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