Meiosis and Meiosis and Gamete FormationGamete Formation

GametesGametes

• Gametes =sex cellsGametes =sex cells• Gametes form from germline cells Gametes form from germline cells

via meiosisvia meiosis– Male gamete =spermMale gamete =sperm– Female gamete =oocyte (egg)Female gamete =oocyte (egg)

• A male and a female gamete come A male and a female gamete come together to form a new individualtogether to form a new individual– Somatic (non-gamete) cells have 46 Somatic (non-gamete) cells have 46

chromosomes (diploid)chromosomes (diploid)– Gametes have 23 chromosomes Gametes have 23 chromosomes

(haploid)(haploid)• ½ + ½ = 1 whole½ + ½ = 1 whole

Human KaryotypeHuman Karyotype

Remember…Remember…

Figure 3.2Homologous pair of chromosomes

Homologous pair of replicated chromosomes

Replicated chromosome(2 sister chromatids connected at centromere)A chromosome

MeiosisMeiosis

• Produces gametes (1n or haploid) Produces gametes (1n or haploid) from somatic germline cells (2n or from somatic germline cells (2n or diploid)diploid)

Figure 3.1

from mother from father child

meiosis reduces genetic content

toomuch!

MeiosisMeiosis

• Produces gametes (1n or haploid) Produces gametes (1n or haploid) from somatic germline cells (2n or from somatic germline cells (2n or diploid)diploid)

• Mixes up genetic informationMixes up genetic information– Diversity is important! Diversity is important!

• Consists of 2 divisionsConsists of 2 divisions– Meiosis I Meiosis I – Meiosis IIMeiosis II

Figure 3.1 Haploid

Diploid

Meiosis I(reductiondivision)

Meiosis II(equationaldivision)

Haploid

Meiosis overview

Meiosis IMeiosis I

• Interphase (CHROMOSOMES Interphase (CHROMOSOMES REPLICATE)REPLICATE)

• Prophase IProphase I• Metaphase IMetaphase I• Anaphase IAnaphase I• Telophase ITelophase I

InterphaseInterphase

• Precedes Meiosis IPrecedes Meiosis I• Cell grows and organelles are madeCell grows and organelles are made• CHROMOSOMES ARE REPLICATEDCHROMOSOMES ARE REPLICATED

Homologous pair of chromosomes Homologous pair of replicated chromosomes

Prophase IProphase I

• Pro =forward, forthPro =forward, forth• Chromosomes condenseChromosomes condense• Nuclear envelope breaks downNuclear envelope breaks down

– Membrane surrounding the nucleusMembrane surrounding the nucleus

• Centrioles move to opposite polesCentrioles move to opposite poles• Spindle fibers growSpindle fibers grow

• Chromosomes synapse and crossing Chromosomes synapse and crossing over occursover occurs

Q4 Prophase

Crossing Over Crossing Over (Recombination)(Recombination)

• Replicated chromosomes pair up with Replicated chromosomes pair up with their homologs their homologs – i.e.i.e. chromosome from mom and chromosome chromosome from mom and chromosome

from dad get togetherfrom dad get together

• Genetic information is exchangedGenetic information is exchanged– Like shuffling a deck of cards and re-dealingLike shuffling a deck of cards and re-dealing

Homologous pair of replicated chromosomes

Synapsis New combinations of genetic info

Q7

Metaphase IMetaphase I

• Meta =after, withMeta =after, with• Homologous chromosome pairs line up at Homologous chromosome pairs line up at

the midlinethe midline– Line up is random so that INDEPENDENT Line up is random so that INDEPENDENT

ASSORTMENT takes placeASSORTMENT takes place

• Spindle attaches to chromosomes at the Spindle attaches to chromosomes at the centromerecentromere– The centromere is where the sister The centromere is where the sister

chromatids are attachedchromatids are attached

Q4 Metaphase

Independent AssortmentIndependent Assortment

• Another way in which meiosis increases Another way in which meiosis increases diversitydiversity

• Chromosomes line up at the midline in a Chromosomes line up at the midline in a random wayrandom way

Q3

Anaphase IAnaphase I

• Ana =againstAna =against• Homologous pairs splitHomologous pairs split• Homologs are pulled to opposite Homologs are pulled to opposite

poles by the shortening of spindle poles by the shortening of spindle fibersfibers

Q4 Anaphase

Telophase ITelophase I

• Telo =completion of a goalTelo =completion of a goal• Spindle breaks downSpindle breaks down• Nuclear envelope forms in each of Nuclear envelope forms in each of

the new cellsthe new cells

Q4 Telophase

Summary of Meiosis ISummary of Meiosis I

Prophase I(early)

(diploid)

Prophase I(late)

(diploid)

Metaphase I(diploid)

Anaphase I(diploid)

Telophase I(diploid)

INTERPHASE

Meiosis IIMeiosis II

• Interphase –Chromosomes Interphase –Chromosomes DO NOTDO NOT replicate!replicate!

• Prophase IIProphase II• Metaphase IIMetaphase II• Anaphase IIAnaphase II• Telophase IITelophase II

Prophase IIProphase II

• Nuclear envelope breaks downNuclear envelope breaks down– Membrane surrounding the nucleusMembrane surrounding the nucleus

• Centrioles move to opposite polesCentrioles move to opposite poles• Spindle fibers growSpindle fibers grow

Q4 PII

Metaphase IIMetaphase II

• Chromosomes line up at the midlineChromosomes line up at the midline• Spindle attaches to chromosomes at Spindle attaches to chromosomes at

the centromerethe centromere– The centromere is where the sister The centromere is where the sister

chromatids are attachedchromatids are attached

Q4 MII

Anaphase IIAnaphase II

• Centromeres splitCentromeres split• Sister chromatids are pulled to Sister chromatids are pulled to

opposite poles by the shortening of opposite poles by the shortening of spindle fibersspindle fibers

Q4 AII

Telophase IITelophase II

• Spindle breaks Spindle breaks downdown

• Nuclear envelope Nuclear envelope forms in each of forms in each of the new cellsthe new cells

• Chromosomes Chromosomes decondensedecondense

Q4 TII

Summary of Meiosis IISummary of Meiosis IIQ2 Entire Overview

Prophase II(haploid)

Metaphase II(haploid)

Anaphase II(haploid)

Telophase II(haploid)

Four nonidentical

haploid daughter cells

Homologous pair of replicated chromosomes

Synapsis New combinations of genetic info

Homologous chromosomes in

germline cells

Square Dance

Males and Females are Males and Females are Different!Different!

Girls have a “flower” and boys have a

“noodle”

Male Reproductive Male Reproductive SystemSystem

• Gametes are made in the gonadsGametes are made in the gonads– Sperm are made in the testesSperm are made in the testes– Sperm are made and stored outside of Sperm are made and stored outside of

the bodythe body• 200-600 million sperm 200-600 million sperm areare released with each released with each

ejaculation!!!ejaculation!!!• Trillions of sperm are Trillions of sperm are made in a lifetimemade in a lifetime

bladder

urethra

testis

vas deferens

epididymis

bulbourethralgland

seminal vesicle

prostate

SpermatogenesisSpermatogenesisQ1

Entire process starts at puberty

Female Reproductive Female Reproductive SystemSystem

• Gametes are made in the gonadsGametes are made in the gonads– Oocytes are made in the ovariesOocytes are made in the ovaries

• A girl is born with just one million A girl is born with just one million oocytes per ovaryoocytes per ovary

• Only 400 eggs will be Only 400 eggs will be

ovulated between ovulated between

puberty and menopausepuberty and menopausevagina

cervix

uterus

ovary

uterine tube

OogenesisOogenesis

Before birthArrested in Prophase I

After puberty (each month)Arrested in Metaphase II

Upon fertilization

Polar BodiesPolar Bodies

• Three polar bodies Three polar bodies are generated due to are generated due to the developing egg the developing egg hogging all of the hogging all of the cytoplasmcytoplasm– Polar bodies usually Polar bodies usually

degeneratedegenerate– If fertilized by a If fertilized by a

sperm, a blighted sperm, a blighted ovum develops and ovum develops and results in miscarriageresults in miscarriage