Chapter 10Chapter 10

Meiosis and Sexual ReproductionMeiosis and Sexual Reproduction

ObjectivesObjectives

1. Contrast asexual and sexual types of reproduction that occur on the cellular and multicellular or ganism levels.

2. Understand the effect that meiosis has on chromosome number.

3. Describe the events that occur in each meiotic phase.

 

ObjectivesObjectives

4. Compare mitosis and meiosis; cite similarities and differences.

5. Contrast meiosis in plant and animal life cycles.

 

Asexual reproduction is easier and fasterAsexual reproduction is easier and faster One parent alone transmits genetic One parent alone transmits genetic

information to offspring. (all clones)information to offspring. (all clones) Sexual reproduction can be an alternative Sexual reproduction can be an alternative

adaption in changing environments. adaption in changing environments. (survival)(survival)

Male and female must find each other and Male and female must find each other and exchange genetic material.exchange genetic material.

10.0 10.0 Why SexWhy Sex

Sexual reproduction has advantages when Sexual reproduction has advantages when other organisms change. (Predators and other organisms change. (Predators and prey, Hosts and pathogens)prey, Hosts and pathogens)

The outcome of sexual reproduction is The outcome of sexual reproduction is offspring that display novel combinations offspring that display novel combinations of traits.(diversity)of traits.(diversity)

Why SexWhy Sex

10.1 Alleles and Sexual Reproduction10.1 Alleles and Sexual Reproduction

Sexual Reproduction involvesSexual Reproduction involves

MeiosisMeiosis

Gamete productionGamete production

FertilizationFertilization Produces genetic variation among Produces genetic variation among

offspringoffspring

Introducing AllelesIntroducing Alleles

Allele – each unique molecular form of the Allele – each unique molecular form of the same gene.same gene.

Such tiny differences affect thousands of Such tiny differences affect thousands of traits.traits.

Alleles are one reason why individuals do Alleles are one reason why individuals do not all look alike.not all look alike.

Sexual reproduction leads to new allelesSexual reproduction leads to new alleles

Homologous Chromosomes Homologous Chromosomes Carry Different AllelesCarry Different Alleles

Cell has two of each chromosome Cell has two of each chromosome

One chromosome in each pair from One chromosome in each pair from

mother, other from fathermother, other from father

Paternal and maternal chromosomes carry Paternal and maternal chromosomes carry

different allelesdifferent alleles

Fig. 10-2, p.156

Homologous Homologous ChromosomesChromosomes

Sexual Reproduction Sexual Reproduction Shuffles AllelesShuffles Alleles

Through sexual reproduction, offspring Through sexual reproduction, offspring inherit new combinations of alleles, which inherit new combinations of alleles, which leads to variations in traitsleads to variations in traits

This variation in traits is the basis for This variation in traits is the basis for evolutionary changeevolutionary change

AllelesAlleles

Section 10.2: What Meiosis DoesSection 10.2: What Meiosis Does

Meiosis is a nuclear division process that divides a parental chromosome number by half in specialized reproductive cells.

Sexual reproduction will not work without it.

Unlike mitosis, meiosis sorts out chromosomes into parcels two times.

Germ cells undergo meiosis and Germ cells undergo meiosis and cytoplasmic divisioncytoplasmic division

Meiosis involves only the sex cells.Meiosis involves only the sex cells. Cellular descendents of germ cells Cellular descendents of germ cells

become gametes, (sperm and egg)become gametes, (sperm and egg) Gametes meet at fertilization Gametes meet at fertilization

Fig. 10-3, p.156

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), they have a ), they have a

pair of each type of chromosome. We call pair of each type of chromosome. We call them homologous chromosomes.them homologous chromosomes.

Gametes are haploid (Gametes are haploid (nn)) Meiosis halves parental chromosome Meiosis halves parental chromosome

numbernumber

Meiosis: Two DivisionsMeiosis: Two Divisions

Two consecutive nuclear divisions Two consecutive nuclear divisions Meiosis IMeiosis I

Meiosis IIMeiosis II

DNA is not duplicated between divisions – DNA is not duplicated between divisions –

NO InterphaseNO Interphase

Four haploid nuclei formFour haploid nuclei form

Meiosis I – Prophase I, Metaphase Meiosis I – Prophase I, Metaphase I, Anaphase I, Telophase II, Anaphase I, Telophase I

Each homologue (matching chromosome) in the cell pairs with its partner,

then the partners separate

p. 158

Meiosis II - Prophase II, Metaphase Meiosis II - Prophase II, Metaphase II, Anaphase II, Telophase IIII, Anaphase II, Telophase II

The two sister chromatids of each duplicated The two sister chromatids of each duplicated chromosome are separated from each otherchromosome are separated from each other

one chromosome (duplicated)

two chromosomes (unduplicated)

p. 158

10.3 Meiosis I -Prophase I10.3 Meiosis I -Prophase I

Each duplicated Each duplicated chromosome pairs with chromosome pairs with homologuehomologue

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 are Chromosomes are pushed and pulled pushed and pulled into the middle of into the middle of cell by microtubulescell by microtubules

The spindle is fully The spindle is fully formedformed

Fig. 10-5, p. 158

Anaphase IAnaphase I

Homologous Homologous

chromosomes chromosomes

separate and begin to separate and begin to

move toward pole.move toward pole.

The sister chromatids The sister chromatids

remain attachedremain attached

Fig. 10-5, p. 158

Telophase ITelophase I

The chromosomes The chromosomes arrive at opposite polesarrive at opposite poles

Usually followed by Usually followed by cytoplasmic division.cytoplasmic division.

Now have two haploid Now have two haploid cells (n).cells (n).

Chromosomes are still Chromosomes are still duplicated.duplicated.

Fig. 10-5, p. 158

Prophase IIProphase II

In each daughter cell In each daughter cell microtubules attach to microtubules attach to the kinetochores of the kinetochores of the duplicated the duplicated chromosomes.chromosomes.

One chromatid of One chromatid of each chromosome each chromosome becomes tethered to becomes tethered to one spindle pole.one spindle pole.

Fig. 10-5, p. 158

Metaphase IIMetaphase II

In each daughter In each daughter cell duplicated cell duplicated chromosomes line chromosomes line up at the spindle up at the spindle equator, midway equator, midway between the polesbetween the poles

Fig. 10-5, p. 158

Anaphase II IIAnaphase II II

In each daughter In each daughter cell sister cell sister chromatids chromatids separate and separate and move toward move toward opposite poles to opposite poles to become become independent independent chromosomes.chromosomes.

Fig. 10-5, p. 158

Telophase II IITelophase II II

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

A nuclear envelope A nuclear envelope forms around each set forms around each set of chromosomes, each of chromosomes, each cell divides in half.cell divides in half.

Four haploid (n) cells.Four haploid (n) cells.

Fig. 10-5, p. 158

Section 10.4: How Meiosis Introduces Section 10.4: How Meiosis Introduces Variations in TraitsVariations in Traits

Crossing over – a molecular interaction between a chromatid of one chromosome and a chromatid of the homologous partner.

This really is gene swapping.

Crossing OverCrossing Over

• During Prophase I 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 After crossing over, each chromosome

contains both maternal and paternal contains both maternal and paternal

segmentssegments

Breaks up old combinations of alleles and Breaks up old combinations of alleles and

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 random, there is no chromosomes are random, there is no particular pattern to the metaphase particular pattern to the metaphase position of chromosomes.position of chromosomes.

Random AlignmentRandom Alignment

Either the maternal or paternal member of Either the maternal or paternal member of a homologous pair can end up at either a homologous pair can end up at either pole. This can also lead to different traits pole. This can also lead to different traits in each new generation.in each new generation.

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

Possible Chromosome Possible Chromosome CombinationsCombinations

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

in a gamete is: in a gamete is:

22nn

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

Possible Chromosome Possible Chromosome CombinationsCombinations

Thus, every time a human sperm or egg Thus, every time a human sperm or egg forms, there is a total of 8,388,608 orforms, there is a total of 8,388,608 or

222323

Possible combinations of maternal and Possible combinations of maternal and paternal chromosomes.paternal chromosomes.

Section 10.5: From Gametes to Section 10.5: From Gametes to OffspringOffspring

The life cycle of most plant species alternates between sporophyte and gametophyte stages.

A sporophyte is a spore producing body that makes spores by the process of meiosis.

A spore is a haploid reproductive cell that undergoes mitosis and gives rise to a gametophyte.

A gametophyte gives rise to gametes, which can then be fertilized and form the zygote.

sporophyte

meiosisdiploid

fertilization

zygote

gametes

gametophytes

spores

haploid

Fig. 10-8a, p.162

Plant Life CyclePlant Life Cycle

Gamete formation in animalsGamete formation in animals

In the male reproductive system, a germ In the male reproductive system, a germ cell develops into four haploid cells, each cell develops into four haploid cells, each becoming a sperm.becoming a sperm.

In the female reproductive system, a germ In the female reproductive system, a germ cells develops into one haploid ovum, or cells develops into one haploid ovum, or egg, and three polar bodies. The polar egg, and three polar bodies. The polar bodies eventually degenerate.bodies eventually degenerate.

When fertilization occurs the diploid When fertilization occurs the diploid number is restored.number is restored.

multicelledbody

meiosisdiploid

fertilization

zygote

gametes

haploid

Fig. 10-8b, p.162

Animal Life CycleAnimal Life Cycle

FertilizationFertilization

Male and female gametes unite and nuclei Male and female gametes unite and nuclei

fusefuse

Fusion of two haploid nuclei produces Fusion of two haploid nuclei produces

diploid nucleus in the zygotediploid nucleus in the zygote

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

Factors Contributing to Variation Factors Contributing to Variation among Offspringamong Offspring

Crossing over during prophase I Crossing over during prophase I (average of 2 or 3 in every human (average of 2 or 3 in every human chromosome)chromosome)

Random alignment of chromosomes at Random alignment of chromosomes at metaphase Imetaphase I

Random combination of gametes at Random combination of gametes at fertilizationfertilization

MitosisMitosis FunctionsFunctions

Asexual reproductionAsexual reproduction Growth, repair Growth, repair

Occurs in somatic Occurs in somatic cellscells

Produces clonesProduces clones

10.6 Mitosis & Meiosis 10.6 Mitosis & Meiosis ComparedCompared

MeiosisMeiosis Function Function

Sexual reproductionSexual reproduction

Occurs in germ cellsOccurs in germ cells

Produces variable Produces variable offspringoffspring

Prophase vs. Prophase I Prophase vs. Prophase I

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

otherother

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

and crossing over occursand 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 separate Sister chromatids of a chromosome separate

from each otherfrom each other

Results of Mitosis and MeiosisResults of Mitosis and Meiosis

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

Repair of DNA breaksRepair of DNA breaks

Checkpoint genes code for proteins that Checkpoint genes code for proteins that can recognize and repair breaks in the can recognize and repair breaks in the double-stranded DNA molecules of double-stranded DNA molecules of chromosomes.chromosomes.

If they detect a problem, there is a pause If they detect a problem, there is a pause in the cycle until the DNA is repaired.in the cycle until the DNA is repaired.

Review of MeiosisReview of Meiosis

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

An Ancestral ConnectionAn Ancestral Connection

Was sexual reproduction a giant evolutionary step from Was sexual reproduction a giant evolutionary step from

aseuxal reproduction?aseuxal reproduction?

Giardia intestinalis – single-celled parasite, does not Giardia intestinalis – single-celled parasite, does not

have a mitochondria, does not form a spindle during have a mitochondria, does not form a spindle during

mitosis, and has never been observed to reproduce mitosis, and has never been observed to reproduce

sexually.sexually.

Chlamydomonas – a single-celled alga, haploid cells Chlamydomonas – a single-celled alga, haploid cells

reproduce asexually by mitosis. They can also fuse and reproduce asexually by mitosis. They can also fuse and

form diploid individuals.form diploid individuals.