Chapter 10Chapter 10

Meiosis and Sexual Meiosis and Sexual ReproductionReproduction

Fig. 10-1b, p.154

Why SexWhy Sex

Why sex?Why sex? Asexual

Easier, faster Big population Indentical Bits can make whole

indv. No new combos All inherit the same

info Clones parthogenesis

Sexual Changing env More variety New combos Involves meiosis

(gametes) and fertilization

allele

Fig. 43-2c, p.756

Cost of Sexual ReproductionCost of Sexual Reproduction

43.1 (p. 756)43.1 (p. 756)Cost of Sexual ReproductionCost of Sexual Reproduction

Specialized cells and structures must be formed Special courtship, and parental behaviors can be

costly Timing of gamete formation and mating Nurturing developing offspring, either in egg or

body, requires resources from mother

10.2 What Meiosis Does10.2 What Meiosis Does

Meiosis – nuclear division that divides parental c-some # by half in specialized reproductive cells Ex: anther, ovules

antherovary

Homologous Chromosomes Homologous Chromosomes Carry Different AllelesCarry Different Alleles

Homologous c-some –

same shape, length

and assortment of

genes, line up with

each other

Paternal and maternal

chromosomes can

carry different alleles

Chromosome NumberChromosome Number

Sum total of chromosomes in a cell

Germ cells are diploid (2n)

Gametes are haploid (n)

Meiosis halves chromosome number

Meiosis: Two DivisionsMeiosis: Two Divisions

Two consecutive nuclear divisions Meiosis I – aligns with partner

Meiosis II – sister chromatids separate

DNA is not duplicated between

divisions

Four haploid nuclei form

10.4 Factors Contributing to 10.4 Factors Contributing to Variation among OffspringVariation among Offspring

Crossing over during prophase I

Independent assortment

Random alignment of chromosomes

at metaphase I

Random combination of gametes at

fertilization

Crossing OverCrossing Over

•Each chromosome

becomes zippered to its

homologue

•All four chromatids are

closely aligned

•Nonsister chromosomes

exchange segments

Effect of Crossing OverEffect of Crossing Over

After crossing over, each

chromosome contains both maternal

and paternal segments

Creates new allele combinations in

offspring

Independent AssortmentIndependent Assortment Microtubules from spindle poles attach to

kinetochores of chromosomes randomly, between Prophase I and Metaphase I

Possible Chromosome Possible Chromosome CombinationsCombinations

As a result of random alignment, the

number of possible combinations of chromosomes in a gamete is:

2n

(n is number of chromosome types)

FertilizationFertilization

Which two gametes unite is random Adds to variation among offspring

Life CyclesLife Cycles

• PlantPlant • AnimalAnimal

sporophyte

meiosisdiploid

fertilization

zygote

gametes

gametophytes

spores

haploid

Fig. 10-8a, p.162

Plant Life Cycle

multicelledbody

meiosisdiploid

fertilization

zygote

gametes

haploid

Fig. 10-8b, p.162

Animal Life Cycle

44.244.2SpermatogenesisSpermatogenesis

Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n)

Meiosis produces haploid spermatids Spermatids mature to become sperm

Figure 44.4Page 775

movie

Male Hormonal ControlMale Hormonal Control

Hypothalamus

Anterior Pituitary

GnRH

LHFSH

Sertoli Cells

Leydig Cells Testes

Testosterone

Inhibin

Formation and Development of Sperm

44.144.1Oocytes Arrested in Meiosis IOocytes Arrested in Meiosis I

Girl is born with primary oocytes already

in ovaries

Each oocyte has entered meiosis I and

stopped

Meiosis resumes, one oocyte at a time,

with the first menstrual cycle

Ovarian Ovarian CycleCycle

secondary oocyte

antrum

primordial follicle

corpus luteum

first polar body

Follicle

grows and

matures

Ovulation

occurs

Corpus

luteum

formsFigure 44.8Page 778

primordial follicle

Ovulation. Mature follicle ruptures and releases the secondary oocyte and the first polar body.

Primary oocyte, not yet released from meiosis I. A cell layer is forming around it. A follicle consists of the cell layer and the oocyte.

A corpus luteum forms from remnants of the ruptured follicle.

A transparent and somewhat elastic layer, the zona pellucida, starts forming around the primary oocyte.

first polar body

secondary oocyte

Mature follicle. Meiosis I is over. The secondary oocyte and first polar body are now formed.

A fluid-filled cavity (antrum) starts forming in the follicle’s cell layer.

The corpus luteum breaks down when the woman doesn’t get pregnant.

Fig. 44-8b, p.778

Female Hormonal ControlFemale Hormonal Control

Hypothalamus

Anterior pituitary

GnRH

LH FSH

OvaryEstrogen

Progesterone,estrogens

follicle growth, oocyte maturation

Rising estrogen stimulates surge in LH

Corpus luteumforms

MitosisMitosis FunctionsFunctions

Asexual Asexual reproductionreproduction

Growth, repair Growth, repair Occurs in somatic Occurs in somatic

cellscells Produces clonesProduces clones

Mitosis & Meiosis Mitosis & Meiosis ComparedCompared

Meiosis Function

Sexual reproduction

Occurs in germ cells

Produces variable offspring

Prophase vs. Prophase I Prophase vs. Prophase I

Prophase (Mitosis)Prophase (Mitosis) Homologous pairs do not interact with Homologous pairs do not interact with

each othereach other

Prophase I (Meiosis) Prophase I (Meiosis) Homologous pairs become zippered Homologous pairs become zippered

together and crossing over occurstogether and crossing over occurs

Anaphase, Anaphase I, and Anaphase, Anaphase I, and Anaphase IIAnaphase II

Anaphase I (Meiosis)Anaphase I (Meiosis)

Homologous chromosomes separate from Homologous chromosomes separate from

each othereach other

Anaphase/Anaphase II (Mitosis/Meiosis)Anaphase/Anaphase II (Mitosis/Meiosis)

Sister chromatids of a chromosome Sister chromatids of a chromosome

separate from each otherseparate from each other

Results of Mitosis and Results of Mitosis and MeiosisMeiosis

MitosisMitosis Two diploid cells producedTwo diploid cells produced

Each identical to parentEach identical to parent

MeiosisMeiosis Four haploid cells producedFour haploid cells produced

Differ from parent and one anotherDiffer from parent and one another

An Ancestral ConnectionAn Ancestral Connection

Was sexual reproduction a giant evolutionary Was sexual reproduction a giant evolutionary

step from aseuxal reproduction?step from aseuxal reproduction?

Giardia intestinalisGiardia intestinalis

ChlamydomonasChlamydomonas

Recombination mechanisms are vital for Recombination mechanisms are vital for

reproduction of euk cells may have evolved from reproduction of euk cells may have evolved from

DNA repair mechanisms in prok ancestorsDNA repair mechanisms in prok ancestors