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Biochemistry 2280A Lecture Notes - Lecture 20: Start Codon, Nonsense Mutation, Initiation Factor

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ochretoad474
21 Nov 2017
School
Western University
Department
Biochemistry
Course
Biochemistry 2280A
Professor
Christopher Brandl

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Document Summary

Combinations of three rna bases result in one codon. One codon spells out the corresponding amino acids. Specific reading frame: starting at aug at on position versus further down the sequence changes the triplets translated. Why is each codon 3 bases: 3 allows specificity. How many codons do you need: 20 codons, one for each amino acid, but we actually have 64 meaning some amino acids coded by more than one codon. Given that any nucleotides can be a, u, g, c how many different 3 base codons are there: 4^3. Universal: all organisms use the codon corresponding to specific sequence, code evolved once and has not undergone much evolutionary change, probably because it is important and not needed to be changed. Nonoverlapping: read in blocks of three and no overlap in code, what if overlapping code, overlapping the codons the codon before will put restriction of what follows so restriction on protein sequence, e. g.

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Related Questions

Fill in the blank. Elongation during translation does NOT involve ____________.

Question 16 options:

the translation of codons according to the genetic code

the formation of bonds catalyzed by the ribosome

complementary base pairing between RNA molecules

amino acids being linked together in a polypeptide

reading the DNA template 3' to 5'

For a given gene, what establishes the reading frame for translation?

Question 17 options:

the location of the enhancer relative to the gene

the first three nucleotides at the 5' end of the mRNA

the first three nucleotides at the 3' end of the mRNA

the start codon in the mRNA

the location of the promoter relative to the gene

Which of the following is the LEAST likely direct consequence of a substitution mutation?

Question 18 options:

changing the length of a protein coded for by a gene

changing one amino acid in a protein

creating a stop codon

eliminating a start codon

changing the length of the DNA molecule containing a gene

Suppose that the pre-mRNA transcript from a eukaryotic gene is 30,000 nucleotides long, and the gene codes for a sequence of 300 amino acids. What is the best explanation for the relationship between these numbers?

Question 19 options:

only the first 900 nucleotides of the pre-mRNA transcript are translated

it takes 100 nucleotides to specify a single amino acid

300 of the nucleotides in the transcript are important, and the rest are "junk"

only the last 900 nucleotides of the pre-mRNA transcript are translated

large portions of pre-mRNA transcripts are cut out during RNA processing

Suppose an individual is born into a population with a novel mutation. Is the new mutation an evolutionary change, and why?

Question 20 options:

no, because it is not a big enough change to count

yes, because new mutations are always adaptive

yes, because the appearance of a new genetic variant is a genetic change in a population

no, because not enough individuals have the mutation for it to matter

no, because most mutations are not adaptive

Its one problem with 7 questions

A) Most eukaryotic cells transcribe a relatively high portion of their genome, but most of those RNA transcripts are likely not encoding for proteins. Elucidating the function(s) of that "junk" RNA is one of the next big challenges in molecular cell biology. You isolate one of those transcripts and determine that its sequence consists of 50% C, 30% U, and 20% A. What is the percentage of Gs in this transcript? (don't write the unit)

B) Which stop codon(s) can be encoded by the RNA in the previous question ("How many times do we need to tell you to stop? 1").
List the codon(s) alphabetically. If you need to enter fewer codons than the alloted number of "blanks", enter N/A .

C) From what you can tell, the sequence of the transcript you isolated in the previous question* appears quite random. Assuming that it is random, you can expect to see the various combinations of any three nucleotides at a probability that depends of the frequency of each of the nucleotides. For example, you naturally should expect to see the triplet CCC more times than AAA. What is the expected percentage of triplets corresponding to stop codon(s) in this transcript (round to one decimal place and enter just the number without "%")? *(How many times do we need to tell you to stop? 1)

D) If the length of the transcript in the previous two questions is 2,500 nucleotides, how many stop codons do you expect to find in it, based on the frequency you calculated above?

E) On average, how many bases separate each of the occurrence of the stop codon(s) (round to the nearest integer).

F) The sequence between two potential stop codons is an "open reading frame" (ORF*). Upon closer examination of the RNA sequence from the previous questions** you determine the presence of an ORF that may encode for a putative protein which is 600 amino acid residues-long. How long is the ORF?

*Open reading frame: a stretch of DNA or RNA which is uninterrupted by a stop codon and may therefore encode for a protein.

**Questions "How many times do we need to tell you to stop" 1 through 5.

27 bases

1,800 bases

2,500 bases

259 bases

600 bases

83 bases

200 bases

G) What may be the ramifications of your analyses of the RNA you isolated*? It is clearly containing an ORF which is longer than what you expected to find based on the sequence analysis you have conducted. What would be a logical conclusions based on the comparison of the lengths of the actual and predicted ORFs? Questions "How many times do we need to tell you to stop" 1 through 6. Pick 1, 2 or 3 below

1) The difference between the actual and predicted sizes of the ORF is not big enough to be significant and therefore your initial hypothesis that sequence is random is clearly random, is supported.

2) Nature is random and your results, using the Bard's words, may be "full of sound and fury" but are really "signifying nothing".

3) The difference between the actual and predicted sizes of the ORF is rather big and may suggest that the sequence of the RNA may not be as random as it first appeared. This raises the hypothesis that what started as piece of "junk" RNA may be actually encoding for a protein.