Agenda 9/26 Rock Pocket Mice Activity Mutations and Genetic Change Lecture Homework: 1. Mutations Video and Notes 2. Dogs decoded viewing guide (on website) 3. Chapter 23 reading and notes Turn in: Video notes FRQ workshop tomorrow morning

Sources of Genetic Variation How does variation in a population or gene pool arise? Mutations, gene duplication and chromosome fusion provide the raw material for evolution. 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. Emphasize the importance of meiosis and sexual reproduction as the driving force of evolution. Meiosis is responsible new phenotypic combinations upon which natural selection can act. Meiosis recombines alleles in new combinations  which results in unique gametes due to the way chromosomes line up on the metaphase plate and crossing over.  Meiosis coupled with fertilization produces offspring with different combinations of alleles.  The genetic complement that the zygote receives will be different from either parent and  is different from any sibling.   Identical twins are genetically identical to one another but the likelihood that two siblings (not identical twins) will be genetically identical is extremely remote 223 x 223.  Tell students that the genetic “shuffling” during meiosis is like getting a new hand when playing cards.

Effect of Sexual Reproduction 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 1/446. Ask students “Why TWO to the 23rd?” First, they should know that humans have 46 chromosomes which can be arranged into 23 pairs with the “last” pair being XX if female and XY if male. Since gametes are haploid, they have only 23 chromosomes. We use 2 because there are 2 chromosomes in a pair, and you have 23 pairs. In order to figure out the probability you raise 2 to the 23rd power, which gives you 8.3 million different possibilities for only the egg or sperm. So, if you take 8.3 million times 8.3 million, you get 64 trillion different possibilities for fertilization, without even considering crossing over. That’s a lot of unique phenotypes upon which natural selection can act! 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 (body) do not affect future generations. Why? Only mutations occurring in gametes (sex cells) affect future generations. Mutations can occur at either the gene or chromosomal level. Examples? Emphasize that mutations lead to genetic variation. These next slides explain each of these mutation types. Students should have prior knowledge of genes, DNA, and mutations from Pre-AP Biology. Mutations may cause a sheep to have a 5th leg. But this is not evolution! 

Review!? DNA- Amino Acid- Protein- Mutation- Chromosome- Define each in 6 words or less DNA- Amino Acid- Protein- Mutation- Chromosome-

Point Mutations: Synonymous vs. Nonsynonymous 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. Students should be aware of DNA, RNA, codons, amino acids and the mechanisms of protein synthesis. If it has been a year or two since their first biology course, you may have to refresh their memory. Also, it’s the synonyms (pun intended) that will confuse students—too bad we can’t decide on a single term to describe a single anomaly!

Point Mutations: Synonymous vs. Nonsynonymous 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: It can result in a protein that does not function as well as the original protein. (This happens most often.) It can result in a protein that functions better than the original protein 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!) Emphasize that intermolecular force interactions between R groups establish the shape of a protein, thus establishing its function.

Gene Duplication 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. Analogy: Evolution is more like editing a book rather than writing a book from scratch. Genes are duplicated and then moved; once duplicated genes are moved, they experience different mutations which can result in different proteins. As you might imagine, there can also be cutting and pasting or copying and pasting errors!

Neutral Mutations 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. This is illustrating that there are some proteins that have been around for a “long time”. That being the case, over time, these proteins experience mutations that do not affect the function of a protein. For example both human and dog hemoglobin found in red blood cells carry oxygen, yet the two proteins vary only slightly in their amino acid sequencing. Cytochrome c is an example of this as well. The average rate of these mutations can be used as an evolutionary clock.

Neutral Mutations 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.

Changes in Cytochrome C Explain that a pseudogene is a gene that has been duplicated but certain mutations have rendered this gene nonfunctional so it is never transcribed or translated. It is part of the genome that is simply conserved. Summarize the significance of this data. What can it tell us? Why is it important?

Cytochrome c Comparison 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? Assign the Amino Acid Sequences and Evolutionary Relationships activity as homework!

Hemoglobin Comparison 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 b chains. 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. Ask the students to speculate as to why soybeans might have a protein similar to hemoglobin. Leghemoglobin removes oxygen that would kill a bacteria living in the root of the soybean. This bacterium “fixes” nitrogen in the roots of the soybean or takes nitrogen from the air and changes it into a usable form that the plant can use.

Frameshift Mutation 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. Interesting example but emphasize that most frameshift mutations do not result in a functional protein.

Chromosomal Rearrangement 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: inversions deletions duplication translocations fusions Chromosomal rearrangement is another source of genetic variation.

Chromosomal Rearrangement 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. Chromosomal rearrangement is another source of genetic variation.