Exam 1 Mitosis Results in two identical daughter cells Direct duplication of genetic material No change in chromosome number Two derivative nuclei exactly like the original nucleus Meiosis Results in four different daughter cells Reshuffle of genetic material Reduction in chromosome number Mitosis division of nucleus Cytokinesis division of the cytoplasm The cell spends 90 of its time in interphase Mitosis occupies less than 10 of the time required to complete the cell cycle G1 Chromosomes unfold organelles multiply membranes are synthesized S synthesis Chromatin is replicated G2 preparation for cell division At the end of replication twice as much DNA in the cell but number of potential chromosomes is the same Centromere Specialized region of the chromosome where the sister chromatids are attached There is only one centromere per chromosome so to determine chromosome number count the centromeres Cytokinesis in plant and animal cells Phragmoplast the cell plate in plants division Cleavage furrow in animals pinching off Ploidy the number of complete sets of unique chromosomes Somatic cells of most animals and many plants are diploid 2N They have two complete sets of unique chromosomes one from the father and one from the mother Chromosomes that carry the same genes are homologs Somatic cells are not gametes or their precursor cells Gametes are haploid N one complete set of chromosomes Homologous chromosomes have the same morphology size and banding patterns and the same genes but the genes will have different alleles Alleles are different forms of the same gene One chromosome of the homologous chromosome pair was contributed by the mother s gamete and one was from the father s Somatic human cells have a total of 46 chromosomes That is there are 23 different pairs of homologous chromosomes N 23 Most of the pairs are autosomes chromosomes that don t determine the sex of the organism One pair is the sex chromosomes that determine the sex of the organism In humans most other mammals some insects and some plants XX result in a female and XY chromosomes result in a male The life cycle the generation to generation sequence of stages in the reproductive history of an organism Meiosis results in a haploid number of chromosomes in each gamete In animals meiosis only occurs in gonads the products of meiosis are gametes Sex Meiosis Crossing over is the reciprocal exchange of genetic material between non sister chromatids of paired homologous chromosomes Novel combinations of alleles result Crossing over combined with independent assortment can lead to novel chromosome combinations or recombinant genes Homologous chromosome pairs line up randomly on the metaphase plate in metaphase 1 Then they separate in Anaphase 1 The random lining up and separation is a process called independent assortment Asexual reproduction If a harmful mutation arises in a diploid mother all the offspring will have at least one copy of the mutation Sexual reproduction If the mother is diploid and she has one allele that doesn t have the mutation and if she mates with a normal male there is a 50 chance that her offspring will not have the mutation If the harmful mutation lowers the individual s ability to grow and reproduce ultimately the normal individuals will have more offspring Natural selection against harmful mutations is purifying selection The genetic variation supplied by crossing over and independent assortment the recombinations allows increased genetic capacity for adapting to changing environments predators competitors pathogens climates etc This is accomplished through selection for novel gene combinations that have higher reproductive output in the new environment MENDELIAN GENETICS Inheritance how are genes passed on from one generation to the next Expression how are these patterns of inheritance expressed in terms of an organisms appearance Genotype the genetic make up of an organism Phenotype the physical physiological traits of an organism that are determined by its genes Inheritance is particulate Parents pass on discrete units to children genes Genes are stretches of DNA that code for proteins A stretch of DNA that codes for a certain protein is called a gene Genes determine which morphological traits appear in the offspring The collection of morphological traits in an organism is the phenotype Therefore your genes impact your phenotype DNA is organized into chromosomes Every gene has a particular location on a chromosome Alleles are different versionsof a gene which produce different phenotypes Gregor Mendel 1822 1884 Basic laws of inheritance pea plants Pisum sativum A contemporary but not an acquaintance of Charles Darwin The importance of his work was not understood until after both men had died His work has become central to our understanding of genetics Mendel s Model smooth or wrinkled 1 Alternative versions of genes account for variations in inherited characters e g alleles for 2 For each character an organism inherits two alleles one from each parent 3 If two alleles differ the dominant allele determines the organism s appearance and the recessive allele has no noticeable effect 4 Law of segregation an organism s two alleles for a character separate during gamete formation and end up in different gametes each gamete carries only one allele for a trait because the alleles have segregated during meiosis When Mendel crossed the F one generation he was performing a cross between two parents known to each carry BOTH alleles for the trait of seed shape wrinkled and smooth Monohybrid cross a hybridization between two options of one trait Locus means location of the gene on the chromosome The plural form is loci Homozygous both alleles for a particular gene are the same Ex genotype RR Homozygous dominant Capital letters dominant alleles Ex genotype rr Homozygous recessive Lowercase letters recessive alleles Heterozygous the two alleles for a different gene are different Ex genotype Rr Heterozygous Punnett Square a simple chart used to predict offspring genotypes and phenotypes NOT usually in the same ratio Dihybrid Cross a hybridization between two options of two traits Principle of Independent Assortment Alleles of different genes are transmitted to the next generation independently of each other Independent assortment results from two genes occurring on different chromosomes so they assort independently of another gene Recombinant genes and phenotypes