Mendel and Meiosis

Chp 10 Pp. 252-279

Contents

• 10-2 Meiosis• 10-1 Mendel

Chromosome

Rod shaped structures made of DNA and proteinsCarrier of genetic materialLocated in the nucleusCopied and passed from generation to generation

Chromosomes

Sex chromosomes: chromosomes that determine the sex of an organismHumans X and Y Females: X XMales: X Y

Autosomes: all other types of chromosomes

Chromosomes

Homologous Chromosomes

2 copies of each autosome Same size and shapeCarry genes for the same traitEx. If 1 homologous chromosome contains gene for eye color the other homologous chromosome will too.

Homologous Chromosomes

Diploid Cell

Cell with 2 sets of chromosomesContains chromosomes for each homologous pairSomatic Cells= Body Cells: DiploidAll human cells except sex cells are diploid. One from each parent 2nHumans 2n = 46

Haploid

Cell containing one of each kind of chromosomeSperm and egg cells = Gametes1 set of chromosomesHalf the number of chromosomes of diploidnSperm + Egg = Zygoten + n = 2n

Mitosis vs. Meiosis

• Mitosis: t = two: Diploid 2n=46

• Meiosis: o = one: Haploid » n = 23

Meiosis

Gametes are produced in specialized body cellsProduces Sperm and Egg Cells2 divisions: Meiosis I and Meiosis IIMeiosis occurs in sex cells, not body cellsResults in 4 daughter cellsEach cell has half the chromosomes of the parent

Meiosis I

Interphase occurs: the cell grows and DNA replicatesMeiosis I begins Original cell produces two new cells

Prophase I

DNA Coils tightly into chromosomesspindle fibers appearEach chromosome lines up next to the homologueSynapis occurs: pairing of homologous chromosomes Tetrad: Each pair of homologous chromosomes

Crossing Over

•Crossing Over: why we do not look Crossing Over: why we do not look exactly like our parents.exactly like our parents.

Portions of the chromatid breaks off and attaches to adjacent chromatids on the

homologous chromosomePermits the exchange of genetic material

between maternal and paternal chromosomes

•Occurs during Prophase

Genetic Recombination

Crossing over produces a new mixture of genetic material

• Occurs during Prophase

Causes of Variation

Chromosomes are assorted randomlyCrossing over may occurCells do not have identical genetic info as each other or the parentGood: more chance of survival and evolutionBad: mistakes more likely

Metaphase I

Tetrads line up randomly along the mid-lineSpindle fibers attach to centromeres

Anaphase IHomologous chromosomes move to the opposite polesRandom separation or Independent Assortment results in separation of maternal and paternal chromosomes.

Telophase I

Chromosomes reach opposite ends of cellCytokinesis beginscell is haploid

Meiosis II

Occurs in each cell formed in Meiosis IInterphase does not occur again

Prophase II

Spindle fibers form and move the chromosomes to the mid-line of the dividing cell

Metaphase II

Chromosomes move to the mid-line of the dividing cell facing opposite poles of the dividing cell

Anaphase II

Chromatids separate and move to opposite poles of the cell

Telophase II

Nuclear membrane forms around the chromosomes in each of 4 new cells

Cytokinesis II

Cytoplasm divides Cell Membrane closes off

End Result:

Four new cells that contain half of the original cells number of chromosomes4 sex cells are created

Meiosis Animation

• Meiosis Animation

Haploid = one of each kind Diploid = two of each kind

• 2n = diploid n = haploid

WHY DO WE NEED HAPLOID?

• EGG

23

SPERM

Female gamete

Male gamete

23

Fertilization restores the diploid number

2n2n

1n1n1n

2n2n

fertilizationmeiosis

Mitosis and cell growth

Gametes

formed by meiosishaploid reproductive cellshumans: meiosis occurs in the testes and ovaries

Spermatogenesis: Male

Oogenesis: FemaleOne Mature

Egg Cell

Human Karotype

Asexual Reproduction

Production of offspring from one parent Does not involve meiosisOffspring are genetically identical to parent

Sexual Reproduction

Production of offspring through meiosis and the union of sperm and eggOffspring are genetically different form parentsGenes are combined in new ways in meiosisEvolutionary advantage is that it enables species to rapidly adapt to new conditions

Mitosis vs. Meiosis

• Mitosis vs. Meiosis Animation

Nondisjunction

• Failure of homologous chromosomes to separate properly during meiosis.

• Both chromosomes of a homologous pair move to the same pole of the cell. • 1 gamete has an extra chromosome

• Or• 1 gamete is missing a chromosome

Nondisjunction Animation

• Nondisjunction Animation

10-1 Mendel

• Gregor Mendel • Austrian monk• Studied how traits are

inherited from parents to offspring

• Father of heredity• Chose garden peas

for his meticulous experiments

Garden Peas

• Reproduce sexually• W/ male & female

gametes (sex cells)• Fertilization results

in zygote • Becomes seed• Pollination male

pollen transferred to female pistil

Monohybrid Cross• Hybrid- offspring of

parents with different forms of a trait

• Tall or short• crossed true-bred

tall plants w/ true-bred short plants to get heterozygous offspring which then self-pollinated

Some Genes are Dominant

• Some Genes Are Dominant click to play

Seven Traits of Peas

Mendel’s Rules

• Alleles- different gene forms

• Rule of Dominance– Dominant –observed

trait– Recessive-

disappearing or hidden trait

Mendel’s Rules

• Law of Segregation • Every individual has 2

alleles of each gene w/ each gamete receiving 1.

• During fertilization, gametes randomly pair to produce four combinations

Phenotypes & Genotypes• Appearance=

Phenotype• Allele combination=

Genotype• Homozygous-Both

alleles alike• Heterozygous-

different alleles, one dominant & one recessive

Mendel’s Dihybrid Cross

Law of Independent Assortment

• Says that genes for different traits are inherited independently from each other.

• The alleles can recombine in four different ways.

Punnett Squares• Shorthand way to find

possible genotypes from crossing two known parents.

• Two heterozygous parents produce 1 homozygous dominant: 2 heterozygous:1 homozygous recessive1BB: 2Bb:1bb