Chapter 10Chapter 10

Meiosis and Sexual Meiosis and Sexual ReproductionReproduction

Fig. 10-1b, p.154

Why SexWhy Sex

Fig. 43-2a, p.756

Why sex?Why sex?

AsexualAsexual SexualSexual

Why sex?Why sex?

AsexualAsexual Easier, fasterEasier, faster Big populationBig population IndenticalIndentical Bits can make whole Bits can make whole

indv.indv. No new combosNo new combos All inherit the same infoAll inherit the same info ClonesClones parthogenesisparthogenesis

SexualSexual Changing envChanging env More varietyMore variety New combosNew combos Involves meiosis Involves meiosis

(gametes) and (gametes) and fertilizationfertilization

alleleallele

Modes of ReproductionModes of Reproduction

Sexual reproductionSexual reproduction Meiosis, gamete formation, and Meiosis, gamete formation, and

fertilizationfertilization Offspring show genetic variationOffspring show genetic variation

Asexual reproductionAsexual reproduction Single parent produces offspringSingle parent produces offspring Offspring are genetically identicalOffspring are genetically identical

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

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

body, requires resources from motherbody, requires resources from mother

Fig. 43-2d, p.756

Cost of Sexual Reproduction

10.2 What Meiosis Does10.2 What Meiosis Does

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

Homologous c-some – same shape, length and Homologous c-some – same shape, length and assortment of genes, line up with each otherassortment of genes, line up with each other

antherovary

Homologous Chromosomes Homologous Chromosomes Carry Different AllelesCarry Different Alleles

Cell has two of each Cell has two of each

chromosome chromosome

One chromosome in One chromosome in

each pair from mother, each pair from mother,

other from fatherother from father

Paternal and maternal Paternal and maternal

chromosomes carry chromosomes carry

different allelesdifferent alleles

Chromosome NumberChromosome Number

Sum total of chromosomes in a cellSum total of chromosomes in a cell

Germ cells are diploid (2Germ cells are diploid (2nn))

Gametes are haploid (Gametes are haploid (nn))

Meiosis halves chromosome numberMeiosis halves chromosome number

Meiosis: Two DivisionsMeiosis: Two Divisions

Two consecutive nuclear divisions Two consecutive nuclear divisions Meiosis I – aligns with partnerMeiosis I – aligns with partner

Meiosis II – sister chromatids separateMeiosis II – sister chromatids separate

DNA is not duplicated between DNA is not duplicated between

divisionsdivisions

Four haploid nuclei formFour haploid nuclei form

10.3 Tour of Meiosis10.3 Tour of MeiosisProphase IProphase I

Each duplicated Each duplicated chromosome pairs with chromosome pairs with homologue (synapse)homologue (synapse)

Homologues swap Homologues swap segments (crossing segments (crossing over)over)

Each chromosome Each chromosome becomes attached to becomes attached to spindlespindle

Fig. 10-5, p. 158

Metaphase IMetaphase I

Chromosomes Chromosomes are pushed and are pushed and pulled into the pulled into the middle of cellmiddle of cell

The spindle is The spindle is fully formedfully formed

Fig. 10-5, p. 158

Anaphase IAnaphase I

Homologous Homologous

chromosomes chromosomes

segregatesegregate

The sister The sister

chromatids chromatids

remain attachedremain attached

Fig. 10-5, p. 158

Telophase ITelophase I

The chromosomes The chromosomes

arrive at opposite arrive at opposite

polespoles

Usually followed Usually followed

by cytoplasmic by cytoplasmic

divisiondivision

Fig. 10-5, p. 158

Meosis II: Prophase IIMeosis II: Prophase II

Microtubules Microtubules attach to the attach to the kinetochores of kinetochores of the duplicated the duplicated chromosomeschromosomes

Attach to one Attach to one chromatid of each chromatid of each chromosomechromosome

Fig. 10-5, p. 158

Metaphase IIMetaphase II

Duplicated Duplicated chromosomes chromosomes line up at the line up at the spindle equator, spindle equator, midway midway between the between the polespoles

Fig. 10-5, p. 158

Anaphase IIAnaphase II

Sister chromatids Sister chromatids separate to separate to become become independent independent chromosomeschromosomes

Attachments Attachments breakbreak

Fig. 10-5, p. 158

Telophase IITelophase II

The chromosomes The chromosomes arrive at opposite arrive at opposite ends of the cellends of the cell

A nuclear A nuclear envelope forms envelope forms around each set around each set of chromosomesof chromosomes

Four haploid cellsFour haploid cells

Fig. 10-5, p. 158

44.244.2SpermatogenesisSpermatogenesis

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

Meiosis produces haploid spermatidsMeiosis produces haploid spermatids Spermatids mature to become spermSpermatids mature to become sperm

Figure 44.4Page 775

movie

Other Testicular CellsOther Testicular Cells

Sertoli cellsSertoli cells Line the seminiferous tubulesLine the seminiferous tubules Nourish the developing spermNourish the developing sperm

Leydig cellsLeydig cells Lie between the seminiferous Lie between the seminiferous

tubulestubules Secrete testosteroneSecrete testosterone

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 Girl is born with primary oocytes already

in ovariesin ovaries

Each oocyte has entered meiosis I and Each oocyte has entered meiosis I and

stoppedstopped

Meiosis resumes, one oocyte at a time, Meiosis resumes, one oocyte at a time,

with the first menstrual cycle 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

10.4 How Meiosis Introduces 10.4 How Meiosis Introduces VariationsVariations

Crossing overCrossing over Independent assortmentIndependent assortment Random alignmentRandom alignment

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 After crossing over, each

chromosome contains both maternal chromosome contains both maternal

and paternal segmentsand paternal segments

Creates new allele combinations in Creates new allele combinations in

offspringoffspring

Random Alignment Random Alignment

During transition between prophase I and During transition between prophase I and metaphase I, microtubules from spindle metaphase I, microtubules from spindle poles attach to kinetochores of poles attach to kinetochores of chromosomes chromosomes

Initial contacts between microtubules and Initial contacts between microtubules and chromosomes are randomchromosomes are random

Randomness cont.Randomness cont.

Either the maternal or paternal member of a Either the maternal or paternal member of a homologous pair can end up at either polehomologous pair can end up at either pole

The chromosomes in a gamete are a mix of The chromosomes in a gamete are a mix of chromosomes from the two parents chromosomes from the two parents

Possible Chromosome Possible Chromosome CombinationsCombinations

As a result of random alignment, the As a result of random alignment, the number of possible combinations of number of possible combinations of

chromosomes in a gamete is: chromosomes in a gamete is:

22nn

((nn is number of chromosome types) is number of chromosome types)

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

FertilizationFertilization

Which two gametes unite is randomWhich two gametes unite is random Adds to variation among offspringAdds to variation among offspring

Factors Contributing to Factors Contributing to Variation among OffspringVariation among Offspring

Crossing over during prophase ICrossing over during prophase I

Random alignment of Random alignment of

chromosomes at metaphase Ichromosomes at metaphase I

Random combination of gametes Random combination of gametes

at fertilizationat fertilization

MitosisMitosis FunctionsFunctions

Asexual Asexual reproductionreproduction

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

cellscells Produces clonesProduces clones

Mitosis & Meiosis Mitosis & Meiosis ComparedCompared

MeiosisMeiosis Function Function

Sexual reproductionSexual reproduction

Occurs in germ Occurs in germ cellscells

Produces variable Produces variable offspringoffspring

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