Warm Up: Discuss the role that mutations play in evolution.

9/21/15

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Sources of Genetic Variation

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How does variation in a population or gene pool arise?

1. Mutations, gene duplication and chromosome fusion provide the raw material for evolution.

2. Meiosis and sexual reproduction produce new recombinants of phenotypes upon which natural selection operates.

The wisteria pictured on the right has a mutation causing it to produce white flowers instead of purple flowers.

Effect of Sexual Reproduction

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Sexual reproduction recombines genes in new ways. This results in unique offspring that differ from either parent or sibling. Humans make 223 different kinds of gametes. Fertilization means that the uniqueness of an individual is 223 223. Or the probability that two siblings will be genetically identical (excluding identical twins) is 446.

Sexual reproduction is like shuffling a deck of cards and every time getting a new and unique hand dealt. It is the major driving force of evolution.

Types of Mutations

• Obviously, mutations occurring in somatic cells do not affect future generations. Why?

• Only mutations occurring in gametes affect future generations.

• Mutations can occur at either the gene or chromosomal level. Examples?

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Mutations may cause a sheep to have a 5th leg.

But this is not evolution! 

Review!?

• Define each in 6 words or less– DNA– Amino Acid– Protein– Mutation– Chromosome

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Point Mutations: Synonymous vs. Nonsynonymous

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Point mutations occur when one nucleotide is substituted for another.

The genetic code contains “synonyms” for the coding of amino acids. For example the DNA codons GGA, GGG, GGT, GGC all code for the amino acid proline.

Therefore, as long as the codon has GG in positions 1 & 2, a mutation in position three has no consequence, proline will be coded for regardless.

This sort of mutation is called a synonymous or silent mutation.

Point Mutations: Synonymous vs. Nonsynonymous

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Point mutations that do result in a different amino acid are called a nonsynonymous or missense mutations.

Missense mutations can affect the protein in one of THREE ways:

1. It can result in a protein that does not function as well as the original protein. (This happens most often.)

2. It can result in a protein that functions better than the original protein

3. It can result in a protein that functions like the original protein. This is usually because the R groups are similar. (both polar or both nonpolar, etc.)

Predict what would happen to each mutation over time (think frequency!)

Gene Duplication

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Genes can be duplicated and occasionally the duplication moves a gene from one chromosome to another. Each gene will accumulate different mutations altering the protein that is subsequently synthesized.

Myoglobin is a protein that binds with oxygen in the muscles. This gene has been duplicated and modified many times. It has given rise to the hemoglobin gene.

Neutral Mutations

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Naturally evolving proteins gradually accumulate mutations while continuing to fold into stable structures.

This process of neutral evolution is an important mode of genetic change and forms the basis for the molecular clock.

•Cytochrome c is a small protein found on the mitochondrial membrane.

•Between mammals and reptiles there are 15 different amino acids or mutations.

Neutral Mutations

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• Mammals and reptiles diverged 265 million years ago.

• That means on average cytochrome c mutated every 17 million years.

• In comparing the evolution of other organisms and their cytochrome c one mutation every 17 million years holds true.

Summarize the significance of this data. What can it tell us? Why is it important?

Changes in Cytochrome C

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Cytochrome c Comparison

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• A dash indicates that the amino acid is the same one found at that position in the human molecule.

• All the vertebrate cytochromes (the first four) start with glycine (Gly).

• The Drosophila, wheat, and yeast cytochromes have several amino acids that precede the sequence shown here (indicated by <<<).

• Again, why is looking at this data important? Why would we be studying mutations at this point in the unit?

Molecular homology of cytochrome c (see three-letter code of amino acids) 1 6 10 14 17 18 20

Human Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys Cys Ser Gln Cys His Thr Val Glu Lys Pig - - - - - - - - - - Val Gln - - Ala - - - - - - -

Chicken - - Ile - - - - - - - Val Gln - - - - - - - - - - Dogfish - - - - - - - - Val - Val Gln - - Ala - - - - - - Asn

Drosophila <<< - - - - - - - - Leu Val Gln Arg Ala - - - - - - Ala Wheat <<< - Asn Pro Asp Ala - Ala - - - Lys Thr - - Ala - - - - - Asp Ala Yeast <<< - Ser Ala Lys - - Ala Thr Leu - Lys Thr Arg - Glu Leu - - - - - -

Hemoglobin Comparison

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• This is a comparison between the differences in the amino acid sequence of human hemoglobin and different species.

• The last three species do not have a distinction between a and bchains.

• There is an inverse relationship between the difference in the amino acid sequence and how closely related the organisms are to humans.

• The b chain of hemoglobin has 146 amino acids.

Human beta chain 0

Gorilla 1

Gibbon 2

Rhesus monkey 8

Dog 15

Horse, cow 25

Mouse 27

Gray kangaroo 38

Chicken 45

Frog 67

Lamprey 125

Sea slug (a mollusk) 127

Soybean (leghemoglobin) 124

Frameshift Mutation

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• A frameshift mutation occurs as a result of either an insertion or deletion of a nucleotide.

• This changes the amino acid sequence of the protein from that point forward.

• Almost all frame shift mutations are deleterious.

• Recently, bacteria were found growing in a pool of nylon wastes. (Flavobacterium)

• These bacteria were actually digesting the nylon waste.

• Upon examining the genome of these bacteria, it was found there was a frameshift mutation in their DNA that caused the production of three different enzymes that could digest the nylon.

With your elbow partner summarize what this image means

Chromosomal Rearrangement

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There have also been major changes in chromosome structure that result in changes within populations which can, in turn, result in the emergence of new species.

These include:

a. inversions

b. deletions

c. duplication

d. translocations

e. fusions

Chromosomal Rearrangement

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Compare the karyotype of a human (H) and a chimpanzee (C).

Notice the great apes have 24 pairs of chromosomes compared to 23 pairs of chromosomes in a human.

Why the difference?

Chromosome #2 in the human is the result of a fusion of two chimpanzee chromosomes.

Human Impact on Gene Pools

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It is well documented that humans have had an impact on certain gene pools.

For example, humans have selected for certain desirable traits within the mustard family and cultivated different agricultural products for human consumption.

Describe other examples of humans selecting for traits

Artificial Selection

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When humans manipulate a gene pool it is called artificial selection. There are often consequences involved in such manipulations. For example in agriculture, farmers try to increase crop production, which may lead to many farmers growing only one variety of a particular crop such as corn. This leads to a loss of genetic diversity. If a disease attacks that particular variety of corn, the farmers growing that variety lose their entire crop.

Antibiotics and Artificial Selection

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• When antibiotics are applied to a population of microorganisms to treat an infection, some of the microorganisms may be naturally immune to the drug.

• Why?

• These resistant microorganisms continue to flourish and cause disease.

• The only remaining option a physician has is to treat the infection with a different antibiotic and hope that none of the surviving microorganisms possess a different random mutation that makes them resistant to the second antibiotic as well.

Antibiotics and Artificial Selection

• The increase in antibiotic-resistant bacteria has caused doctors to reduce the number of prescriptions written for antibiotics in general.

• About 70% of pathogenic bacteria are resistant to at least one antibiotic and are called “super bugs” or MDR bacteria. (multidrug resistant)

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MRSA or Methicillin-resistant Staphylococcus aureus

• MDR bacteria do not respond to “first line of defense” antibiotics.

• These types of bacteria are most commonly found in hospitals.

• Skin boils or similar lesions that do not heal often result.

• MDR bacteria can attack internal organs upon gaining entry into the body.

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Reducing or Eliminating Gene Pools

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• Human activities often augment genetic drift and diminish gene flow for many species.

• This reduces genetic variation thereby disrupting adaptive processes both locally and globally within a species.

• This impact is illustrated within populations of collared lizards (Crotaphytus collaris) living in the Missouri Ozarks.

• Forest fire suppression has reduced habitat and disrupted gene flow in this lizard, thereby altering the balance toward drift and away from gene flow. This balance can be restored by managed landscape burns.