Meiosis and Sexual Meiosis and Sexual ReproductionReproduction

Chapter 7Chapter 7

7.1 Meiosis7.1 Meiosis In some organisms haploid gametes join to In some organisms haploid gametes join to

form a diploid zygote form a diploid zygote If gametes weren’t haploid imagine how the If gametes weren’t haploid imagine how the

number of chromosomes would increase of number of chromosomes would increase of timetime

Meiosis is the form of cell division that makes Meiosis is the form of cell division that makes diploid cells haploid when forming gametesdiploid cells haploid when forming gametes

There are 2 divisions in Meiosis – Meiosis I There are 2 divisions in Meiosis – Meiosis I and Meiosis IIand Meiosis II

Before Meiosis begins DNA is replicated so Before Meiosis begins DNA is replicated so cell has homologous chromosomescell has homologous chromosomes

Prophase IProphase I

Chromosomes Chromosomes condensecondense

Nuclear envelope Nuclear envelope breaks downbreaks down

Homologous Homologous chromosomes pair chromosomes pair upup

Crossing over occursCrossing over occurs Chromatids exchange Chromatids exchange

Metaphase IMetaphase I

Homologous pairs Homologous pairs move to equator by move to equator by spindlespindle

Anaphase IAnaphase I

Homologous pairs Homologous pairs separate and separate and pulled towards pulled towards polespoles

Chromatids do not Chromatids do not separate; still separate; still connected by connected by centromerescentromeres

Telophase ITelophase I

Individual Individual chromosomes chromosomes gather at polesgather at poles

Most organisms Most organisms now have now have cytokinesiscytokinesis

2 new cells formed2 new cells formed Chromosomes do Chromosomes do

not replicate againnot replicate again

Prophase IIProphase II

New spindle formsNew spindle forms

Metaphase IIMetaphase II

Chromosomes line Chromosomes line up along equator up along equator and are still and are still attached at attached at centromerescentromeres

Anaphase IIAnaphase II

Centromeres divide Centromeres divide Chromatids Chromatids

separate; Now separate; Now called called chromosomeschromosomes

Telophase IITelophase II

Nuclear envelope Nuclear envelope forms around forms around chromosome setschromosome sets

Meiosis in ActionMeiosis in Action

Meiosis and Genetic Meiosis and Genetic Variation Variation

Three key ways meiosis creates new Three key ways meiosis creates new genetic variationsgenetic variations Independent AssortmentIndependent Assortment Crossing OverCrossing Over Random FertilizationRandom Fertilization

Independent AssortmentIndependent Assortment Most organisms have Most organisms have

more than 1 more than 1 chromosomechromosome

In humans the gamete In humans the gamete gets 1 chromosome gets 1 chromosome from the 23 pairsfrom the 23 pairs

Which of the pair you Which of the pair you get is chanceget is chance

This random distribution This random distribution is called Independent is called Independent AssortmentAssortment

Mathematically 2Mathematically 223 23 = = over 8.3 million over 8.3 million combinationscombinations

Crossing OverCrossing Over Portion of 1 Portion of 1

homologous homologous chromosome is chromosome is broken off and broken off and exchanged with exchanged with the the corresponding corresponding portion of its portion of its homologous homologous partnerpartner

Random FertilizationRandom Fertilization

What egg and sperm meet is also What egg and sperm meet is also completely randomcompletely random

Mathematically that means that the Mathematically that means that the possible combinations of the sperm possible combinations of the sperm (about 8.3 million) can be multiplied (about 8.3 million) can be multiplied by the combinations for the egg by the combinations for the egg (about 8.3 million) giving 70 trillion (about 8.3 million) giving 70 trillion possible combinationspossible combinations

Meiosis generates variation in a Meiosis generates variation in a species very quicklyspecies very quickly

Pace of evolution is sped up due to Pace of evolution is sped up due to variationvariation

A + B = C, a new individual not A + B = C, a new individual not identical to its parentsidentical to its parents

Gametogenesis – Formation of Gametogenesis – Formation of GametesGametes

Spermatogenesis – Spermatogenesis – sperm production sperm production occurs in testesoccurs in testes Diploid cell Diploid cell

increases in size increases in size and becomes and becomes immature germ cellimmature germ cell

Meiosis I = 2 Meiosis I = 2 immature sperm immature sperm cellscells

Meiosis II = 4 Meiosis II = 4 haploid cellshaploid cells

Develop tails and Develop tails and become spermbecome sperm

Oogenesis – egg production Oogenesis – egg production occurs in ovariesoccurs in ovaries Diploid cell increases and Diploid cell increases and

becomes germ cellbecomes germ cell Meiosis I = after cytokinesis Meiosis I = after cytokinesis

cytoplasm is not equally dividedcytoplasm is not equally divided Smaller one called polar bodySmaller one called polar body

Meiosis II = larger cell divides Meiosis II = larger cell divides again, again cytoplasm not again, again cytoplasm not equally dividedequally divided

Forms another polar bodyForms another polar body Polar bodies will diePolar bodies will die

Ovum (pl. = ova) is larger cellOvum (pl. = ova) is larger cell More cytoplasm means more More cytoplasm means more

stored energystored energy

7.2 Sexual Reproduction7.2 Sexual Reproduction

Asexual ReproductionAsexual Reproduction Single parent passes on copies to offspring called Single parent passes on copies to offspring called

“clone”“clone” Identical to parentIdentical to parent Prokaryotes use binary fissionProkaryotes use binary fission

Many eukaryotes Many eukaryotes use asexual use asexual reproductionreproduction Fission – parent into Fission – parent into

2 equal sized 2 equal sized offspringoffspring

Fragmentation Fragmentation Body into several Body into several

piecespieces Missing parts grow Missing parts grow

backback Budding – new split Budding – new split

off from old; may off from old; may stay attachedstay attached

Genetic DiversityGenetic Diversity Asexual reproduction is simplest methodAsexual reproduction is simplest method In stable environment can produce many In stable environment can produce many

offspring in short time’offspring in short time’ Don’t need to waste energy making gametes Don’t need to waste energy making gametes

or find a partneror find a partner DNA varies very littleDNA varies very little In changing environment may not be able to In changing environment may not be able to

adaptadapt

Sexual ReproductionSexual Reproduction 2 parents form 2 parents form

reproductive cells with reproductive cells with half the number of half the number of chromosomeschromosomes

Diploid Mother makes Diploid Mother makes haploid gametehaploid gamete

Diploid Father makes Diploid Father makes haploid gametehaploid gamete

Haploid gametes make Haploid gametes make diploid offspring diploid offspring different from parentsdifferent from parents

Evolution of Sexual Evolution of Sexual ReproductionReproduction

May have come as a way for protists May have come as a way for protists to repair DNAto repair DNA

Only diploid cells can repairOnly diploid cells can repair Protists form diploid cells under Protists form diploid cells under

stressstress Many enzymes that repair DNA are Many enzymes that repair DNA are

involved in meiosisinvolved in meiosis

Sexual Life Cycle in Sexual Life Cycle in EukaryotesEukaryotes

Life Cycle – span from 1 generation to Life Cycle – span from 1 generation to nextnext

All sexually reproductive organisms All sexually reproductive organisms have a diploid-haploid-diploid lifecyclehave a diploid-haploid-diploid lifecycle

Only 3 Types of Sexual Life CyclesOnly 3 Types of Sexual Life Cycles Only difference is which phase Only difference is which phase

becomes multicellularbecomes multicellular

Haploid – simplest and most Haploid – simplest and most commoncommon

Zygote is the only diploid cell and it Zygote is the only diploid cell and it undergoes meiosis immediatelyundergoes meiosis immediately

Haploid cells in multicellular organism Haploid cells in multicellular organism produce gametes through mitosisproduce gametes through mitosis

Fusion – gametes fuse to form zygote Fusion – gametes fuse to form zygote and meiosis followsand meiosis follows

Meiosis allows for damage repairMeiosis allows for damage repair 2 homologous chromosomes are lined 2 homologous chromosomes are lined

up, special repair enzymes remove up, special repair enzymes remove damaged sections of DNA and fill in damaged sections of DNA and fill in gapsgaps

Protists, Fungi, AlgaeProtists, Fungi, Algae

Haploid Life CycleHaploid Life Cycle

DiploidDiploid

Adults are diploid, inherit from 2 Adults are diploid, inherit from 2 parentsparents

Diploid cell then meiosis then haploid Diploid cell then meiosis then haploid gametegamete

Gametes join during fertilization Gametes join during fertilization forming diploid zygoteforming diploid zygote

Zygote divides by mitosisZygote divides by mitosis Gametes are the only haploid cellsGametes are the only haploid cells

Diploid Life CycleDiploid Life Cycle

ParthenogenesisParthenogenesis New individual forms from New individual forms from

unfertilized eggunfertilized egg Hypothesized that Hypothesized that

mothers chromosomes mothers chromosomes are copied instead of are copied instead of getting fathers getting fathers chromosomeschromosomes

Produces clonesProduces clones Honeybees provide Honeybees provide

drones this way (haploid)drones this way (haploid) Whiptail lizards are all Whiptail lizards are all

femalesfemales Doesn’t seem possible in Doesn’t seem possible in

mammalsmammals Only mammal clones are Only mammal clones are

twins from split eggtwins from split egg

Alternation of GenerationsAlternation of Generations Regularly alternates between haploid and Regularly alternates between haploid and

diploiddiploid Plants – diploid phase called sporophyte Plants – diploid phase called sporophyte

produces sporesproduces spores Spore forming cells produce haploid spore that Spore forming cells produce haploid spore that

can form adult without another cellcan form adult without another cell Gametophyte is haploid phaseGametophyte is haploid phase

Produces gametes by mitosisProduces gametes by mitosis Gametes fuse and produce diploid phaseGametes fuse and produce diploid phase

Diploid – Haploid – Diploid – Haploid – etc…Diploid – Haploid – Diploid – Haploid – etc… Moss – haploid spores develop at top of Moss – haploid spores develop at top of

sporophyte stalksporophyte stalk