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MeiosisMeiosisFormation of Formation of

GametesGametes (Eggs & (Eggs & Sperm)Sperm)

Important Terminology

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Diploid (2N): Diploid (2N): “two sets”; a cell that “two sets”; a cell that has 1 set of chromosomes from its has 1 set of chromosomes from its female parent and 1 set from its female parent and 1 set from its male parent (=2 copies of each male parent (=2 copies of each chromosome)chromosome)

Homologous: Homologous: each chromosome each chromosome from one parent has a from one parent has a corresponding chromosome from corresponding chromosome from the other parent.the other parent.

Haploid (N or 1N): Haploid (N or 1N): the number of chromosomes in a the number of chromosomes in a

gametegamete ( (sex cell – egg or spermsex cell – egg or sperm)) Only one copy of each chromosome Only one copy of each chromosome

Polypoidy

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What would this mean???What would this mean??? More than diploid (more than 2 sets of More than diploid (more than 2 sets of

chromosomes)chromosomes) Triploid (3N)Triploid (3N) Tetraploid (4N)Tetraploid (4N)

Not viable (“survivable”) in humansNot viable (“survivable”) in humans Seedless watermelons are triploid, Seedless watermelons are triploid,

formed from a cross between diploid and formed from a cross between diploid and tetraploid watermelons.tetraploid watermelons.

Because the 3 copies of the Because the 3 copies of the chromosomes won’t line up correctly chromosomes won’t line up correctly during meiosis, meiosis is halted, so no during meiosis, meiosis is halted, so no gametes (seeds) are formed.gametes (seeds) are formed.

Definition of Meiosis

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The process of The process of reduction reduction divisiondivision in which the number of in which the number of chromosomes per cell is cut in chromosomes per cell is cut in half (half (reductionreduction) through the ) through the separation (separation (divisiondivision) of ) of homologous chromosomes homologous chromosomes in a in a diploid celldiploid cell

Homologous chromosomes Homologous chromosomes (“homologs”):(“homologs”): “matched” “matched” chromosomes (not necessarily chromosomes (not necessarily identical); both before and identical); both before and after replication!after replication!

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Facts About MeiosisFacts About MeiosisPreceded by interphase which Preceded by interphase which

includesincludes chromosome replicationchromosome replicationTwoTwo meiotic divisions --- meiotic divisions --- Meiosis Meiosis

I and Meiosis III and Meiosis IICalled reduction divisionCalled reduction divisionOriginal cell is diploid (2n)Original cell is diploid (2n)Four daughter cellsFour daughter cells produced produced

are are monoploid (1n) (AKA, monoploid (1n) (AKA, haploid)haploid)

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Facts About MeiosisFacts About MeiosisDaughter cellsDaughter cells contain half the contain half the

number of chromosomesnumber of chromosomes as the as the original cell.original cell.

ProducesProduces gametesgametes (eggs & (eggs & sperm)sperm)

Occurs in the Occurs in the testestestes in males in males

((SpermatogenesisSpermatogenesis))ovariesovaries in females ( in females (OogenesisOogenesis))

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Human cells start with Human cells start with 46 46 doubledouble strandedstranded chromosomes chromosomes (2n).(2n).After 1After 1stst division - division - 23 23 doubledouble stranded chromosomes (n) in stranded chromosomes (n) in each of 2 cellseach of 2 cellsAfter 2After 2ndnd division - division - 23 23 singlesingle stranded chromosomes (n) in stranded chromosomes (n) in each of 4 cellseach of 4 cells  Occurs in our Occurs in our germ cellsgerm cells that that produce gametesproduce gametes

More Meiosis FactsMore Meiosis Facts

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Why Do We Need Why Do We Need Meiosis?Meiosis?

It is the fundamental basis It is the fundamental basis of of sexual reproduction.sexual reproduction.

Two haploid (1n) gametes Two haploid (1n) gametes are brought together are brought together through through fertilizationfertilization to form to form a diploid a diploid (2n) zygote.(2n) zygote.

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Fertilization – “Putting Fertilization – “Putting it all together”it all together”

1n =31n =3

2n = 62n = 6

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Replication of Replication of ChromosomesChromosomesReplicationReplication is the is the

process of process of duplicating a duplicating a chromosomechromosome

Occurs Occurs prior to prior to division division (interphase)(interphase)

Replicated copies Replicated copies are called are called sister sister chromatids.chromatids.

Held together at Held together at centromerecentromere

Occurs in Occurs in InterphasInterphas

ee

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A Replicated A Replicated ChromosomeChromosome

HomologsHomologs (same genes, (same genes, different alleles)different alleles)

SisterSisterChromatidsChromatids(same genes,(same genes,same alleles)same alleles)

Gene XGene X

Homologs separate in meiosis I and Homologs separate in meiosis I and therefore different alleles separate.therefore different alleles separate.

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Meiosis Forms Haploid Meiosis Forms Haploid GametesGametes

Meiosis must Meiosis must reduce the chromosome number by half.reduce the chromosome number by half. Fertilization then restoresFertilization then restores the 2n number. the 2n number.

from mom from dad child

meiosis reducesgenetic content

toomuch!

The right number!

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Meiosis: Two Part Cell Meiosis: Two Part Cell DivisionDivision

HomologsHomologsseparateseparate

Sister Sister chromatidschromatidsseparateseparate

DiploidDiploid

MeiosisMeiosis I I

MeiosisMeiosisIIII

DiploidDiploid

HaploiHaploidd