Ch.11.4 - Meiosis

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Ch.11.4 - Meiosis
Formation of Gametes (egg sperm)
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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
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Chromosome Numbers
Vary in organisms
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A closer look at Chromosome Pairs
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Karyotype

• Picture of chromosomes arranged by size
• Pairs 1-22 autosomes
• Pair 23 sex chromo
• Detects abnormalities
• XX female or XY male

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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

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

Homologous Pair ?
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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)

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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

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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)

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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)

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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!
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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
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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!)
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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
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Meiosis I Reduction Division
Chromo pair up
Early Prophase I (Chromosome number doubled)
Late Prophase I
Metaphase I
Anaphase I
Telophase I (diploid)
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Meiosis II Reducing Chromo
Prophase II
Metaphase II
Telophase II
Anaphase II
4 genetically different haploid cells
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Interphase I

• SAME as MITOSIS
• Chromosomes will double
• G1, S, G2

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Prophase I

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

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Tetrads Form in Prophase I
Homologous chromosomes Pair up(each with sister
chromatids)  
Join to form a TETRAD
Called Synapsis
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Crossing-Over occurs in Prophase I

• Tetrad Forms
• Definition Pieces of chromosomes or genes are
  exchanged
• Advantage of sexual reproduction genetic
  variation!

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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!
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Genetic Variability is due to

• Crossing over
• Independent Assortment
• Random Segregation
• Random Fertilization
• Meaning You are unique for these 4 reasons!

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• 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

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Independent Assortment
Random Segregation
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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.
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Telophase I and Cytokinesis
Nuclear envelopes reassemble. Spindle
disappears. Cytokinesis divides cell into two
new diploid cells.
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Meiosis II-occurs in 2 cells
Gene X

• Only one homolog of each chromosome is present in
  the cell.

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Meiosis II Reducing Chromo
Prophase II
Metaphase II
Telophase II
Anaphase II
4 genetically different haploid cells
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• Metaphase II
• Chromosomes align along the equator

• Prophase II
• Nuclear envelope disappears
• Spindle fibers form
• No Crossing Over

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Anaphase II
Equator
Pole
Sister chromatids separate and move to opposite
poles.
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Telophase II
Nuclear envelope reforms. Chromosomes loosen
into chromatin. Spindle breakdown. Cytokinesis
breaks the cells into 2 new daughter cells
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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
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Prophase II
Prophase I
Metaphase II
Metaphase I
Anaphase II
Anaphase I
Telophase II
Telophase I
Cytokinesis
Cytokinesis
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Meiosis Animation
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Overview of Meiosis
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(No Transcript)
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THE PROCESS OF MAKING EGG AND SPERM

• Oogenesis Spermatogenesis

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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

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Spermatogenesis in the Testes
Spermatid
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Spermatogenesis
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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

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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)
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Oogenesis
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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

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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

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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
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Whats the difference between Mitosis Meiosis ?