Changes in DNA

We have learned that DNA codes for proteins, and that the code lies within the order of the nitrogen bases. (Example: TAC does not mean the same thing as TAG). But something else is at work when it comes to DNA and how proteins are made. They are called Introns and Exons. Intron: a sequence of nitrogen bases that do not code for a protein. Exon:a sequence of nitrogen bases that do code for a protein.

Modeling Introns and Exons Mini Lab You need a length of masking tape and a pen or pencil. Stick the tape to your desk. Write on the tape as shown, filling in the tape from end to end. Appropriately joined Cut the tape to separate the differently colored letters. Keeping the pieces of tape in order, group the tape by color.

You should end up with the tape grouped like this: Apprialyjoed protein The first group of letters DO NOT code for a useful, functional protein. This is like introns in a strand of DNA. The second group of letters DO code for a functional protein. This is like exons in a strand of DNA.

What happens if the exons were missing just one nitrogen base, or a whole bunch of them? What if one nitrogen base in a strand of DNA got copied twice? Mutations are changes in DNA sequence.

Analogy We will be using a sentence as an analogy representing a strand of DNA. Our sentence is: The fat cat ate the wee rat.   If this sentence represents a strand of DNA, what does each word represent? What does each letter represent?

Substitution The fat cat ate the wee rat. The fat cat ate the wet rat.   What changed? What does this represent? How would this type of change affect the protein? Substitution mutation - Nitrogen bases are substituted for others, changing the codons. What changed: The letter “e” changed to “t”. What does this represent: It represents a substitution of a single nucleotide in a codon. How would this type of change affect the protein: It may change one amino acid in the protein, or it may not change anything; depending upon the specific nucleotide change. For example, if AAA becomes AAG, they both still code for the same amino acid (phenylalanine).

Deletion The fat cat ate the wee rat. The fat cat att hew eer at.   What changed? What does this represent? How would this type of change affect the protein? Deletion mutation - Nitrogen bases are left out of the sequence. What changed: The “e” in “ate” was deleted. This caused all the letters to shift and changed all of the remaining codons. This represents a deletion of a nucleotide that causes a change in all the remaining triplets/codons. How would this type of change affect the protein: Because all of the remaining codons are changed, it may shorten the protein or make it ineffective.

Insertion The fat cat ate the wee rat. The fab tca tat eth ewe era t.   What changed? How would this type of change affect the protein? Insertion mutation - Nitrogen bases are added to the sequence. What changed: There was a “b” inserted between the “at” of “fat”. How would this type of change affect the protein: Again, it changes everything else remaining, so it will likely make the protein shorter or ineffective.

Point Mutation Point mutation - change in only ONE nitrogen base. This may or may not change the amino acid coded for.

Frameshift Mutation Frameshift mutation: A change in the sequence of codons following an insertion or deletion mutation.

Point Mutations vs. Frameshift Mutations The substitution example was a point mutation. These last two examples of deletion and insertion were frameshift mutations.   What’s the difference between a point mutation and frameshift mutation? Which has the most significant impact on the protein? Why? A point mutation only changes one single point/nucleotide in the DNA sequence and so, therefore, may have a smaller impact. I either causes no change in the amino acid or a change in only a single amino acid. A frameshift mutation changes all of the remaining codons in the sequence, so therefore will likely have a much larger impact because it changes the translation of the sequence.

Synonymous (“Silent”) Point Mutations What do you think a synonymous (“silent”) point mutation is? Silent point mutations do not cause a change in the amino acid sequence Generally, do not cause a change in the protein—however, can reduce the amount of a specific protein the cell makes or cause the structure of the protein to be changed in a manner that disrupts its functioning in the body Example: TAT changed to TAC—both still code from Tyrosine CTC changed to CTA—both still code for Leucine **You may wish to discuss with students that research is showing that even these “silent” point mutations cause changes. Scientists used to believe that synonymous point mutations were “no big deal”; however, new research is proving them wrong. Articles to consider are: http://news.sciencemag.org/sciencenow/2006/12/22-02.html http://www.fda.gov/BiologicsBloodVaccines/ScienceResearch/ucm271385.htm http://www.nature.com/scientificamerican/journal/v300/n6/full/scientificamerican0609-46.html

Inversion The fat cat ate the wee rat. The fat tar eew eht eta tac.   What changed? How would this type of change affect the protein? Inversion mutations happen when part of the nitrogen base sequence flips (reverses). What changed: The last portion of the sentence was completely inverted (reversed). How would this affect the protein: It will invert the amino acid sequence so it may result in an ineffective protein.

Nitrogen base mutations vs. Chromosomal mutations Chromosomal mutations cause change to a chromosome.

Chromosomal Mutations What changed? How would this type of change affect the protein? Duplication – section of a chromosome copied twice. This graphic represents a chromosome level duplication mutation. A portion of the chromosome is duplicated. This may result in no noticeable change; however, it is also how some cancers spread.

Chromosomal Mutations What changed? How would this type of change affect the protein? Deletion – a section of a chromosome was left out. This graphic represents a chromosome level deletion mutation. A portion of the chromosome is deleted. Deletions of larger portions of a chromosome often lead to genetic disorders (ex: Duchenne Muscular Dystrophy, Cri du chat syndrome, and Spinal Muscular Atrophy) and some deletions may be fatal. (Note to students the difference between a base sequence deletion when just one or a few nucleotides are deleted vs. a chromosomal deletion in which a large portion of a chromosome is deleted.)

Chromosomal Mutations What changed? How would this type of change affect the protein? This graphic represents a chromosome level translocation mutation. This is the rearrangement of chromosomal sections with non-homologous chromosomes. Possible affects include: destroying the gene’s function; alterations in gene expression (ex. Burkitt’s Lymphoma) or creation of a hybrid gene (ex. Chronic Myelogenous Leukemia). Translocation – Parts of two chromosomes switch places.

Non-Disjunction Ex: Trisomy 21 or Down Syndrome Non-disjunction - an error in meiosis when egg or sperm cells divide resulting in too many or too few chromosomes. How do you think this type of change would impact an organism? Causes an abnormal number of chromosomes and often results in genetic disorders (ex. Down Syndrome, Turner’s Syndrome, Triple-X Syndrome). This can also result in death of the fetus.

Mutation Impact The impact of a mutation on an individual also depends on where and when it occurs. If there was a mutation in the DNA of a zygote, how would that impact the individual? How might a mutation in a skin cell affect an individual?