Chapter 10.2

Genes:Organisms have thousands of genes that determine individual traits

They are lined up on chromosomes1 chromosome holds 100-1000 genes

Traits:Hair color, length of leg, size of hand, shape of eyes, etc.Each trait can have different forms (called alleles)

Examples:Trait Possible alleles

Hair color = blonde, brunette, red, black

Shape of nose = pointy, round, squareEye Color = brown, blue, hazel,

green

Genes & TraitsGenes & Traits

ChromosomesChromosomesChromosome: organelle that contains DNA

Carries genetic material that is copied and passed from generation to generation of cellsDarkly colored when stained so it is easy to see under the microscope

Chromosome Number:A specific number of chromosomes that an organism has

Examples:Humans have 46 chromosomes or 23 pairsPea Plants have 14 chromosomes or 7 pairsDogs have 78 chromosomes or 39 pairs

Diploid vs. HaploidDiploid vs. HaploidBody Cells

Chromosomes occur in pairs

Diploid Cell: cell with two of each type of chromosome

Contains a diploid, or 2n, number of chromosomesOffspring receive ½ of their parents DNA

One chromosomes from male parentOne chromosome from female parent

Haploid Cells: a cell with one of each kind of chromosome

Contains a haploid, or n, number of chromosomes

Chromosomes are matched pairs because they carry the same trait

Not necessarily the same allele!

Example:

Still matched pairs even because they are for the same trait, even though they are different

Trait Allele from Mom Allele from Dad

Chromosome pair #1 Hair color Brown Blonde

Chromosome pair #2 Nose Shape Pointy Pointy

Chromosome pair #3 Eye Color Blue Green

Homologous Chromosomes:Paired chromosomes of a diploid cell.Each pair has genes for same traits not necessarily alleles

How do offspring have the same number of How do offspring have the same number of chromosomes as their parents? Why aren’t chromosomes as their parents? Why aren’t they doubled?they doubled?

Meiosis: cell division that produces gametes Meiosis: cell division that produces gametes containing half the number of chromosomes as containing half the number of chromosomes as a parent’s cella parent’s cell

Occurs in specialized cells that produce gametesOccurs in specialized cells that produce gametesGamete – male and female sex cells (sperm and eggs)Gamete – male and female sex cells (sperm and eggs)

Phases of MeiosisPhases of MeiosisMeiosis has two separate divisionsMeiosis has two separate divisions

Meiosis I and Meiosis IIMeiosis I and Meiosis II

Meiosis IMeiosis IStarts with one diploid (2n) cellStarts with one diploid (2n) cell

At end of Meiosis II At end of Meiosis II 4 haploid cells (n) 4 haploid cells (n) Called sex cells (gametes)Called sex cells (gametes)

Male gametes – spermMale gametes – spermFemale gametes – eggsFemale gametes – eggs

Phases of MeiosisPhases of Meiosis

Some is same as MitosisSome is same as MitosisSpindle fibers formSpindle fibers formCytoplasm splits Cytoplasm splits

InterphaseInterphaseCell replicates its chromosomesCell replicates its chromosomes

Each chromosome consists of two identical sister Each chromosome consists of two identical sister chromatidschromatidsHeld together by a centomereHeld together by a centomere

Meiosis Animation

Prophase IProphase IChromosomes coil up and spindle fibers formChromosomes coil up and spindle fibers form

Each pair of homologous chromosomes come Each pair of homologous chromosomes come togethertogether

Matched gene by geneMatched gene by geneFor a four part structure called a For a four part structure called a tetradtetrad

Tetrad:Tetrad:Consists of two homologous chromosomes , each Consists of two homologous chromosomes , each made of 2 sister chromatidsmade of 2 sister chromatidsPair so tightly some genetic information is actually Pair so tightly some genetic information is actually exchanged, called exchanged, called Crossing OverCrossing OverAverage of 2-3 crossovers on every homologous Average of 2-3 crossovers on every homologous chromosomeschromosomes

Metaphase IMetaphase ICentromere is attached to the spindle fiberCentromere is attached to the spindle fiber

Spindle fibers pull the tetrads into the equator of the Spindle fibers pull the tetrads into the equator of the cellcellLine up as tetrads, not homologous chromosomesLine up as tetrads, not homologous chromosomes

Anaphase I & Telophase IAnaphase I & Telophase I

Anaphase I:Anaphase I:Homologous Chromosomes separate and move to Homologous Chromosomes separate and move to opposite polesopposite poles

Telophase I:Telophase I:Spindle fibers breakdownSpindle fibers breakdownChromosomes uncoilChromosomes uncoilCytoplasm divides (cytokinesis)Cytoplasm divides (cytokinesis)

We still need another cell division because we We still need another cell division because we have doubled chromosomeshave doubled chromosomes

Anaphase I & Telophase IAnaphase I & Telophase I

Anaphase IAnaphase I

Telophase ITelophase I

Meiosis IIMeiosis IIShort interphase periodShort interphase period

Then it follows the same steps of mitosisThen it follows the same steps of mitosisProphase IIProphase IIMetaphase IIMetaphase IIAnaphase IIAnaphase IITelophase IITelophase II

You now have 4 (four) haploid cells have been formed You now have 4 (four) haploid cells have been formed from 1 (one) diploid cell from 1 (one) diploid cell

Each haploid cell contains one chromosome from each Each haploid cell contains one chromosome from each homologous pairhomologous pairBecome gametesBecome gametes

Meiosis Animation

Meiosis IIMeiosis II