Transcript Cell Cycle & Cell Division

MEIOSIS
FORMATION 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
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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
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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 X
Homologs
Same genes,
different alleles
(traits)
Sister
Chromatids
Same genes,
Same alleles (traits)
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
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right
number!
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MEIOSIS: TWO PART CELL DIVISION
Sister
chromatids
separate
Homologs
separate
Meiosis
I
Meiosis
II
Diploid
Diploid
Haploid
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MEIOSIS I: REDUCTION DIVISION
Nucleus
Early Prophase I
(Chromosome
number
doubled)
Spindle
fibers
Nuclear
envelope
Late
Prophase I Metaphase I
Anaphase I Telophase I
(diploid)
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PROPHASE I
Early prophase
Homologs pair.
Crossing over occurs
Late prophase
Chromosomes condense.
Spindle forms.
Nuclear envelope
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fragments.
TETRADS FORM IN PROPHASE I
Homologous chromosomes
(each with sister chromatids)
Join to form a
TETRAD
Called Synapsis 11
CROSSING-OVER
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

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
Homologous pairs of
chromosomes align
along the equator of
the cell
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ANAPHASE I
Homologs separate and move
to opposite poles.
Sister chromatids remain
attached at their centromeres.
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TELOPHASE I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into
two.
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MEIOSIS II
Gene X
Only one homolog of each
chromosome is present in the
cell.
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and thus one
copy of each gene.
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MEIOSIS II: REDUCING CHROMOSOME
NUMBER
Prophase II Metaphase II
Anaphase II
Telophase II
4 haploid
cells 19
PROPHASE II
Nuclear envelope
fragments.
Spindle forms.
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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
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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GAMETOGENESIS
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SPERMATOGENESIS

Occurs in the testes
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
Two divisions produce
4 spermatids
Spermatids mature
into sperm
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Men produce about
250,000,000 sperm per
day
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SPERMATOGENESIS
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OOGENESIS

Occurs in the ovaries
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Two divisions produce 3
polar bodies that die and 1
egg
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Polar bodies die because of
unequal division of cytoplasm
Immature egg called oocyte
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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
Mitosis
Oogonium
(diploid)
A
X
X
Primary
oocyte
(diploid)
X
a
X
a
a
Polar
bodies
die
X
Meiosis I Meiosis II
(if fertilization
A
occurs)
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg)
A
Mature
egg
X
Second
polar body
(haploid)
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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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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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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?
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