Chapter 6.2 Notes

Chapter 6.2Meiosis and Sexual ReproductionChapter 6Meiosis and SexualReproductionThis picture showsgametes ___________unitingThere are many smallsperm cells competingto _____________ fertilize alarger egg cellChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Section MeiosisObjectives• Summarize the events that occur during meiosis.• Relate crossing-over, independent assortment, andrandom fertilization to genetic variation.• Compare spermatogenesis and oogenesis.Chapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Meiosis• Crossing over• Independent assortment• Spermatogenesis• Sperm• Oogenesis• ovumChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 6Formation of Haploid Cells• Meiosis is a form of cell division that __________ halvesthe number of chromosomes when formingspecialized reproductive cells, such as __________ gametesor _____________spores– These specialized cells are _______________haploids• Since the chromosome number is being______________ reduced by half, this process is often calledcell __________________reductionChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• What are the two types of human gametes?Sperm and egg cells• What do gametes do?They fuse to create a fertilized zygote• Why is it important that they contain half the numberof chromosomes as found in a diploid cell?So that they create a diploid cell when theyfuseChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Meiosis involves two divisions of the nucleus called…– _________________Meiosis I– _________________Meiosis II• These divisions are very similar to the process of_________________, mitosisbut with a few changes• Before meiosis begins, the DNA of the original cellmust be copied during _______________interphase– This original cell is a _________________ diploid cellChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Meiosis I includes….– Prophase ___ I– Metaphase ___ I– Anaphase ___ I– Telophase ___ I• Meiosis II includes– Prophase ____ II– Metaphase ____ II– Anaphase ____ II– Telophase ____ IIChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Prophase I• Nuclear ______________membraneand _______________ nucleolus breakdown• _____________ Spindle is arranged• __________________Chromosomesbecome visible– How many chromosomesare in the cell?4– How many chromatids?8Chapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Prophase I• This stage differs from mitosisbecause the homologouschromosomes ________ pair up ina process called ___________synapsisMeiosisMitosisChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Review...what are homologous chromosomes?– They are a pair of chromosomes that are similar in________________ structure and that also contain genes for thesame ____________– You inherit one from ______ and one from ______– They are not ________________• For example…dad’s chromosome may have a_________ gene while mom’s has a __________ genetalltraitsmomidenticaldadshortChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Since they are not identical,the homologouschromosomes are______________ colored differently• However, the identicalcopies within eachchromosome are coloredthe same…what are theycalled?sister chromatidsChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• The homologous structures pair up so ____________ tightlythat they form a four part structure called a ________ tetrad• Within a tetrad, a processcalled ________________crossing overmay occurChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• During crossing over, areas on _______________nonsisterchromatids may exchange genetic material• This picture showsa ___________ singlecross overLetters = genesChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Before crossing over, how many different genecombinations existed? 2• After crossing over,how many differentgene combinationsexist?4Chapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Metaphase I• The __________ pairs of homologouschromosomes line up along the_______________equator• This arrangement is also_____________ unique to meiosisMeiosisMitosisChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Anaphase I• Homologous_______________ chromosomes arepulled apart by spindlefibers and movedtowards______________ opposite polesof the cellChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Telophase I and CytokinesisProphase I• Opposite of ______________• Cytokinesis splits the__________________ cytoplasm andseparates the ______ 2 new cells• How many chromosomes are ineach new cell?• The chromosome number has beenreduced by _______ half and the cellsare now _____________2haploidChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

After Meiosis IAfter Mitosis4 chromosomes per cell2 chromosomes per cellChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

VERY IMPORTANT!!!!!!!!• These two cells do NOT enter ________________interphaseagain and do NOT copy their ________ DNA again!• They both proceed with meiosis ____. IIChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Metaphase II• Now, _______________individualchromosomes line upalong the equator ineach cellChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Anaphase II• Sister _____________chromatidsare pulled towards theopposite poles of eachcell• After the chromatidsseparate, what are theycalled?chromosomesChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Telophase II and Cytokinesis• In each cell, the nuclearmembrane andnucleolus ___________ reform• The spindle______________dissolves• Chromosomes will____________uncoil• Cytokinesis splits thecells into ______ 4 uniquehaploid cellsChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Meiosis and Genetic Variation• Meiosis is an important process that allows for therapid generation of new genetic ________________.combinations• Three mechanisms make key contributions to thisgenetic variation:– _________________________Independent Assortment– _________________________Random fertilization– _________________________Crossing overChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Independent Assortment• Refers to the _____________ random distribution ofhomologous chromosomes during meoisis IChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• In ____________________, Metaphase I the homologous chromosomesmay pair and line up randomly on either sideChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

segregategametes• This affects how the chromosomes ________________and are passed along to the ________________Chapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• Since humans have 23 pairs of homologouschromosomes…each pair has ____ 2 ways it may facewhile lining up• Mathematically, the total number of possible gametesformed is…2 n or 2 23 = 8,388,608 gametesChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Random fertilizationfuse• Each of the possible gametes (2 23 ) must _______ withanother one of the possible gametes (2 23 ) duringfertilization– Mathematically, the total number of possible zygotesis…2 23 x 2 23 = 70,368,744,000,000 zygotes• So many different ________________ with great_________________ may resultvariationoffspringChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Crossing over• Since homologous chromosomes may __________ exchangegenetic information within a tetrad, the number ofpossible gametes and zygotes is actually___________________unlimitedChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Importance of Genetic Variation• Meiosis and the joining of gametes are essential to__________________evolution• The pace of evolution is sped up by genetic_______________.recombination– Each offspring contains a combination of genesthat is ________________ different from each parentChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Gamete Formation in Males and FemalesMeiosis in Males• The process by which sperm are produced in maleanimals is called _______________________spermatogenesis• Spermatogenesis occurs in the male reproductiveorgan, called the _______________testesChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• First, an immaturediploid cell, calleda ___________ germcell, begins togrow• After meiosis, four_____________uniquehaploid cells areformedChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• These haploidcells must______________,differentiatemeaning theymust changeslightly in formand develop_________, flagella tobecome spermcellsChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Meiosis in Females• The process by which gametes are produced infemale animals is called ____________________oogenesis• Oogenesis occurs in the female reproductive organs,called _______________ ovaries and produces femalegametes called eggs or ________ ova (ovum –singular)Chapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

• In oogenesis, thecytoplasm does notdivide ____________, equallyresulting in…one– _____ large eggcell that containsmore cytoplasm and______________nutrients3– and ______ smallercells called second_________ polar___________, bodieswhich will all dieChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Objective 1• Summarize the events that occur during meiosis.At the beginning of Meiosis I, homologous chromosomes________ pair tightly to form ___________ tetrads in a process called_____________. synapsis ____________ Crossing over may occur. Whenthe homologous chromosomes separate, two____________ cells are formedhaploidchromatidsDuring Meiosis II, the sister ______________ separate,forming _____ 4 different gametes.Chapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Objective 2• Relate crossing-over, independent assortment, andrandom fertilization to genetic variation.Crossing over –Changes gene combinations onchromosomesIndependent Assortment –The homologous chromosomes line up andsegregate randomly during Meiosis I.Random fertilization -The possible gametes fuse randomlyChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.

Objective 3• Compare spermatogenesis and oogenesis.Creates four motilesperm cellsCreates one large eggcell and three smallerpolar bodiesChapter menuResourcesCopyright © by Holt, Rinehart and Winston. All rights reserved.