MeiosisMeiosis

Chapter 8Chapter 8

Part IIPart II

AgendaAgenda

Textbook: Pages 136-143 Meiosis and Textbook: Pages 136-143 Meiosis and crossing over, Pages 181-193 The crossing over, Pages 181-193 The structure of genetic material.structure of genetic material.PowerpointPowerpoint

Quiz datesQuiz dates

12/7 Germ cell vs Somatic cell, 12/7 Germ cell vs Somatic cell, Meiosis, crossing over, Homologous Meiosis, crossing over, Homologous chromosomes.chromosomes.

12/9 TBA12/9 TBA

Octopus Sex Octopus Sex

Male reaches under his mantle with Male reaches under his mantle with tentacle, removes packet of sperm, tentacle, removes packet of sperm, and insert it into female’s egg and insert it into female’s egg chamberchamber

Eggs are fertilized and give rise to Eggs are fertilized and give rise to new octopusesnew octopuses

Limpet SexLimpet Sex

Larva can become adult of either sexLarva can become adult of either sex

Depends on whether other limpets are Depends on whether other limpets are

present and what their sex ispresent and what their sex is

Adults can change sex in response to Adults can change sex in response to

new arrivalsnew arrivals

Slipper snailsSlipper snails

HermaphroditicHermaphroditic Stacked on top of each otherStacked on top of each other Young are always maleYoung are always male Can stay male or change to femaleCan stay male or change to female Cannot go back to maleCannot go back to male

Sexual ReproductionSexual Reproduction

Chromosomes are duplicated in Chromosomes are duplicated in germ cellsgerm cells

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

Cellular descendents of germ cells Cellular descendents of germ cells become gametesbecome gametes

Gametes meet at fertilization Gametes meet at fertilization

Meiosis

Asexual Reproduction Asexual Reproduction

Single parent produces offspringSingle parent produces offspring

All offspring are genetically identical All offspring are genetically identical to one another and to parentto one another and to parent

Sexual ReproductionSexual Reproduction

InvolvesInvolves

MeiosisMeiosis

Gamete productionGamete production

FertilizationFertilization Produces genetic variation among Produces genetic variation among

offspringoffspring

Homologous Homologous Chromosomes Carry Chromosomes Carry

Different AllelesDifferent 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 Paternal and maternal chromosomes

carry different allelescarry different alleles

Alleles- Different forms of a Alleles- Different forms of a genegene

What is a gene?What is a gene?

Chemical factors that determine Chemical factors that determine traitstraits

Sexual Reproduction Sexual Reproduction Shuffles AllelesShuffles Alleles

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

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

Gamete FormationGamete Formation

Gametes are sex cells (sperm, eggs)Gametes are sex cells (sperm, eggs) Arise from germ cellsArise from germ cells

testes

ovaries

anther ovary

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 IMeiosis I

Meiosis IIMeiosis II

DNA is NOT duplicated between DNA is NOT duplicated between

divisionsdivisions

Four haploid nuclei are formedFour haploid nuclei are formed

Meiosis IMeiosis I

Each homologue in the cell pairs with its partner,

then the partners separate

Meiosis IIMeiosis II

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

one chromosome (duplicated)

two chromosomes (unduplicated)

Stages of MeiosisStages of Meiosis

Meiosis IMeiosis I

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

Meiosis IIMeiosis II

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

Meiosis I - StagesMeiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

92

46x2 46x2 46x2 23x4

Prophase IProphase I

Each duplicated, condensed Each duplicated, condensed chromosome pairs with its homologuechromosome pairs with its homologue

Homologues swap segments !!!!!!! Homologues swap segments !!!!!!! Take note !!!!Take note !!!!

Each chromosome becomes attached Each chromosome becomes attached to microtubules of newly forming to microtubules of newly forming spindlespindle

Crossing-Over

Crossing-Over

Crossing-Over

Crossing OverCrossing Over

•Each chromosome

becomes zippered to its

homologue

•All four chromatids are

closely aligned

•Non-sister 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 parental segmentsand parental segments

Creates new allele combinations in Creates new allele combinations in

offspringoffspring

Metaphase IMetaphase I

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

Sister chromatids of one homologue Sister chromatids of one homologue orient toward one pole, and those of orient toward one pole, and those of other homologue toward opposite pole other homologue toward opposite pole

The spindle is now fully formedThe spindle is now fully formed

Anaphase IAnaphase I

Homologous chromosomes Homologous chromosomes

segregate from each other segregate from each other

The sister chromatids of each The sister chromatids of each

chromosome remain attachedchromosome remain attached

Telophase ITelophase I

The chromosomes arrive at opposite The chromosomes arrive at opposite

polespoles

The cytoplasm dividesThe cytoplasm divides

There are now two haploid cellsThere are now two haploid cells

This completes Meiosis IThis completes Meiosis I

Meiosis II - StagesMeiosis II - Stages

Prophase II Metaphase II Anaphase II Telophase II

Prophase IIProphase II

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

Motor proteins drive the movement Motor proteins drive the movement of chromosomes toward the of chromosomes toward the spindle’s equatorspindle’s equator

Metaphase IIMetaphase II

All of the duplicated chromosomes All of the duplicated chromosomes are lined up at the spindle are lined up at the spindle equator, midway between the equator, midway between the polespoles

Anaphase IIAnaphase II

Sister chromatids separate to Sister chromatids separate to become independent chromosomesbecome independent chromosomes

Motor proteins interact with Motor proteins interact with microtubules to move the separated microtubules to move the separated chromosomes to opposite poleschromosomes to opposite poles

Telophase IITelophase II

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

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

The cytoplasm dividesThe cytoplasm divides There are now four haploid cells There are now four haploid cells

Random AlignmentRandom Alignment

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

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

Possible Possible ChromosomChromosom

eeCombinatioCombinatio

ns ns

or

or

or

1 2 3

FertilizationFertilization

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

nuclei fusenuclei fuse

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 Factors Contributing to Variation Among Variation Among

OffspringOffspring 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

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

Pre-Natal Tests - help to determine if an at risk baby is healthy

Sex chromosomes

Human Karyotype

Sex Chromosomes

Down Syndrome (trisomy 21),

47,XY,+21.

47: the total number of chromosomes (46 is normal).

XY: the sex chromosomes (male).

+21: designates the extra chromosome as a 21.

Klinefelter’s Syndrome

47, XXY

Turner’s SyndromeTurner’s Syndrome

Only one complete XOnly one complete X The second could also be incompleteThe second could also be incomplete In almost all cases, unable to repro. In almost all cases, unable to repro.

Or menstruate Or menstruate

HermaphroditeHermaphrodite

XX/XY or XOXX/XY or XO 350-400 Known cases350-400 Known cases

Gene Therapy - an absent or faulty gene is replaced by a working gene

Has not been very successful