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MEIOSISFORMATION OF GAMETES
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FACTS ABOUT MEIOSIS Daughter cells contain
half the number of chromosomes as the original cell (haploid or 1n).
Produces gametes Egg and sperm Occurs in the testes in
males spermatogenesis
Occurs in the ovaries in females Oogenesis 22
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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 33
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FERTILIZATION: PUTTING IT ALL TOGETHER
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1n =31n =3
2n = 62n = 6
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REPLICATION OF CHROMOSOMES
Replication is the process of duplicating a chromosome Occurs prior to
division Replicated copies
are called sister chromatids
Held together at centromere
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Occurs in Occurs in InterphasInterphas
ee
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A REPLICATED CHROMOSOME
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Homologs Same genes, different alleles (traits)
SisterChromatidsSame genes, Same alleles (traits)
Gene X
Homologs separate in meiosis I and therefore different alleles (traits)
separate.
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MEIOSIS FORMS HAPLOID GAMETES
Meiosis must reduce the chromosome number by half Fertilization then restores the 2n number
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from mom from dad child
meiosis reducesgenetic content
toomuch!
The right number!
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MEIOSIS: TWO PART CELL DIVISION
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Homologsseparate
Sister chromatidsseparate
Diploid
Meiosis I
MeiosisII
Diploid
Haploid
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MEIOSIS I: REDUCTION DIVISION
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Nucleus Spindlefibers Nuclear
envelopeEarly Prophase I
(Chromosome number doubled)
Late Prophase
IMetaphas
e IAnaphas
e ITelophase I
(diploid)
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PROPHASE I
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Early prophase Homologs pair. Crossing over occurs
Late prophaseChromosomes condense.Spindle forms.Nuclear envelope fragments.
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TETRADS FORM IN PROPHASE I
1111
Homologous chromosomes(each with sister
chromatids)
Join to form a TETRAD
Called Synapsis
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CROSSING-OVER Homologous
chromosomes in a tetrad cross over each other
Pieces of chromosomes or genes are exchanged
Produces genetic recombination in the offspring
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HOMOLOGOUS CHROMOSOMES DURING CROSSING-OVER
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CROSSING-OVER
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METAPHASE I
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Homologous pairs of
chromosomes align along the equator of the
cell
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ANAPHASE I
1616
Homologs separate and move to opposite poles.
Sister chromatids remain attached at their centromeres.
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TELOPHASE I
1717
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
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MEIOSIS II
Only one homolog of each chromosome is present in the cell.
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Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry identical genetic
information.
Gene X
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MEIOSIS II: REDUCING CHROMOSOME NUMBER
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Prophase II
Metaphase II
Anaphase II
Telophase II 4
haploid cells
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PROPHASE II
2020
Nuclear envelope fragments.
Spindle forms.
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METAPHASE II
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Chromosomes align along equator of cell.
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ANAPHASE II
2222
Sister chromatids separate and move to opposite poles.
Equator
Pole
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TELOPHASE II
2323
Nuclear envelope assembles.
Chromosomes uncoil.
Spindle disappears.
Cytokinesis divides cell into two.
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RESULTS OF MEIOSIS
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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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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 die and 1 egg Polar bodies die because of unequal division of cytoplasm
Immature egg called oocyte Starting at puberty, one oocyte matures into an ovum (egg) every 28 days
2828
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OOGENESIS
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Oogonium(diploid)
Mitosis
Primaryoocyte(diploid)
Meiosis I
Secondaryoocyte(haploid)
Meiosis II(if fertilizationoccurs)
First polar bodymay divide (haploid) Polar
bodiesdie
Ovum (egg)
Secondpolar body(haploid)
a
A
X
X
a
X
A X
a
X
a
X
Matureegg
A
X
A
X
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COMPARISON OF DIVISIONS
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Mitosis MeiosisNumber of
divisions1
2
Number of daughter cells
2 4
Genetically identical?
Yes No
Chromosome #Same as parent
Half of parent
Where Somatic cells Germ cells
When Throughout lifeAt sexual maturity
RoleGrowth and
repairSexual
reproduction
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WWhat are the four steps to the cell cycle?
In which step does the nucleus content divide?
What is it called when you divide the cytoplasm?
T/F All cell division happens at the same rate.
What limits the size of cell?
In mitosis, cytokinesis produces two identical>>>>
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3333
TheThe process of organizing and condensing long strands of DNA into
compact form takes place when?
What are the group of proteins called that organizes and condenses the long strands of
DNA into tight coils?
During which phase of mitosis do sister chromatids separate from each other?
What stage completes the cell cycle?
A liver cell is an example of which type of cell?
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How many chromosomes are in a human gamete?
What does it mean to be homologous?
When are haploid gametes produced?
At fertilization, what happens to the nucleus?
What is the result of Meiosis I?
What happens to sister chromatids in meiosis II?
Gametogenesis is the term for????
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Does the egg or the sperm carry the organelles for the fertilized cell?
What is replication?
What is the main function of DNA polymerase?
What does it mean to be semi-conservative?
What is the ‘Central Dogma’?
What is the RNA strand for GTAGTCA?
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What is the term for a three nucleotide sequence that codes for an amino acid?
How many amino acids are used to make the proteins in the human body?
A tRNA that carries the amino acid methionine pairs with what type of
codon?
What is the name for converting the mRNA into a polypeptide?
Where does translation occur?