What determines a cell's shape? function? DNA!
Where would you expect Humans to land? What about a plant, like an apple? How many chromosomes do you think cats and dogs have?
Modified by Liz LaRosa 2011
...and this is WITHOUT crossing over, and ONLY TWO alleles! Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine.
A. Chromosome number of various organisms: Man = 46 , Penicillium = 4, Mosquito = 6, Fruit fly = 8, Garden pea = 14, Adder's tongue fern = 1262,Frog = 26, Vampire bat 28, Chimpanzee = 48, Duck = 80.
Meiosis How Reproductive Cells Divide & Pass on DNA
Why do Cells Divide?• To replace old/worn out or damaged cells • skin, red blood cells, intestines• To fight disease • white blood cells identical copies• To reproduce • gametes (sex cells) • male sperm cell • female ova cell variation
DNA: Secret Code of the Cell Traits are characteristics inherited from parents • i.e.: leaf shape, eye/hair color, tree height, etc. Instructions that determine these traits are found in sections of DNA called genes • most cells have two genes that control each trait • one gene comes from the mother (maternal) • one gene comes from the father (paternal) This gene might give instructions for nose width in a human.
Mama Genes & Papa Genes Homologous chromosomes • every body (somatic) cell has a set of two similar chromosomes • one paternal, one maternal • genes of homologous chromosomes code for the same traits • genes for similar traits are usually found at the same location on the two different homologous chromosomes
Homologues Same length Same centromere position Carry genes that control the same inherited traits • i.e.: both carry the gene for hair color - one is for brown hair and one is for red hair One is from mother, one is from father = a matching set NOT identical
Haploid vs. Diploid Cells Cells with only one set of chromosomes (one gene for each trait) are haploid • symbolized by N • all gametes (sex cells) are haploid • haploid cells are the result of meiosis Cells having homologous chromosomes (two genes for each trait) are diploid • symbolized by 2N • all somatic cells (body cells) are diploid • diploid cells are created through mitosis
Sexual Reproduction The nuclei of haploid gametes combine in fertilization, resulting in a diploid zygote haploid gametesdiploidzygote
Does Chromosome # determine complexity?Comparative Number of Chromosomes Organism Body Cell (2n) Gamete (n)Fruit fly 8 4Garden pea 14 7Flatworm 16 8Corn & Green algae 20 10Frog 26 13Apple 34 17Cat 38 19Monkey 42 21Human 46 23Potato & Chimpanzee 48 24Dog 78 39Adders tongue fern 1260 630
Meiosis Interphase • Chromosomes replicate • Chromatin condenses Prophase I Interphase • Replicated homologous chromosomes pair up (synapsis) • Chromosomes each consist of two identical sister chromatids • Nuclear membrane breaks down • Spindles form • Crossing over begins Prophase I
Meiosis Crossing over • chromosomal segments are exchanged between pairs of homologous chromosomes • produces variation in genetic information passed to offspring Prophase I
Meiosis Metaphase I • Chromosome kinetochores attach to spindle fibers. • Homologous chromosome pairs line up at the Metaphase I equatorial plate.
Meiosis Anaphase I • Homologues separate and move to opposite poles of the cell • Sister chromatids Anaphase I remain attached!
Meiosis Telophase I • Spindles break down • Chromosomes uncoil • Two nuclei form Telophase I • Cytoplasm divides (cytokinesis) Cells have only half as many chromosomes (they are now haploid) • homologous pairs have separated into two cells • chromosomes have already replicated
Meiosis Prophase II • A second set of phases begins in both haploid daughter cells • Spindle apparatus forms Prophase II • Chromosomes condense • Nuclear membrane breaks down No DNA replication occurs between Meiosis I and Meiosis II (interphase is skipped)
Meiosis Metaphase II • A haploid number of chromosomes line up at the equatorial plate • Spindle fibers attach to Metaphase II kinetochores
Meiosis Anaphase II • Sister chromatids are pulled apart at the centromere • Spindle fibers pull Anaphase II chromatids toward the opposite poles of the cell Telophase II • chromosomes reach the poles • nuclear membranes and nuclei reform Telophase II
Meiosis Cytokinesis • Plasma membranes and cytoplasm of cells divide • Four gametes are the result, each with n number of chromosomes Cytokinesis
Meiosis - summary Germs cells (reproductive cells produced in the testes & ovaries in humans) divide by meiosis In meiosis, a cell divides twice to produce four daughter cells Cells arising from meiosis are haploid, which means they half the number of chromosomes Provides genetic variability, because daughter cells are not identical (as in mitosis)