Download Meiosis notes

MEIOSIS
FORMATION OF GAMETES
https://www.youtube.com/watch?v=A2Xcm0t-kEM
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HOMEWORK- DUE FRIDAY
Take
notes over chapter 10 in
your STARR text.
At least 3 bullet points per
section
One drawing per section
There are 6 sections in this
chapter so you should end up
with 18 facts and 6 pictures.
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WHY DO WE NEED MEIOSIS?

It is the fundamental
basis of sexual
reproduction


Two haploid (1n)
gametes are brought
together through
fertilization to form a
diploid (2n) zygote
Promotes genetic
diversity (aka a better
gene pool)
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FACTS ABOUT MEIOSIS


Daughter cells contain half
the number of chromosomes
as the original cell
(haploid/1n)
Germ Cells- produce
gametes

Makes sperm in the testes in
males


spermatogenesis
Makes eggs in the ovaries in
females

Oogenesis
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MEIOSIS: TWO PART CELL DIVISION
Sister
chromatids
separate
Homologs
separate
Meiosis
I
Meiosis
II
Diploid
Haploid
Haploid
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FERTILIZATION: PUTTING IT ALL
TOGETHER
2n = 6
1n =3
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REPLICATION OF CHROMOSOMES

Replication is the
process of duplicating
a chromosome



Occurs in
Interphase
Occurs prior to
division
Replicated copies are
called sister
chromatids
Held together at
centromere
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A REPLICATED CHROMOSOME
Gene: specific region of DNA that codes for a particular protein.
Allele: any alternative form of a gene
Gene X
Homologs
AKA Homologous pair
Same genes,
different alleles
(versions of genes)
Sister
Chromatids
Same genes,
Same alleles
Homologs separate in meiosis I and therefore
different alleles separate.
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THE ORDER OF MEIOSIS
Interphase
Meiosis
I
PMAT C
Meiosis II
PMAT C
Number of chromosomes
 In interphase the
chromosome number
duplicates 46x2 = 92



Meiosis I the chromosome
number divides 92/2 = 46
Meiosis II the chromosome
number divides again
46/2=23
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MEIOSIS I: GENETIC RECOMBINATION
Nucleus
Early Prophase I
(Chromosome
number
doubled from
interphase)
Spindle
fibers
Nuclear
envelope
Late
Prophase I Metaphase I
Anaphase I Telophase I
(haploid-only
one 10
chromosome
type per cell)
PROPHASE I
Early prophase
•
•
•
Chromosomes
condense.
Spindle forms.
Nuclear envelope
fragments.
Late prophase
• Homologs pair forming
tetrad.
• Crossing over occurs
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TETRADS FORM IN PROPHASE I
Homologous chromosomes
(each with sister chromatids)
Join to form a
TETRAD
Synapsis = putting12
the tetrads together
CROSSING-OVER

Sister chromatids in
a tetrad cross over
each other
Pieces of
chromosomes or
genes are
exchanged
 Produces genetic
recombination in
the offspring


These are called
homologous
chromosomes
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HOMOLOGOUS CHROMOSOMES DURING
CROSSING-OVER
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CROSSING-OVER
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METAPHASE I
Homologous pairs
(homologs) of
chromosomes align
along the equator of
the cell
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ANAPHASE I
Homologous pairs
(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.
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Each cell is now haploid
MEIOSIS II: REDUCING CHROMOSOME
NUMBER
Prophase II Metaphase II
Anaphase II
Telophase II
4 haploid
cells 19
PROPHASE II
Nuclear envelope
fragments.
Spindle forms.
DNA is NOT
replicated.
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METAPHASE II
Chromosomes align
along equator of cell.
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ANAPHASE II
Equator
Pole
Sister chromatids
separate and move to
opposite poles.
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TELOPHASE II AND CYTOKINESIS
Nuclear envelope
assembles.
Chromosomes uncoil.
Spindle disappears.
Cytokinesis divides cell
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into two.
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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Short intro
https://www.youtube.com/watch?v=D1_-mQS_FZ0
 Good detailed version of meiosis
https://www.youtube.com/watch?v=rqPMp0U0HOA

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GAMETOGENESIS
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SPERMATOGENESIS

Occurs in the testes


Two divisions produce
4 spermatids
Spermatids mature
into sperm

Men produce about
250,000,000 sperm per
day
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SPERMATOGENESIS
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OOGENESIS

Occurs in the ovaries

Two divisions produce 3
polar bodies that are
recycled and 1 large egg


Polar bodies aren’t viable
because of unequal division of
cytoplasm
Immature egg called oocyte

Starting at puberty, one
oocyte matures into an ovum
(egg) every 28 days
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OOGENESIS
First polar body
may divide
(haploid)
a
X
a
a
X
a
A
X
X
Meiosis I
Meiosis II
(if fertilization
occurs)
X
Primary
oocyte
(diploid)
Polar
bodies
recycle
A
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg)
(haploid)
Mature
egg
A
X
polar body
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COMPARING MITOSIS
AND MEIOSIS
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COMPARISON OF DIVISIONS
Mitosis
Meiosis
2
Number of
divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Germ cells
When
Throughout life
At sexual maturity
Genetically
identical?
Chromosome #
Role Growth and repair
Sexual reproduction
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