Chapter 13 Basic unit the gene Heredity transmission of traits from one generation to the next Locus on a chromosome location Sequence of DNA that codes for specific enzymes and other proteins that produce an organism s Inherited traits Passed to offspring during MEIOSIS 1 Asexual Reproduction a Parent reproduces exact copy of itself clone i Examples Bacteria binary fission singled celled eukaryotes vegetative reproduction regeneration budding b Advantages i Offspring carries 100 of parents genes ii Organism does not need to find a mate iii Can reproduce numerous offspring quickly c Disadvantages i NO VARIATION all or none all offspring are alike and may not be suited for the environment ii Mutations may accumulate in each successive generation iii More likely to catch a disease because no variation 2 Sexual Reproduction a Haploid 1n gametes from each parent fuse sygamy to form a new diploid 2n zygote i Example found in most diploid organisms humans dogs cats plants ii Haploid gametes produced by MEIOSIS iii Meiosis produces variation two parents give rise to offspring that have unique combinations of genes inherited from the parents b Advantages variation i All offspring are different new combinations of genes even if the environments changes some offspring may be suited to it ii May prevent the accumulation of harmful mutations in the genome by chance some offspring won t have them iii Allows offspring to be more resistant to disease c Disadvantages i Need to find a mate ii Each of your offspring only carries 50 of your genes 3 Mitosis Compared to Meiosis a Mitosis identical i DNA is duplicated ii iii RESULT IF PARENT CELL IS DIPLOID Identical sets sister chromatids separate into 2 cells 1 1 diploid cell 2n becomes 2 identical diploid cells b Meiosis reductional cannot occur in haploid i 1st division MEIOSIS 1 1 Homologous Chromosomes separate this reduces the total number of chromosomes by half from 2n to n ii 2nd division MEIOSIS 2 1 Identical sister chromatids separate a RESULT 1 diploid becomes 4 un identical haploid cells a Diploid phase dominates life cycle of humans and other animals only haploid cells are gametes and haploid cells do not divide b Haploid phase dominates fungi and some protists diploid zygote does not divide 4 Types of lifecycles 5 Alternation of generations a Both haploid and diploid cells divide by mitosis i Plants and Algae ii haploid generation gametophyte produces gametes iii Diploid generation sporophyte produces spores 6 Meiosis 1 reduction from diploid to haploid Interphase before meiosis the chromosomes are replicated to form sister chromatids a b single chromosome is replicated i Prophase 1 1 Synapsis chromosomes condense and homologous chromosomes pair up to form tetrads set of four 2 Spindle forms from each centrosome connects to kinetochores on 3 Crossing over chromosomes tetrads start to move shaped regions called chiasmata sister chromatids of homologous chromosomes cross X 4 Homologous portions of two non sister chromatids trade places a Occurs 2 3 times per chromosome pair in humans 1 Tetrads arranged at the metaphase plate 2 Homologue on each side of the metaphase plate 1 Homologous chromosomes separate and are pulled toward opposite ii Metaphase 1 iii Anaphase 1 poles iv Telophase 1 1 Homologous chromosomes continue moving until they reach the poles 2 Cytokinesis occurs simultaneously c Meiosis 2 separation of sister chromatids very similar to Mitosis only starting with half the number of chromosomes i Prophase 2 1 Spindle forms and attaches to kinetochores of each sister chromatid 2 Kinetochore of one pole attaches to one sister chromatid the other pole to the other sister chromatid 7 Sexual Reproduction produces variation each other ii Metaphase 2 iii Anaphase 2 iv Telophase 2 1 Sister chromatids are arranged on either side of the metaphase plate 1 Centromeres of sister chromatids separate 2 Chromatids move toward opposite poles of the cell 1 Sister chromatids arrive at opposite poles of the cell 2 Nuclear envelop beings to form 3 Cytokinesis occurs 4 4 haploid daughter cells are produced unlike the parent cell and unlike a Meiosis allows cells to combine desirable traits and increase genetic diversity without increasing chromosome number b Diversity is created in two ways during meiosis i Crossing over shuffles genes on homologous chromosomes creates new ii combinations Independent assortment of chromosomes makes sure that no two gametes are ever identical c INDEPENDENT ASSORTMENT pairs at metaphase 1 i Each homologous pair of chromosomes is positioned independently of the other ii Chance of a gamete getting either member of homologous pair is 50 50 iii Number of possible chromosome combinations is 2n n is the haploid number of the organism iv Humans n 23 v 2 23 about 8 million possible combinations 1 Crossing over produces recombinant chromosomes which combine d Siblings are different share approximately half their genes but genes inherited from each parent i Independent assortment 1 Egg is 1 of 8 million possible chromosome combinations 2 Successful sperm is 1 of 8 million different possibilities ii Random Fertilization 1 Egg and successful sperm 2 The resulting zygote is composed of 1 in 70 trillion possible combinations of chromosomes iii Crossing over and recombination provides even more variation to this