Download File - MS Barnes` Biology 12

DNA Mutations during Protein Synthesis – Notes
Complete the questions and tasks below while reading section 5.6 in your Biology 12 textbook (pp 259263). You will be responsible for this material on your test and exam.
Introduction:
What is a mutation in the genetic code?
A mutation is a mistake in the genetic code (DNA sequence) that can be passed on to the individuals’
offspring. A mutation may affect the synthesis of proteins, or it may have no effect.
Is a mutation always a negative event in an organism? Give an example of when a mutation might be
negative and when it might be positive.
A negative side effect of a mutation might result when necessary proteins are not able to be produced
(an important gene is not “expressed”). Mutations also drive evolution, though, and if they result in
advantageous physical characteristics they will be naturally selected for and passed on to future
offspring.
Types of mutations:
Use your own words (it will help you remember better) to describe the following types of mutations and
their consequences:
Silent Mutation: What does it mean for a mutation to be silent? There are two ways that a mutation can
be silent, what are they?
A mutation is silent if it does not affect the overall structure and function of the protein produced. Silent
mutations might happen in the introns, which are cut out of the mRNA transcript and have no effect on
the resulting protein or they might result in a change of codons that does not change the amino acid
coded for.
Non-silent mutations:
Missense mutation:
A non-silent mutation resulting from the substitution of one base pair for another and changing the
amino acids coded for. This will change the polypeptide produced.
Nonsense mutation:
Results in a protein that is too short (too few amino acids) and therefore unusable due to the conversion
of a codon that would have coded for an amino acid into a stop codon.
What is substitution? How is it related to missense and nonsense mutations?
Substitution is the replacement of one base pair with another. It is the cause of missense and nonsense
mutations.
Deletion: a mutation that happens when base pairs are removed from the genetic code. This will
change the amino acids coded for and results in a frame-shift mutation.
Insertion: the addition of an extra base pair in the genetic sequence, resulting in different amino
acids and a frame-shift mutation.
Frame-shift mutation:
These mutations cause a shift in how the DNA sequence is read. Since the codons are read in groups of 3,
any change in the sequence involving the adding or taking away of a base pair (or 2) will result in a
change in the groups of 3 – a shift. Adding or deleting 3 bases (or multiples of 3) does not shift the frame.
Point mutation:
A mutation that only involves one base pair change.
Translocation:
The movement of segments of DNA along a chromosome or between chromosomes. Not the same as
crossing over, because it involves non-homologous chromosomes and will result in changes in the
organism’s genome.
What is a “jumping gene”?
A “jumping gene” is a complete gene that can move from one region of DNA to another. Also called a
“transposable element”, it can cause problems if it interrupts the expression of another gene.
Inversion:
This event happens when a segment of the chromosome breaks off and reattaches in the opposite
direction.
Causes of Genetic Mutations:
What is the difference between a spontaneous mutation and an induced mutation?
A spontaneous mutation results from errors that occur during DNA replication, or during protein
synthesis. An induced mutation is caused by an environmental factor called a mutagenic agent or
“mutagen”.
What are some examples of mutagens? Describe the effects of at least three mutagens.
X-rays: high frequency, high energy radiation that breaks the backbone of DNA molecules
UV light: radiation, can cause point mutations
Pesticides: can cause deletions in the genetic sequence
Why is cancer considered to be a genetic disease?
Cancer is caused by a mutation in the genetic sequence. Genes that control cell growth and division are
affected by mutagens (such as the chemicals in cigarette smoke) and are transformed into oncogenes.
The regulations that restrict unchecked cell division are no longer in place.
A geneticist found that a particular mutation had no effect on the protein coded by a gene. What do you
think is the most likely type of mutation in this gene? Why?
This type of mutation is a silent mutation because it has no effect on the final protein. It is most likely a
point mutation, specifically substitution because that is the most likely option if the amino acid sequence
is not influenced.
You have a DNA sequence that codes for a protein and is 105 nucleotides long. A frame-shift mutation
occurs at the 85th base - how many amino acids will be correct in this protein?
105/3 = 35 codons = 34 amino acids
84/3 = 28 codons  possibility of having at least 28 correct amino acids
Do question #6 & 7 from p 263 in your textbook below.
#6 a) Original sequence: Met – Phe – Leu – Pro – Tyr – His – Arg
Mutated sequence: Met – Phe – Leu – Pro – Tyr – His – Arg  no change in aa sequence, silent mutation,
not harmful
b) Met – Phe – Leu – Pro – Stop  Protein is too short, premature stop codon, nonsense mutation
c) Met – Phe – Leu – Ala – Leu – Ser – Ser  insertion (frame-shift) mutation, changes the aa sequence &
the structure of the protein
d) Met – Phe – Leu – Pro – Ile – Ile  deletion (frame-shift) mutation, changes aa sequence
e) Cys – Tyr – Tyr – Ser – Val – Phe – Val  inversion – completely different protein. Also, no start or stop
codon, so this sequence may not be translated at all
#7 Which aa changes can result from a single base pair substitution?
a) Arg  Leu, Yes, CGU  CUU, CGC  CUC, CGA  CUA, CGG  CUG
b) Cys  Glu, No
c) Ser  Thr, Yes, UCU  ACU, UCC  ACC, UCA  ACA, UCG  ACG, AGU  ACU, AGC  ACC
d) Ile  Ser, No
Below is the DNA base sequence for the normal protein for normal hemoglobin and the base sequence
for (abnormal) sickle cell hemoglobin:
Normal: GGG CTT CTT TTT
Sickle: GGG CAT CTT TTT
A. Transcribe and translate the normal and sickle cell DNA.
Normal: CCC GAA GAA AAA
Pro – Glu – Glu – Lys
Sickle: CCC GUA GAA AAA
Pro – Val – Glu – Lys
B. Identify this as a point or frameshift mutation. Explain.
This is a point mutation because it involves the substitution of A for T at the 5 th base pair in the DNA
sequence and there is no shift in the reading frame.
C. If the base sequence read GGG CTT CTT AAA instead, would this result in sickle cell hemoglobin?
Explain
CCC GAA GAA UUU
Pro – Glu – Glu – Phe
This is still a point mutation that would still affect the structure and shape of the protein hemoglobin.
Do questions #8 & 9 from your textbook below.
#8 A food dye that has been identified as a chemical mutagen poses greater dangers for a developing
fetus than for an adult. A fetus is undergoing rapid developmental growth within the uterus. The rate of
mitosis for all cells is much faster than that within an adult. The effects of the mutagen can vary
depending on which stage of development the fetus is in. If the mutagen affects undifferentiated cells
(cells that will eventually become specialized cells such as liver, heart, kidney etc.), it may impart a
serious mutation that will lead to abnormal development. An adult also undergoes mitosis but does so to
replace existing cells. If a mutation takes place in an adult, chances are it will be limited to the one cell
and its daughter cells in the future; therefore, it will be localized to one area. In a fetus, a mutation will
affect all cells since undifferentiated cells can become many different types of cells.
#9 There are lots of precautions that you can take that would reduce your chances of experiencing an
induced genetic mutation. These include (but are not limited to): wearing sunscreen, not using pesticides
on lawns, washing all food thoroughly before ingestion, avoiding cigarette smoke and avoiding
unnecessary X-rays.