independent Assortment
  • The way a pair of chromosomes lines up during
    metaphase is not dependent on other pairs.
  • Aka Moms chromosomes dont necessarily all line
    up on one side
  • Random Segregation
  • Random chromatids separate into the newly forming
    eggs/sperm
  • Aka if you inherit moms hair color, you might
    get her brown hair trait OR her blonde hair trait

26
Independent Assortment
Random Segregation
27
Metaphase I and Anaphase I
Metaphase I Homologous pairs of chromosomes align
along the equator
Anaphase I -Homologs separate and move to
opposite poles. -Sister chromatids remain
attached at their centromeres.
28
Telophase I and Cytokinesis
Nuclear envelopes reassemble. Spindle
disappears. Cytokinesis divides cell into two
new diploid cells.
29
Meiosis II-occurs in 2 cells
Gene X
  • Only one homolog of each chromosome is present in
    the cell.

30
Meiosis II Reducing Chromo
Prophase II
Metaphase II
Telophase II
Anaphase II
4 genetically different haploid cells
31
  • Metaphase II
  • Chromosomes align along the equator
  • Prophase II
  • Nuclear envelope disappears
  • Spindle fibers form
  • No Crossing Over

32
Anaphase II
Equator
Pole
Sister chromatids separate and move to opposite
poles.
33
Telophase II
Nuclear envelope reforms. Chromosomes loosen
into chromatin. Spindle breakdown. Cytokinesis
breaks the cells into 2 new daughter cells
34
Results of Meiosis
Gametes (egg sperm) form Four haploid cells
with one copy of each chromosome One allele of
each gene Different combinations of alleles for
different genes along the chromosome
35
Prophase II
Prophase I
Metaphase II
Metaphase I
Anaphase II
Anaphase I
Telophase II
Telophase I
Cytokinesis
Cytokinesis
36
Meiosis Animation
37
Overview of Meiosis
38
(No Transcript)
39
THE PROCESS OF MAKING EGG AND SPERM
  • Oogenesis Spermatogenesis

40
Spermatogenesis
  • Creation of sperm
  • Testes
  • 2 divisions produce
  • 4 viable haploid spermatids
  • Spermatids mature into sperm
  • Men produce about 250,000,000 sperm per day

41
Spermatogenesis in the Testes
Spermatid
42
Spermatogenesis
43
Oogenesis
  • Creation of Eggs
  • Ovary
  • 2 divisions produce 3 polar bodies that die 1
    viable egg
  • Polar bodies die because of unequal division of
    cytoplasm
  • Starting at puberty, if unfertilized, one
    immature oocyte matures into an ovum (egg) every
    28 days ? MENSTRUAL CYCLE

44
Oogenesis in the Ovaries
Egg cells are specialonly one egg is made
every time meiosis occurs the other 3 cells
(polar bodies) that are made are much smaller
are discarded (Remember the egg cell is the
largest cell youll come across)
45
Oogenesis
46
Meiosis is IMPORTANT Genetic Variability
  • 1. Independent assortment
  • (late pro/early meta III)
  • -chromosomes line up randomly
  • 2. Law of Segregation
  • (Late meta/ana III)
  • -spindle fibers attach randomly to chromosomes
    and separate.
  • 3. Crossing-over
  • (Pro I)
  • pieces of sister chromatids are switched
  • 4. Random fertilization
  • Random sperm random egg are combined

47
What are the possibilities?
  • Total possible chromosome combinations due to
    independent assortment 2n for humans 223
    8,388,608
  • Total possible chromosomally different zygotes
    due to fertilization (223)2
    70,368,744,000,000
  • Possible genetically different zygotes per
    couple if crossing-over occurs only once (423)2
    4,951,760,200,000,000,000,000,000,000
  • --advantageous b/c variability needed for
    evolution

48
Comparison of Divisions
Mitosis Meiosis
of divisions 1 2
Number of daughter cells 2 4
Genetically identical? Yes No
Chromosome Same as parent Half of parent
Where Somatic cells Gamete cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction
49
Whats the difference between Mitosis Meiosis ?
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