1.3-Meiosis and Gametogenesis

8/12/2019 1.3-Meiosis and Gametogenesis

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Lesson#1.3Meiosis


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Mitosis/Meiosis Overview

Difference between Mitosis and Meiosis

Mitosis Reproduction

Asexual, required 1 parent cell

Division increases population

Growth

Organism develops from a single cell

100 trillion cells in a human body from single cell

RBC reproduce1 million/second

Repair

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Mitosis/Meiosis Overview

Meiosis Reproduction

Sexual, requires TWO parents

Produces gametes: egg and sperm cellsLeads to genetic variability and diversity

within species

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Meiosis

Formation of Gametes(Eggs & Sperm)


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Facts About Meiosis

Preceded by interphase whichincludeschromosome replication

Goes through Twomeiotic divisions

Meiosis I and Meiosis II

Original cell is diploid (2n)

Four daughter cellsproduced, arehaploid (1n)


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Facts About MeiosisDaughter cellscontain half the

number of chromosomesas theoriginal cell

Producesgametes(eggs & sperm)

Occurs in the testesin males

Occurs in the ovariesin females


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Start with 46 double strandedchromosomesAfter 1 division - 23 double

stranded chromosomesAfter 2nd division - 23 singlestranded chromosomes Occurs in our germ cellsthatproduce gametes

More Meiosis Facts


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Why Do we Need Meiosis?

It is the fundamental basis ofsexual reproductionTwo haploid (1n) gametes arebrought together throughfertilizationto form a diploid(2n) zygote

F ili i P i i


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Fertilization Putting itall together

1n =3

2n = 6


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Replication of ChromosomesReplicationis the

process ofduplicating achromosomeOccurs prior todivisionReplicated copiesare called sisterchromatidsHeld together atcentromere

Occurs inInterphase


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A Replicated Chromosome

Homologouschromosomes

(same genes,

different alleles)

SisterChromatids

(same genes,same alleles)

Gene X


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Meiosis Forms Haploid GametesMeiosis must reduce the chromosome number

by halfFertilization then restoresthe 2n number

from mom from dad child

meiosis reducesgenetic content

too

much!

The right

number!


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Meiosis: Two Part CellDivision

Homologsseparate

Sisterchromatidsseparate

Haploid

MeiosisI

MeiosisII

Diploid

Haploid


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Meiosis I: Reduction Division

Nucleus Spindlefibers Nuclear

envelopeEarlyProphase I

(Chromosomenumberdoubled)

LateProphase

I

MetaphaseI Anaphase

ITelophase I

P h I


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Prophase I

Early prophaseHomologs pair.Crossing overoccurs.

Late prophase

Chromosomes condense.Spindle forms.Nuclear envelopefragments.


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Tetrads Form in Prophase I

Homologous chromosomes(each with sister

chromatids)

Join to form aTETRAD


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Crossing-OverHomologous

chromosomesina tetrad crossover each other

Pieces of

chromosomes orgenes areexchanged

ProducesGeneticrecombinationinthe offspring


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Homologous ChromosomesDuring Crossing-Over


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Crossing-over multiplies the already hugenumber of different gamete types

produced by independent assortment

Crossing-Over

M t h I


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Metaphase I

Homologous pairsof

chromosomes alignalong the equatorof

the cellMicrotubules from one pole

are attached to thecentromere of onechromosome of each tetrad,

while those from the otherpole are attached to theother.

A h I


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Anaphase I

Homologs separateand

move to opposite poles.

Sister chromatids remain

attached at theircentromeres.

T l h I


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Telophase I

-Homologous chromosomes move

until there is a haploid set ateach pole.-Each chromosome consists oflinked sister chromatids.-Cytokinesis by the samemechanisms as mitosisusually occurs simultaneously

Nucleus reforms, but there isno further replicationof chromosomes.


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Meiosis II

NO interphase betweenMeiosis I and Meiosis II

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M i i R d i


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Meiosis II: ReducingChromosome Number

ProphaseII

MetaphaseII

AnaphaseII

TelophaseII 4 unique

haploid cells

P h II


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Prophase IIDuring prophase II a spindle

apparatus forms, attachesto centromeres of eachsister chromatids, andmoves them around.

Spindle fibers from one poleattach to the centromere ofone sister chromatid andthose of the other pole tothe other sister chromatid.

M t h II


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Metaphase II

Chromosomes align

along equatorof cell.

A h II


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Anaphase II

Sister chromatidsseparate andmove to opposite

poles.

Equator

Pole

T l h II


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Telophase II

Nuclear envelopeassembles.

Chromosomesdecondense.

Spindle disappears.

Cytokinesis dividescell into two.

R lt f M i i


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Results of Meiosis

-Gametes (egg & sperm)form

-Four haploid cells with one

copy of each chromosome

-One allele of each gene isinherited from each parent

-Different combinations ofalleles for different genesalong the chromosome


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Lesson#1.3

Gametogenesis


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Gametogenesis

Oogenesis

orSpermatogenesis

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SpermatogenesisOccurs in the

testesbegins atpuberty

Two divisions

produce 4spermatids

Spermatids mature

into spermMen produce about250,000,000

sperm per day

Spermatogenesis


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Spermatogenesis

O i


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OogenesisOccurs in the ovaries

Two divisions produce 3 polar bodiesthat die and 1 egg

Polar bodies die because of unequaldivision of cytoplasm

Immature egg called oocyte

Starting at puberty, one oocytematures into an ovum (egg)every 28days

O i


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Oogenesis

Oogonium(diploid)

Mitosis

Primaryoocyte(diploid)

Meiosis I

Secondaryoocyte(haploid)

Meiosis II

(if fertilizationoccurs)

First polar body

may divide(haploid) Polarbodiesdie

Ovum (egg)

Secondpolar body(haploid)

a

A

X

X

a

X

A X

a

X

a

X

Matureegg

A

X

A

X

P tti it ll t th


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Putting it all together All your body cells (somatic cells) go through mitosis for

repair and growth 46 chromosomes, 2n (diploid)

At puberty, your sex cells (gametes) go through meiosisto prepare for fertilization 23 chromosomes, n (haploid)

FERTILIZATION

1 haploidegg + 1 haploidsperm = 1 diploidzygote

embryobabyAdult

mitosis

mitosismitosis

Mitosis forrepair


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Comparing

Mitosis andMeiosis

C mparis n f Divisi ns


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Mitosis Meiosis

Number ofdivisions

12

Number ofdaughter cells

2 4

Geneticallyidentical?

Yes No

Chromosome # Same as parent Half of parent

Where Somatic cells Germ cells

When Throughout life At sexual maturity

R l

Growth and

i l d i

Comparison of Divisions

Documents

1.3-Meiosis and Gametogenesis