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MEIOSISFORMATION OF GAMETES

11

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

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

FERTILIZATION: PUTTING IT ALL TOGETHER

44

1n =31n =3

2n = 62n = 6

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

55

Occurs in Occurs in InterphasInterphas

ee

A REPLICATED CHROMOSOME

66

Homologs Same genes, different alleles (traits)

SisterChromatidsSame genes, Same alleles (traits)

Gene X

Homologs separate in meiosis I and therefore different alleles (traits)

separate.

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!

MEIOSIS: TWO PART CELL DIVISION

88

Homologsseparate

Sister chromatidsseparate

Diploid

Meiosis I

MeiosisII

Diploid

Haploid

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)

PROPHASE I

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Early prophase Homologs pair. Crossing over occurs

Late prophaseChromosomes condense.Spindle forms.Nuclear envelope fragments.

TETRADS FORM IN PROPHASE I

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Homologous chromosomes(each with sister

chromatids)   

Join to form a TETRAD

Called Synapsis

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

1313

CROSSING-OVER

1414

METAPHASE I

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Homologous pairs of

chromosomes align along the equator of the

cell

ANAPHASE I

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Homologs separate and move to opposite poles.

Sister chromatids remain attached at their centromeres.

TELOPHASE I

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Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

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

MEIOSIS II: REDUCING CHROMOSOME NUMBER

1919

Prophase II

Metaphase II

Anaphase II

Telophase II 4

haploid cells

PROPHASE II

2020

Nuclear envelope fragments.

Spindle forms.

METAPHASE II

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Chromosomes align along equator of cell.

ANAPHASE II

2222

Sister chromatids separate and move to opposite poles.

Equator

Pole

TELOPHASE II

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Nuclear envelope assembles.

Chromosomes uncoil.

Spindle disappears.

Cytokinesis divides cell into two.

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

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

2727

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

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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

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

3131

3232

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>>>>

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?

3434

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?

3636

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?