6.1 Chromosomes and Meiosis

6.1 Chromosomes and MeiosisKEY CONCEPTGametes have half the number of chromosomesthat body cells have.

6.1 Chromosomes and MeiosisYou have body cells and gametes.• Body cells are also called somatic cells.• Sex cells develop into gametes.– Gametes are sex cells: egg and sperm.– Gametes have DNA that can be passed tooffspring.body cells sex cells (sperm) sex cells (egg)

6.1 Chromosomes and MeiosisYour cells have autosomes and sex chromosomes.• Your body cells have 23 pairsof chromosomes.– Homologous pairs ofchromosomes have thesame structure.– For each homologous pair,one chromosome comesfrom each parent.• Chromosome pairs 1-22 areautosomes.• Sex chromosomes, X and Y,determine gender in mammals.

6.1 Chromosomes and MeiosisBody cells are diploid; gametes are haploid.• Fertilization between egg and sperm occurs in sexualreproduction.• Diploid (2n) cells have two copies of everychromosome.– Body cells are diploid.– Half the chromosomes come from each parent.

6.1 Chromosomes and Meiosis• Haploid (n) cells have one copy of everychromosome.– Gametes are haploid.– Gametes have 22 autosomes and 1 sexchromosome.

6.1 Chromosomes and Meiosis• Chromosome number must be maintained in animals.• Many plants have more than two copies of eachchromosome.• Mitosis and meiosis are types of nuclear division thatmake different types of cells.• Mitosis makesmore diploid cells.

6.1 Chromosomes and Meiosis• Meiosis makes haploid cells from diploid cells.– Meiosis occurs in sex cells.– Meiosis produces gametes.

6.1 Chromosomes and MeiosisWord RootsKnowing word roots can improveyour vocabulary!• a- = not or without (asexual: type ofreproduction not involvingfertilization)• auto- = self (autosome: thechromosomes that do not determinegender)

6.1 Chromosomes and MeiosisKEY CONCEPTDuring meiosis, diploid cells undergo two celldivisions that result in haploid cells.

6.1 Chromosomes and MeiosisCells go through two rounds of division in meiosis.• Meiosis reduces chromosome number and createsgenetic diversity.

6.1 Chromosomes and MeiosisREMEMBER: Homologous chromosomes• The twochromosomes ofeach pair in adiploid cell arecalledhomologouschromosomes.

6.1 Chromosomes and Meiosis• Meiosis I and meiosis II each have four phases, similar tothose in mitosis.– Pairs of homologous chromosomes separate inmeiosis I.– Homologous chromosomes are similar but not identical.– Sister chromatids divide in meiosis II.– Sister chromatids are copies of the same chromosome.homologous chromosomessisterchromatidssisterchromatids

6.1 Chromosomes and Meiosis• Meiosis I occurs after DNA has been replicated.• Meiosis I divides homologous chromosomes in four phases.

6.1 Chromosomes and Meiosis• Meiosis II divides sister chromatids in four phases.• DNA is not replicated between meiosis I andmeiosis II.

6.1 Chromosomes and MeiosisProphase I – crossing over• Chromatids are wrapped sotightly the chromosomescan actually break andexchange genetic materialin a process known ascrossing over.• Crossing over results innew combinations of alleleson a chromosome.

6.1 Chromosomes and MeiosisMetaphase I – (Same as before)• The centromere of eachchromosome attachesto a spindle fiber.• The spindle fibers pullthe tetrads into themiddle, or equator, ofthe spindle.

6.1 Chromosomes and MeiosisAnaphase I – (Same as before)• Homologouschromosomes separateand move to oppositeends of the cell.• This critical stepensures that each newcell will receive only onechromosome from eachhomologous pair.

6.1 Chromosomes and MeiosisTelophase I – (Same as before)• The spindle is brokendown, the chromosomesuncoil, and the cytoplasmdivides to yield two newcells.• Each cell has half the DNAas the original cell becauseit has only onechromosome from eachhomologous pair.

6.1 Chromosomes and MeiosisProphase II• A spindle forms ineach of the twonew cells and thespindle fibersattach to thechromosomes.

6.1 Chromosomes and MeiosisMetaphase II.• The chromosomes,still made up ofsister chromatids,are pulled to thecenter of the celland line uprandomly at theequator.

6.1 Chromosomes and MeiosisAnaphase II• The centromereof eachchromosomesplits.• The sisterchromatids toseparate andmove to oppositepoles.

6.1 Chromosomes and MeiosisTelophase II• Finally nuclei reform,the spindlesbreakdown, and thecytoplasm divides.• Four haploid cellshave been formedfrom one diploidcell

6.1 Chromosomes and Meiosis• Meiosis differs from mitosis in significant ways.– Meiosis has two cell divisions while mitosis has one.– In mitosis, homologous chromosomes never pair up.– Meiosis results in haploid cells; mitosis results in diploidcells.

6.1 Chromosomes and MeiosisWhy meiosis is important1. Forms gametes forsexual reproduction2. Crossing over duringmeiosis whichrearranges allelecombinations so thatthe offspringgenerations aregenetically differentthan the parents.

6.1 Chromosomes and MeiosisHaploid cells develop into mature gametes.• Gametogenesis is the productionof gametes.• Gametogenesis differs betweenfemales and males.– Sperm become streamlined andmotile.– Sperm primarily contribute DNAto an embryo.– Eggs contribute DNA, cytoplasm,and organelles to an embryo.– During meiosis, the egg getsmost of the contents; the othercells form polar bodies.

6.1 Chromosomes and MeiosisWord RootsKnowing word roots can improve yourvocabulary!• di- = two (diploid: cells that containtwo homologous sets ofchromosomes)• fertil- = fruitful (fertilization: processof fusion of a haploid sperm and ahaploid egg cell)