Understanding Sources of Variation. Part 1: Variation Overview (

Transcription

1 Name: Per. Date: Understanding Sources of Variation Part 1: Variation Overview ( After watching the variation presentation, answer the following questions: Variation 1. In your own words, what do you think vary means? 2. In a strictly scientific sense, what do you think variation means? Mutations 3. In your own words, define mutation based on the information given in the slides. 4. Which of the follow are mutations? Circle all that apply. a) duplicating DNA chunks d) inserting a letter b) deleting DNA chunks e) skipping a letter c) miscopying a letter 5. Which of the following are true? a) mutations are random b) mutations are common c) mutations are planned 6. Explain your answer choices: Recombination 7. In which process does recombination occur: a) Mitosis b) Meiosis 8. Explain your answer above: 9. Why are mutation and recombination considered to be random? 1

2 Part 2: Mutation Whie you are reading, highlight or underline vocabulary. Highlight or underline the main ideas from each section. Mutation Reading A change in the sequence of bases in DNA or RNA is called a mutation. Does the word mutation make you think of science fiction and bug-eyed monsters? Think again. Everyone has mutations. In fact, most people have dozens or even hundreds of mutations in their DNA. Mutations are essential for evolution to occur. They are the ultimate source of all new genetic material - new alleles - in a species. Although most mutations have no effect on the organisms in which they occur, some mutations are beneficial. Even harmful mutations rarely cause drastic changes in organisms. Types of Mutations There are a variety of types of mutations. Two major categories of mutations are germline mutations and somatic mutations. Germline mutations occur in gametes. These mutations are especially significant because they can be transmitted to offspring (future generations) and every cell in the offspring will have the mutation. Somatic mutations occur in other (non-sex) cells of the body. These mutations may have little effect on the organism because they are confined to just one cell and its daughter cells. Somatic mutations cannot be passed on to offspring. Mutations also differ in the way that the genetic material is changed. Mutations may change the structure of a chromosome or just change a single base. Substitution Mutations A point mutation is a change in a single base in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as shown in the table below. The effects of point mutations depend on how they change the genetic code. Type Description Example Effect Silent mutated codon codes for the same amino acid CAA (glutamine) CAG (glutamine) none Missense mutated codon codes for a different amino acid CAA (glutamine) CCA (proline) variable Nonsense mutated codon is a premature stop codon CAA (glutamine) UAA (stop) usually serious 2

3 Frameshift Mutations A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. This means that the codons are read differently because a letter is inserted or deleted in the sequence of DNA nucleotides. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA: AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine Now, assume an insertion occurs in this sequence. Let s say an A nucleotide is inserted after the start codon AUG: AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine Even though the rest of the sequence is unchanged, this insertion changes the reading frame and thus all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mrna are read. This can have a drastic effect on the protein product. What Causes Mutations? Mutations have many possible causes. Some mutations seem to happen spontaneously without any outside influence. They can occur when mistakes are made during DNA replication or transcription. Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is known as a mutagen. Examples of mutagens are pictured in the figure here. 3

4 Mutation Computer Activity Go to and click on Run Model. This computer simulation shows what could happen to proteins if DNA is mutated during DNA replication. To review what happens during protein synthesis, click on Transcribe. 1. What is happening during this step? Now click on Translate. 2. What is happening during this step? Original DNA Protein Substitution Insertion Deletion Above is a picture of the protein produced with the original DNA strand. Now mutate the DNA in the following ways and draw what happens to the resulting protein in the table above. Between each mutation, click on Stop to draw your picture of the protein, then Reset to return to the beginning. Substitution: Substitute a nucleotide on your strand by clicking on any of the existing nucleotides and choosing from the pop-up menu. Insertion: Insert a nucleotide on your strand by clicking on any of the existing nucleotides and choosing from the pop-up menu. Deletion: Delete a nucleotide on your strand by clicking on any of the existing nucleotides and choosing from the pop-up menu. 3. Which type of mutation will affect future offspring: germline mutations or somatic mutations? 4. When a point mutation (change in a single nucleotide) happens in DNA, what could happen to the resulting amino acid sequence and protein? 5. Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of existing alleles. Which category does mutation fall into, and why? 4

5 Mutation Practice Complete the boxes below. Classify each as either Deletion, Insertion, or Substitution AND as either frameshift, missense, silent or nonsense (hint: deletion or insertion will always be frameshift). Original DNA Sequence: mrna Sequence: Amino Acid Sequence: T A C A C C T T G G C G A C G A C T Mutated DNA Sequence #1: T A C A T C T T G G C G A C G A C T What is the mrna sequence (Circle the change) What will be the amino acid sequence? Will there likely be effects? What kind of mutation is this? Mutated DNA Sequence #2: T A C G A C C T T G G C G A C G A C T What is the mrna sequence (Circle the change) What will be the amino acid sequence? Will there likely be effects? What kind of mutation is this? Mutated DNA Sequence #3: T A C A C C T T A G C G A C G A C T What is the mrna sequence (Circle the change) What will be the amino acid sequence? Will there likely be effects? What kind of mutation is this? Mutated DNA Sequence #4: T A C A C C T T G G C G A C T A C T What is the mrna sequence (Circle the change) What will be the amino acid sequence? Will there likely be effects? What kind of mutation is this? Mutated DNA Sequence #5: T A C A C C T T G G G A C G A C T What s the mrna sequence (Circle the change) What will be the amino acid sequence? Will there likely be effects? What kind of mutation is this? 6. Which type of mutation is responsible for new variations of a trait? 7. Which type of mutation results in abnormal amino acid sequence? 8. Which type of mutation stops the translation of the mrna? 5

6 Part 3: Recombination (Crossing Over and Independent Assortment) Crossing-over Reading Introduction Recall that chromosomes come in pairs. Each chromosome pair has the same set of genes, but those genes may be different alleles. There can be many genes on a single chromosome. Pairs of chromosomes are called homologous chromosomes. This is a picture of a human karyotype, which is all the chromosomes present in a single cell. Humans have 23 homologous chromosomes, or a total of 46 chromosomes. For sexual reproduction to happen, each parent must make gametes (eggs or sperm) that contain half the number of chromosome90s. That way, when the gametes come together during fertilization, the resulting baby will have the same number of chromosomes as the parent. The formation of gametes is called meiosis. Meiosis is a type of cell division in which the number of chromosomes in the daughter cells is reduced by half. It occurs only in certain special cells of sexually reproducing organisms. Two cell divisions occur during meiosis, and a total of four daughter cells are produced. Those daughter cells become eggs (in females) or sperm (in males). Genetic recombination can happen during meiosis because of the way that chromosomes line up and then separate into daughter cells. There are two ways genetic recombination occurs, crossing-over and independent assortment. Crossing Over Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome. Crossing-over happens during meiosis when homologous chromosomes line up in pairs before being separated into different gametes. The chromosomes come very close to each other and swap segments of DNA. That is, the material breaks off and reattaches at the same position on the homologous chromosome. This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes. Crossing-over occurs when homologous chromosomes pair up during meiosis. The chromosomes can exchange pieces of DNA or whole genes. 6

7 Crossing-over Practice Go to one or both of the following websites to give you a detailed visual model of crossing over during meiosis: In the circles below, which represent the 4 gametes that will be produced from meiosis, draw (including gene labels) the chromosomes that will end up in each gamete. Over Homologous Chromsomes Not Crossed Over Homologous Chromsomes Crossed- 9. How does crossing-over increase variation in gametes? 10. Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of existing alleles. Which category does crossing-over fall into, and why? 7

8 Independent Assortment Reading When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells. Before they separate into daughter cells, homologous chromosomes line up in the middle of the cell (similar to metaphase in mitosis). But the order in which they line up is completely random. This is called independent assortment. It results in gametes that have unique combinations of chromosomes. Notice how the two different line-ups of chromosomes could result in different gametes. This is called independent assortment. In humans, there are over 8 million ways in which the chromosomes can line up during metaphase I of meiosis. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Genetic recombination can also occur during fertilization. In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring. This is known as random fertilization. Because of these three genetic recombination processes, more possibilities for genetic variation exist between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. In humans, over 8 million (2 23 ) chromosome combinations exist in the production of gametes in both the male and female. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing-over. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations! 8

9 Independent Assortment Practice Go to the following website to give you a detailed visual model of crossing over during meiosis: In the blank cell in the Metaphase I section, diagram another way the homologous chromsomes can line up in Metaphase I that will lead to genetic variation In gametes. In the four cells beneath each metaphase I cell that represent gametes after meiosis, draw (including gene labels) the chromosomes that will end up in each gamete. 1. How does independent assortment increase variation in gametes? 2. Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of existing alleles. Which category does independent assortment fall into, and why? 3. How is independent assortment represented in: a. Punnet Squares? b. Ugly Baby? 9

10 Extension: Mutation Outcomes Go to This website shows some examples of how mutations can lead to new or different traits in organisms. Choose 3 of the following to describe using the diagrams below: peas, cattle, cats, curly hair, or red hair. Read each tab carefully! What does the protein from this DNA do? What trait do we see as a result? Original DNA Mutated DNA: What does the protein from this DNA do? What trait do we see as a result? Original DNA Mutated DNA: What does the protein from this DNA do? What trait do we see as a result? Original DNA Mutated DNA: 10