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Bio 1309 DNA as the The Ways of Change Slide 1 DNA – What is it? • DeoxyriboNucleic Acid, • Large, complex molecule in most living cells (not in RBC) • Double Helix • Stores “Data” about you • Involved in new cell and organism reproduction and protein creation Slide 2 1 DNA - Structure Always same width! Slide 3 DNA – Reproduction vs Proteins Slide 4 2 DNA & Cell Replication • Cell makes a copy to transmit information from one generation to the next • DNA Replication Slide 5 DNA & Cell Replication • the cell divides and each new cell get a copy of the DNA • new "daughter" cells cells contain same genetic info as the parent cell Slide 6 3 DNA Construction • DNA – made of small subunits called nucleotides • Four types of nucleotides, each contains a different base= A, T, G, C • A always pairs with T • G always pairs with C Bases: A= Adenine T= Thymine G= Guanine C= Cytosine Slide 7 DNA – Nucleotide Bases • DNA is made from the same four nucleotides for all organisms Slide 8 4 DNA – Order Differences • Nucleotides - like beads on a string • Differences in order results in intra and inter species differences. Slide 9 Slide 10 5 DNA - chromosome • • DNA is organized as a chromosome w/ 2 strands Each species - characteristic number of chromosomes – 46 for example! Slide 11 DNA & Genes • chromosome subdivided into functional regions called genes • Each chromosome has thousands of genes Slide 12 6 DNA Gene Protein • Each gene determines the structure of one or more proteins • Each protein performs a specific function in the cell Slide 13 DNA & Variations • For any gene, there can be variations • Variations in a gene are called alleles • Example: freckles - an allele that codes for freckles and an allele that codes for no freckles Slide 14 7 DNA& How It Works in Eukaryotic Cells • one allele from father and another from mother • one or two alleles for freckles = will have freckles • two alleles for no freckles = no freckles Slide 15 DNA is Universal • Important facts about evolutionary history of life on the earth: – all living things have same basic DNA structure – All organisms use DNA the same way Slide 16 8 Reproduction • Parents pass a copy of their DNA to each offspring • In asexual reproduction, offspring from single parent = offspring has identical copy of DNA so genetically identical to their parents (remember Bonnie Bassler?) Slide 17 Asexual Reproduction • genetic variation for asexual reproduction, only from mutation • Offspring are copies, or clones, of their parents Slide 18 9 Sexual Reproduction • Sexual reproduction = new genetic combinations so offspring are genetically different from their parents • Each provides ½ the genetic information Slide 19 Sexual Reproduction & Meiosis • Sexual reproduction - more complex than asexual • To prevent too many chromosomes in each new generation, organisms make special reproductive cells with 1/2 the regular chromosome number, yet containing one full set of genes • Called meiosis (a reduction division) • Ploidy (haploid or diploid) refers to the chromosome number state Slide 20 10 Meiosisn • Meiosis starts in diploid cells in sex organs – Ovaries in females – Testes in males • Diploid cells have two full sets of chromosomes Slide 21 Meiosis = shuffling • • Meiosis separates the chromosomes into two complete but separate sets original chromosomes that came from each parent are shuffled and dealt out randomly to the new cells being formed – known as recombination or shuffling – for example: Slide 22 11 Haploid Cells • new cells with only one set of chromosomes are called haploid cells – Eggs in females – Sperm in males Slide 23 Fertilization • haploid egg fuses with haploid sperms to produce a new individual called a zygote • zygote has two full sets of chromosomes, it is diploid Slide 24 12 Sources of Genetic Variation • Natural selection only occurs if genetic variation among the individuals of a population Slide 25 Evolution and Genetic Variation • Evolution happens within a group of same species individuals • natural selection favors traits that enable individuals to best survive and reproduce in a given environment – for example? Thanks to Sheri Amsel Slide 26 13 Where does Genetic Variation come from? • Genetic variation stems from two processes: – Changes in the DNA (mutations) – Gene shuffling (recombination) during sexual reproduction Slide 27 What is a Mutation? • random and accidental, permanent changes in DNA • Some are very small-only one unit of DNA (one nucleotide) is miscopied Slide 28 14 Small Mutations-• change may seem minor, but human diseases such as cystic fibrosis and sickle cell anemia caused by this kind of mutation Slide 29 Larger Mutations • larger errors in copying that can involve large parts of the DNA being either lost, duplicated, or put in the wrong place where it does not work correctly Spontaneous – random change Induced – chemical, radiation. Point – change a single base Nonsense – change a normal codon into a stop codon – Back-mutation – mutation is reversed – Frameshift – reading frame of the mRNA changes – – – – Slide 30 15 Mutations- Explained Slide 31 Environmental Sources of Mutations Direct Damage Caused by Exposure of Cells to Radiation or Harmful Chemicals • Exposure to ultraviolet light from the sun, radioactivity or certain chemicals can change the DNA molecules so that they don't work right Slide 32 16 Recombination - heh Slide 33 Recombination Possibilities • • • • Humans = two sets of 23 chromosomes > eight million different chromosome combinations each for eggs and sperm That means = at least 64 trillion possible combinations of egg and sperm, or 64 trillion possible different genetic combinations for offspring from two human individuals 64,000,000,000,000 ! (twelve zeros!) Slide 34 17 Phenotypes? • Alleles physical manifestation, for example - freckles or not? • Phenotypes were envisioned by Darwin when he used the term trait Slide 35 Genotype? • actual alleles inherited from parents • For each gene, inherit one allele from mother and one from father Slide 36 18 Example- Blood Typing Slide 37 Studying Genetic Variation Allele Frequencies • An individual has two alleles for a gene – One from each parent • Populations, on the other hand, can have many different alleles for the same gene Slide 38 19 Studying Genetic Variation Allele Frequencies • Let’s look at flower color • There are four possible colors (alleles) of a flower: – – – – Red White Blue Yellow Slide 39 Studying Genetic Variation Allele Frequencies • In order to determine how common an allele is in a population, scientists measure allele frequency – The word frequency means the same as proportion Slide 40 20 Studying Genetic Variation Allele Frequencies • A percent is a type of proportion - tells how many out of 100 • Population geneticists = scientists studying how allele frequencies in populations change over time Slide 41 Allele Frequency Oddities • • • Sometimes allele frequencies change in unexpected ways from one generation to the next In 1908, scientists developed a mathematical model to help explain how allele frequencies can change over time - called the Hardy Weinberg Equilibrium model Hardy Weinberg Equilibrium model shows what happens in a sexuallyreproducing population that is not evolving Slide 42 21 Hardy Weinberg Equilibrium model & unchanging frequencies • For allele frequencies staying the same from generation to generation - set up a model of a population that does not change to identify factors that could cause changes in allele frequencies • These factors are the underlying assumptions of the model • What are these assumptions? Slide 43 Hardy Weinberg Equilibrium model assumptions • Hardy Weinberg assumes: – No mutations are occurring – Large population size – No movement of individuals and their alleles into or out of the population – Random mating, where each individual has an equal chance of mating with another individual – No natural selection Slide 44 22 Hardy Weinberg Equilibrium model… • model serves as foundation for understanding evolution • By varying these factors, scientists can figure out the various mechanisms of evolution Slide 45 Genetic Drift ? • the change in the frequency of an allele in a population • alleles in the offspring are a sample of those in the parents. • chance plays a role in determining whether a given individual survives and reproduces Slide 46 23 Genetic Drift & allele frequency • population allele frequency= fraction copies of one gene that share a particular form • genetic drift may cause alleles to disappear completely from a population and reduces genetic variation Slide 47 Example: Genetic Drift Simulation Slide 48 24 Evolutionary Fitness • In natural selection, the environment acts on populations to “select” the individuals with the traits (phenotypes) that best help them to survive and reproduce • Limiting the gene pool? Slide 49 Evolutionary Fitness • Natural selection only occurs when individuals in a population differ in reproductive success, or evolutionary fitness • Zimmer describes evolutionary fitness as “the rate at which a genotype increases in a population” Slide 50 25 Evolutionary Fitness • Focuses on genotypes within a population – not on individuals and their offspring Slide 51 Evolutionary Fitness & Offspring • This means that when comparing members of the same population or species to each other, the ones with the most offspring have the highest evolutionary fitness Tribbles … thanks to Star Trek Slide 52 26 Evolutionary Fitness – so what? • an organism that is very physically "fit" may have a very low evolutionary fitness if it does not reproduce • It is true that individuals who are more physically fit may have a higher chance of surviving in a difficult environment, and that may also give them higher evolutionary fitness. • But not necessarily!!! Slide 53 Studying Genetic Variation Evolutionary Fitness • Natural selection is not survival of the strongest • It is the survival and reproduction of those individuals best suited to the existing environment Slide 54 27 Studying Genetic Variation Evolutionary Fitness • Even if the biggest and strongest members of a population survive, they don't always reproduce as well as smaller and weaker members Slide 55 Sexual Selection • natural selection acts on traits that contribute to an organism's mating success rather than survival • Example: Male cardinals have bright red feathers and females have duller orange-brown feathers Slide 56 28 Sexual Selection – brighter is better? • Male bright color attracts females, so the brightest males have more offspring • This means= males with bright red feathers have higher evolutionary fitness than males with less colorful feathers Slide 57 Sexual Selection - compromise • Evolution is often a compromise between different kinds of selection • A trait that gives a mating advantage to the male increases his evolutionary fitness because he will mate with more females Slide 58 29 Sexual Selection – might be dangerous! • the same trait (red feathers?) might be a serious liability when evading predators • A small reading that helps: http://www.eebweb.arizona.edu/Animal_behavior/chase/chaseaway2a.htm Slide 59 Sexual Selection – mating vs survival • the fitness advantage of the trait (e.g. mating with more females) must offset the survival disadvantage of things like shortened lifespan Yeah. Yeah. Whatever! Slide 60 30 Studying Genetic Variation Sexual Selection http://www.pbs.org/wgbh/evolution/library/01/ 6/quicktime/l_016_09_56.html This link might not work…. Looking for a new one. Slide 61 31