MeiosisMeiosis

Section 10.1 (pp. 270-276)Section 10.1 (pp. 270-276)

Terms to KnowTerms to Know

GenesGenes – a segment of DNA that controls – a segment of DNA that controls for the production of one proteinfor the production of one protein

ChromosomesChromosomes – DNA that becomes – DNA that becomes tightly coiled before cell divides; one tightly coiled before cell divides; one chromosome contains a thousand or more chromosome contains a thousand or more genesgenes

Homologous ChromosomesHomologous Chromosomes Homologous Chromosomes-chromosomes that Homologous Chromosomes-chromosomes that

make up a pair, one from each parent.make up a pair, one from each parent. A parent will randomly give one copy of their A parent will randomly give one copy of their

chromosome to their offspringchromosome to their offspring They are the same length and have the same They are the same length and have the same

centromere position. They carry genes that centromere position. They carry genes that control the control the samesame traits. traits.

Homologous ChromosomesHomologous Chromosomes

Haploid CellHaploid Cell Haploid cell Haploid cell – a cell that contains a haploid, or – a cell that contains a haploid, or n,n,

number of chromosomesnumber of chromosomes

Has one of each kind of chromosomeHas one of each kind of chromosome

Gametes- haploid cell that contain half the Gametes- haploid cell that contain half the chromosomes. chromosomes.

(Ex: Sex Cells = 23 chromosomes)(Ex: Sex Cells = 23 chromosomes)

Diploid NumberDiploid Number

Diploid cell Diploid cell – a cell that contains a diploid, – a cell that contains a diploid, or 2or 2n,n, number of chromosomes number of chromosomes

One chromosome in each pair comes from One chromosome in each pair comes from the male parent and the other from the the male parent and the other from the female parent.female parent.

Fertilization- process which one haploid Fertilization- process which one haploid gamete combines with another haploid gamete combines with another haploid gamete, creating a diploid cellgamete, creating a diploid cell

Chromosome NumberChromosome Number Each species of Each species of

organism contains a organism contains a characteristic number characteristic number of chromosomesof chromosomes

The chromosome The chromosome number of a species is number of a species is notnot related to the related to the complexity of the complexity of the organismorganism

OrganismOrganismBody Body cell cell (2n)(2n)

GametGamete (n)e (n)

Fruit flyFruit fly 88 44

Garden peaGarden pea 1414 77

CornCorn 2020 1010

TomatoTomato 2424 1212

Leopard frogLeopard frog 2626 1313

AppleApple 3434 1717

HumanHuman 4646 2323

ChimpanzeeChimpanzee 4848 2424

DogDog 7878 3939

Adder’s tongue Adder’s tongue fernfern

12601260 630630

MeiosisMeiosis

Meiosis- process which gametes are Meiosis- process which gametes are formed.formed.

Meiosis is cell division that Meiosis is cell division that halveshalves the the number of chromosomes. The gametes number of chromosomes. The gametes come together in fertilization so the new come together in fertilization so the new cell has the correct number of cell has the correct number of chromosomeschromosomes

MeiosisMeiosis

Consists of Consists of twotwo separate divisions separate divisions Meiosis IMeiosis I Meiosis IIMeiosis II

Meiosis I begins with 1 diploid (2n) cellMeiosis I begins with 1 diploid (2n) cell By the end of meiosis II there are By the end of meiosis II there are fourfour

haploid (n) cellshaploid (n) cells

Sexual ReproductionSexual Reproduction

Haploid cells are called gametes -sex cellsHaploid cells are called gametes -sex cells Sperm - male gametes (Sperm - male gametes (nn)) Eggs – female gametes (Eggs – female gametes (nn)) Zygote – a cell resulting from the Zygote – a cell resulting from the

fertilization of the egg by a sperm (2fertilization of the egg by a sperm (2nn)) The zygote can develop into a multicellular The zygote can develop into a multicellular

organism by mitosisorganism by mitosis

Meiosis IMeiosis I

Meiosis Intro ClipMeiosis Intro Clip

Phases of Meiosis IPhases of Meiosis IInterphaseInterphase Cell replicates its chromosomes Cell replicates its chromosomes

Prophase IProphase I Replicated chromosomes (sister chromatids) Replicated chromosomes (sister chromatids) are visible. Tetrad forms (pair of are visible. Tetrad forms (pair of homologous chromosomes) in a process homologous chromosomes) in a process called synapsis. Crossing over may occurcalled synapsis. Crossing over may occur

Metaphase IMetaphase I Centromeres attach to spindle fibers, tetrad Centromeres attach to spindle fibers, tetrad is pulled to equatoris pulled to equator

Anaphase IAnaphase I Homologous chromosomes (2 sister Homologous chromosomes (2 sister chromatids) separate to opposite endschromatids) separate to opposite ends

Telophase ITelophase I Spindle breaks down, chromosomes uncoil, Spindle breaks down, chromosomes uncoil, cytoplasm divides to form 2 new cells (sister cytoplasm divides to form 2 new cells (sister chromatids)chromatids)

Phases of Meiosis IIPhases of Meiosis II

InterphaseInterphase Short phase, chromosomes do not replicateShort phase, chromosomes do not replicate

Prophase IIProphase II Spindles form in each of 2 new cellsSpindles form in each of 2 new cells

Metaphase IIMetaphase II Centromeres attach to spindle fibers and Centromeres attach to spindle fibers and pulls sister chromatids to equatorpulls sister chromatids to equator

Anaphase IIAnaphase II Sister chromosomes separate to opposite Sister chromosomes separate to opposite polespoles

Telophase IITelophase II Nuclei re-form, spindle breaks down, Nuclei re-form, spindle breaks down, chromosomes uncoil, cytoplasm divideschromosomes uncoil, cytoplasm divides

Mitosis Vs. MeiosisMitosis Vs. Meiosis

MitosisMitosis MeiosisMeiosis

One divisionOne division 2 divisions -Meiosis I &II2 divisions -Meiosis I &II

DNA replicates DNA replicates DNA replicates only onceDNA replicates only once

No SynapsisNo Synapsis Synapsis occur in Prophase ISynapsis occur in Prophase I

2 identical cells are formed2 identical cells are formed 2 haploid cells are formed2 haploid cells are formed

Daughter cells are identicalDaughter cells are identical Daughter cells not Daughter cells not identicalidentical

Mitosis occurs in body cellsMitosis occurs in body cells Meiosis occurs Meiosis occurs reproduction (somatic cells)reproduction (somatic cells) cellscells

Genetic VariationGenetic Variation

Genetic recombination Genetic recombination – the reassortment – the reassortment of chromosomes by crossing over or of chromosomes by crossing over or independent segregation of homologous independent segregation of homologous chromosomeschromosomes

Is a major source of variation among Is a major source of variation among organismsorganisms

Source of genetic change in speciesSource of genetic change in species

Crossing OverCrossing Over Late in prophase I, the homologous Late in prophase I, the homologous

chromosomes come together to form chromosomes come together to form tetrads. Arms of nonsister chromatids wind tetrads. Arms of nonsister chromatids wind around each other and genetic material may around each other and genetic material may be exchangedbe exchanged

Meiosis ClipMeiosis Clip

How many different kinds of gametes How many different kinds of gametes can an organism produce?can an organism produce?

Pea plantPea plant 7 pair chromosomes7 pair chromosomes Each of seven pairs can line up at the equator in Each of seven pairs can line up at the equator in

two different ways (2two different ways (277)) 128 different kinds of sperm are possible128 different kinds of sperm are possible 2277 = 128 = 128 When fertilization occurs, 128When fertilization occurs, 128 XX 128128 or 16,384 or 16,384

different zygotes are possible!different zygotes are possible!

How many different kinds of gametes How many different kinds of gametes can an organisms produce?can an organisms produce?

HumanHuman 23 pair chromosomes23 pair chromosomes Each of 23 pairs can line up at the equator in two Each of 23 pairs can line up at the equator in two

different ways (2different ways (22323)) More than 8 million different kinds of sperm are More than 8 million different kinds of sperm are

possible possible When fertilization occurs, 2When fertilization occurs, 223 23 XX 2223, 23, or 70 trillion or 70 trillion

different zygotes are possible!different zygotes are possible!

Polyploidy (p. 285)Polyploidy (p. 285) Polyploids – organisms with more than the Polyploids – organisms with more than the

usual number of chromosome setsusual number of chromosome sets It is rare in animals…causing deathIt is rare in animals…causing death It occurs in plants. Flowers and fruits of these It occurs in plants. Flowers and fruits of these

plants are larger and healthier than normal plants are larger and healthier than normal

Ex: Tripolid (3n) Ex: Tripolid (3n)

Tetrapolid (4n)Tetrapolid (4n)

Mistakes (Polyploids) Mistakes (Polyploids) can be beneficialcan be beneficial

Tetraploid (4n)- coffeeTetraploid (4n)- coffee Triploid (3n)- applesTriploid (3n)- apples Polyploid chrysanthemums (2n,4n,6n,8n,10n)Polyploid chrysanthemums (2n,4n,6n,8n,10n) Polyploid (6n)- wheat, oats and sugar canePolyploid (6n)- wheat, oats and sugar cane Polyploid (8n)- strawberriesPolyploid (8n)- strawberries Plant breeders artificially produce polyploid Plant breeders artificially produce polyploid

plants using chemicals.plants using chemicals.