Chapter 10.2

Genes: Organisms have thousands of genes that determine individual traits They are lined up on chromosomes 1 chromosome holds genes Traits: Hair color, length of leg, size of hand, shape of eyes, etc. Each trait can have different forms (called alleles) Examples: TraitPossible alleles Hair color=blonde, brunette, red, black Shape of nose =pointy, round, square Eye Color=brown, blue, hazel, green Genes & Traits

Chromosomes Chromosome: organelle that contains DNA Carries genetic material that is copied and passed from generation to generation of cells Darkly 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 pairs Pea Plants have 14 chromosomes or 7 pairs Dogs have 78 chromosomes or 39 pairs

Diploid vs. Haploid Body Cells Chromosomes occur in pairs Diploid Cell: cell with two of each type of chromosome Contains a diploid, or 2n, number of chromosomes Offspring receive ½ of their parents DNA One chromosomes from male parent One 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 TraitAllele from MomAllele from Dad Chromosome pair #1Hair colorBrownBlonde Chromosome pair #2Nose ShapePointy Chromosome pair #3Eye ColorBlueGreen

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 chromosomes as their parents? Why aren’t they doubled? Meiosis: cell division that produces gametes containing half the number of chromosomes as a parent’s cell Occurs in specialized cells that produce gametes Gamete – male and female sex cells (sperm and eggs)

Phases of Meiosis Meiosis has two separate divisions Meiosis I and Meiosis II Meiosis I Starts with one diploid (2n) cell At end of Meiosis II 4 haploid cells (n) Called sex cells (gametes) Male gametes – sperm Female gametes – eggs

Phases of Meiosis Some is same as Mitosis Spindle fibers form Cytoplasm splits Interphase Cell replicates its chromosomes Each chromosome consists of two identical sister chromatids Held together by a centomere Meiosis Animation Meiosis Animation

Prophase I Chromosomes coil up and spindle fibers form Each pair of homologous chromosomes come together Matched gene by gene For a four part structure called a tetrad Tetrad: Consists of two homologous chromosomes, each made of 2 sister chromatids Pair so tightly some genetic information is actually exchanged, called Crossing Over Average of 2-3 crossovers on every homologous chromosomes

Metaphase I Centromere is attached to the spindle fiber Spindle fibers pull the tetrads into the equator of the cell Line up as tetrads, not homologous chromosomes

Anaphase I & Telophase I Anaphase I: Homologous Chromosomes separate and move to opposite poles Telophase I: Spindle fibers breakdown Chromosomes uncoil Cytoplasm divides (cytokinesis) We still need another cell division because we have doubled chromosomes

Anaphase I & Telophase I Anaphase I Telophase I

Meiosis II Short interphase period Then it follows the same steps of mitosis Prophase II Metaphase II Anaphase II Telophase II You now have 4 (four) haploid cells have been formed from 1 (one) diploid cell Each haploid cell contains one chromosome from each homologous pair Become gametes Meiosis Animation Meiosis Animation

Meiosis II