cell division- meiosis
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Cell Division: Mitosis and Meiosis
(4 Periods)
State StandardsGenetics State Standards10.4 In sexually reproducing organism, each offspring
contains a mix of characteristics inherited from both parents.
Genetics CAPT StrandsD 34. Describe, in general terms, how the genetic
information of organisms can be altered to make them produce new materials.
D 35. Explain the risks and benefits of altering the genetic composition and cell products of existing organisms.
D 36. Explain how meiosis contributes to the genetic variability of organisms.
ObjectivesUnderstand that most organisms have two
genes for each trait. Illustrate how meiosis contributes to genetic
variability. Create a visual representation of the stages
of meiosis.
The process to make cells with half the number of chromosomes for sexual reproduction
Occurs in our germ cells that produce gametesSperm & egg
Results in four cells genetically different from parent cell and from each other
Haploid (n) cells
Meiosis
Why Do We Need Meiosis?It is the fundamental basis of sexual reproduction
Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote
If egg and sperm had the same number of chromosomes as other body cells then the offspring would have too many chromosomes.
Meiosis must reduce the chromosome number by half (n)
Fertilization then restores the 2n number
from mom from dad child
meiosis reducesgenetic content
toomuch!
The right number!
Meiosis
Fertilization
n =3
2n = 6
Replication is the process of duplicating a chromosome
Occurs prior to division
Replicated copies are called sister chromatids
Held together at centromere
Meiosis-Replication
Homologous chromosomes (same genes, different alleles)
SisterChromatids(same genes,same alleles)
Gene Z
Homologous chromosomes are the same size & shape
Carry genes for the same trait but are not identical!
Meiosis
Checkpoint Meiosis results in ____.
1. 4 identical cells
2. 4 genetically different cells
3. 2 genetically different cells
4. 2 identical cells
Checkpoint Homologous chromosomes are _____.
1. Same in size
2. Same in shape
3. Carry genes for the same trait
4. All of the above
Checkpoint Why do we need meiosis?
A: To reduce the chromosome number in gametes, otherwise offspring would have too many chromosomes
Homologous chromosomesseparate
Sister chromatidsseparate
Diploid
Meiosis I Meiosis II
Diploid
Haploid
Meiosis
Nucleus Spindlefibers
Nuclearenvelope
Early Prophase I(Chromosome # doubled)
Late Prophase I Metaphase I Anaphase ITelophase I (haploid)
Meiosis I
Early prophaseHomologous
pair.Crossing over
occurs.
Late prophaseChromosomes
condense.Spindle forms.Nuclear envelope
fragments.
Prophase I
Homologous chromosomes(each with sister chromatids)
Join to form a TETRAD
Prophase I : Crossing-OverSynapsis – the pairing of homologous chromosomes
Group of 4 chromatids
Homologous chromosomes in a tetrad cross over each other
Pieces of chromosomes or genes are exchanged
Produces genetic recombination in the offspring
Prophase I : Crossing-Over
Homologous chromosomes during crossing-over
Prophase I : Crossing-Over
Homologous pairs of chromosomes align along the equator of the cell
Metaphase I
Homologous chromosomes separate and move to opposite poles.
Sister chromatids remain attached at their centromeres.
Anaphase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
Telophase I
Only one of the homologous pair of chromosome is present in the cell.
Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry identical genetic information.
Gene Z
Meiosis II
Prophase II Metaphase II
Anaphase II Telophase II 4 different haploid cells
Meiosis II
Nuclear envelope fragments.
Spindle forms.
Prophase II
Chromosomes align along equator of cell.
Metaphase II
26
Sister chromatids separate and move to opposite poles.
Equator
Pole
Anaphase II
Nuclear envelope assembles.
Chromosomes unwind.
Spindle disappears.
Cytokinesis divides cell into two.
Telophase II
Gametes (egg & sperm) form
Four haploid cells (n) with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome
Results of Meiosis
Checkpoint Which best describes crossing-over?
1. Pieces of genes are exchanged
2. Allow genetic variation
3. Occurs in Prophase I
4. All of the above
Checkpoint During anaphase I in meiosis what is pulled
apart?
1. Centromere
2. Homologous chromosomes
3. Sister chromatids
4. Spindle fibers
Checkpoint Which best describes a haploid cell?
1. n
2. Half the genetic information
3. Gametes
4. All of the above
Occurs in the testes
Two divisions produce 4 spermatids
Spermatids mature into sperm
Men produce about 250,000,000 sperm per day
Spermatogenesis
Spermatid
Spermatogenesis
Occurs in the ovariesTwo divisions produce 3 polar bodies that die and 1
eggPolar bodies die because of unequal division of
cytoplasmImmature egg called oocyteStarting at puberty, one oocyte matures into an ovum
(egg) every 28 days
Oogenesis
Oogenesis
Oogonium(diploid)
Mitosis
Primaryoocyte(diploid)
Meiosis I
Secondaryoocyte(haploid)
Meiosis II
Polar body (haploid) Polar
Bodies(haploid)
Ovum
Secondpolar body(haploid)
a
A
X
X
a
X
A X
a
X
a
X
MatureEgg (haploid)
A
X
A
X
Oogenesis
Checkpoint How many chromosomes would a sperm or egg
contain if either one resulted from the process of mitosis?
A: 46 Chromosomes
Checkpoint Oogenesis results in______.
1. 4 polar bodies
2. 3 polar bodies
3. 1 ovum
4. Both 2 & 3
Checkpoint Why is the sperm so small compared to the
egg?
A: The egg has all the starting nutrients, organelles and building blocks. The sperm only provides DNA. That is why oogenesis only produces one ‘good’ egg.
Mitosis MeiosisNumber of
divisions1
2
Number of daughter cells
2 4
Genetically identical?
Yes No
Chromosome # 46 23
Where Somatic cells Germ cells
When Throughout life At sexual maturity
RoleGrowth and
repairSexual reproduction
STUDENT
The process to make cells with ___________ the number of chromosomes for sexual reproduction
Occurs in our germ cells that produce gametes___________ & ______________
Results in ____________ cells genetically different from parent cell and from each other
Haploid (____) cells
Meiosis
Why Do We Need Meiosis?It is the fundamental basis of sexual reproduction
Two ____________ (n) gametes are brought together through fertilization to form a _______________ (2n) _______________
If egg and sperm had the same number of chromosomes as other body cells then the offspring would have too many chromosomes.
Meiosis must ____________ the chromosome number by ____________ (n)_____________________ then restores the 2n
number
from mom from dad child
meiosis ____________genetic content
toomuch!
The right number!
Fertilization
n =____
2n = ____
Replication is the process of duplicating a ______________________
Occurs prior to division
Replicated copies are called sister chromatids
Held together at __________________
Meiosis-Replication
Homologous chromosomes (same _________, different ________)
SisterChromatids(same ________,same _________)
Gene
Homologous chromosomes are the same ________ & ___________
Carry genes for the same trait but are _______ identical!
Meiosis results in ____.
1. 4 identical cells 2. 4 genetically different
cells
3. 2 genetically different cells 4. 2 identical cells
√CHECKPOINT
Homologous chromosomes are _____.
1. Same in size 2. Same in shape
3. Carry genes for the same trait 4. All of the
aboveWhy do we need meiosis?
Homologous chromosomesseparate
Sister chromatidsseparate
Meiosis I Meiosis II
______________
____________________________
Early prophase IHomologous pair.Crossing over
occurs.
Late prophase IChromosomes
condense.Spindle forms.Nuclear envelope
fragments.
Homologous chromosomes(each with sister chromatids)
Join to form a ________________
Prophase I : Crossing-Over_____________ – the pairing of homologous chromosomes
Group of 4 __________________
Homologous chromosomes in a tetrad ____________________ each other
Pieces of chromosomes or genes are ____________________
Produces genetic recombination in the ____________________
Metaphase I________________________________of chromosomes align along the equator of the cell
Anaphase IHomologous chromosomes _______________
Sister chromatids remain attached at the _____________
Telophase INuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into ________.
Only _______ of the homologous pair of chromosome is present in the cell.
Sister chromatids carry _________________ genetic information.
Gene Z
Meiosis II
Meiosis II produces _______________ with one copy of each chromosome and thus one copy of each _________.
Prophase IINuclear envelope fragments.
Spindle forms.
Metaphase IIChromosomes align along ________________ of cell.
Anaphase II________________________________separate and move to opposite poles.
Telophase IINuclear envelope assembles.
Chromosomes ____________.
Spindle disappears.
Cytokinesis divides cell into ________.
_____________(egg & sperm) form
Four haploid cells (___) with one copy of each chromosome
One ________ of each gene
________________ combinations of alleles for different genes along the chromosome
Results of Meiosis
Which best describes crossing-over?
1. Pieces of genes are exchanged 2. Allow genetic variation3. Occurs in Prophase I 4. All of the above
√CHECKPOINT
Which best describes a haploid cell?1. n 2. Half the genetic
information
3. Gametes 4. All of the above
During anaphase what is pulled apart?
1.Centromere 2. Homologous
chromosomes
3. Sister chromatids 4. Spindle fibers
Occurs in the ___________
Two divisions produce 4 __________________
Spermatids mature into __________________
Men produce about 250,000,000 sperm per day
Spermatogenesis
Spermatid
Occurs in the _________________
Two divisions produce: _______ __________ bodies that die
because of unequal division of cytoplasm
_____ egg- Immature egg called _____________
Starting at puberty, one oocyte matures into an ovum (egg) every 28 days
Oogenesis
How many chromosomes would a sperm or egg contain if either one resulted from the process of mitosis?
√CHECKPOINT
Why is the sperm so small compared to the egg?
Oogenesis results in______.
1. 4 polar bodies 2. 3 polar bodies
3. 1 ovum 4. Both 2 & 3
Mitosis Meiosis
Number of divisions
Number of daughter cells
Genetically identical?
Chromosome #
Where
When
Role