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Slide 1

Bio 1309 DNA as the The Ways of Change

Slide 2

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

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Slide 3

DNA - Structure

Always same width!

Slide 4

DNA – Reproduction vs Proteins

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Slide 5

DNA & Cell Replication

• Cell makes a copy to transmit information from one generation to the next

• DNA Replication

Slide 6

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

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Slide 7

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= AdenineT= ThymineG= GuanineC= Cytosine

Slide 8

DNA – Nucleotide Bases

• DNA is made from the same four nucleotides for all organisms

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Slide 9

DNA – Order Differences

• Nucleotides - like beads on a string

• Differences in order results in intra and inter species differences.

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Slide 11

DNA - chromosome• DNA is organized as a chromosome w/ 2 strands

• Each species - characteristic number of chromosomes – 46 for example!

Slide 12

DNA & Genes • chromosome

subdivided into functional regions called genes

• Each chromosome has thousands of genes

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Slide 13

DNA Gene Protein

• Each gene determines the structure of one or more proteins

• Each protein performs a specific function in the cell

Slide 14

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

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Slide 15

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 nofreckles = no freckles

Slide 16

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

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Slide 17

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?)

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Asexual Reproduction

• genetic variation for asexual reproduction, only from mutation

• Offspring are copies, or clones, of their parents

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Slide 19

Sexual Reproduction

• Sexual reproduction = new genetic combinations so offspring are geneticallydifferent from their parents

• Each provides ½ the genetic information

Slide 20

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

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Slide 21

Meiosisn

• Meiosis starts in diploid cells in sex organs

– Ovaries in females

– Testes in males

• Diploid cells have two full sets of chromosomes

Slide 22

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:

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Slide 23

Haploid Cells

• new cells with only one set of chromosomes are called haploid cells

– Eggs in females

– Sperm in males

Slide 24

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

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Slide 25

Sources of Genetic Variation

• Natural selection only occurs if genetic variation among the individuals of a population

Slide 26

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

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Slide 27

Where does Genetic Variation come from?

• Genetic variation stems from two processes:

– Changes in the DNA (mutations)

– Gene shuffling (recombination) during sexual reproduction

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What is a Mutation?

• random and accidental, permanent changes in DNA

• Some are very small--only one unit of DNA (one nucleotide) is miscopied

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Slide 29

Small Mutations--

• change may seem minor, but human diseases such as cystic fibrosis and sickle cell anemia caused by this kind of mutation

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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

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Slide 31

Mutations- Explained

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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

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Slide 33

Recombination - heh

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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!)

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Slide 35

Phenotypes?

• Alleles physical manifestation, for example - freckles or not?

• Phenotypes were envisioned by Darwin when he used the term trait

Slide 36

Genotype?• actual alleles inherited from

parents

• For each gene, inherit one allele from mother and one from father

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Slide 37

Example- Blood Typing

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Studying Genetic VariationAllele 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

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Slide 39

Studying Genetic VariationAllele Frequencies

• Let’s look at flower color

• There are four possible colors (alleles) of a flower:

– Red

– White

– Blue

– Yellow

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Studying Genetic VariationAllele Frequencies

• In order to determine how common an allele is in a population, scientists measure allele frequency

– The word frequencymeans the same as proportion

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Slide 41

Studying Genetic VariationAllele 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

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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 sexually-reproducing population that is not evolving

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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 44

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

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Slide 45

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 46

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

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Slide 47

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 48

Example: Genetic Drift Simulation

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Slide 49

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 50

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”

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Slide 51

Evolutionary Fitness

• Focuses on genotypes within a population –not on individuals and their offspring

Slide 52

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

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Slide 53

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 54

Studying Genetic VariationEvolutionary Fitness

• Natural selection is not survival of the strongest

• It is the survival and reproduction of those individuals best suited to the existing environment

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Slide 55

Studying Genetic VariationEvolutionary 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 56

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

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Slide 57

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 58

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

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Slide 59

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 60

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!

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Slide 61

Studying Genetic VariationSexual 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.