Theme 2- Module 2 (The Genetic Code) Flashcards
How many different amino acids are there?
20
What are the four nucleotides in mRNA?
A
G
C
U
Who created the RNA tie club?
George Gamow
What is the RNA tie club?
Consisted of 20 members (one for each amino acid) + four honorary members (one for each nucleotide in nucleic acids)
What did George Gamow suggest? (in terms of RNA and shit)
(If one nucleotide coded for one amino acid, then DNA could only code for four amino acid residues– much less than needed for the 20 amino acids used by the cells)
Determined that a 1-base code would not be enough to code for all 20 amino acids
It must be a 3-base code
Why must there be 3-base codes (and not 1, 4, 6, etc)?
3 base codes make 64 distinct combinations of nucleotides
It’s more than we need but the 3-base code is the smallest group that will accomodate the 20 combos that we need. However, bc of this, multiple unique triplets would code for the same amino acid
Why can’t there be 4, 5, 6, etc. base codes if all you need are 20 combos?
It would be unnecessary
Would increase the redundancy of the code
If a codon were made of groups of 4 nucleotide bases, how many combos of nucleotides would there be?
4 x 4 x 4 x 4
= 256
Describe Nirenberg and Mattheai’s cell-free system experiment and results
Placed all of the components they believed were necessary for protein synthesis. Started by making a simple nucleic acid made up of a string of uracils
This produced a repeated, simple polypeptide sequence that contained identical amino acids (phenylalanine).
Suggests that the triplet UUU codes for phe
Codons are always written in which direction? 5’–>3’ or 3’–>5’?
5’–>3’
Why is the non-template strand often referred to as the ‘coding strand’?
Bc the RNA transcript has the same sequence as the non-template strand (except for U in place of T)
What is the START codon? (nucleotides + amino acid name)
AUG
Methionine
Why is the genetic code referred to as “unambiguous”?
Bc a unique codon triplet will always code for a specific amino acid and will never code for more than one amino acid
What is another amino acid that is only coded by one codon (besides methionine)?
Tryptophan
UGG
What are the STOP codons? Which amino acids do they code for?
UAA
UAG
UGA
They do not code for any amino acids
What is an open reading frame?
The entire cont’ seq of a gene that begins at a triplet start codon and ends at a stop codon
How many reading frames are there and why are there several?
6
Open reading frame’s first triplet is ACU. Translation can start from A, C, or U, thus changing the amino acids made.
BOTH strands on the double helix could be the template strand, so instead of 3 reading frames, you have six (3 in one direction, 3 in other direction on opposite strand)
What conclusive evidence showed us that the code was read in triplets?
If a single nucleotide was added or removed in a sequence, the code was misread (frameshift mutation)
How could you minimize the effects of a frameshift mutation?
Some sequences repeat in a certain way so that if you take away one codon later in the code, the original reading frame could still be restored, but the middle would have a section of wrong amino acids (min. the effect)
What would be the effect of adding a whole codon?
Would simply add amino acid to the protein; reading frame (and most of the remainder of the polypeptide) would be conserved
What does the RNA world hypothesis suggest?
Some type of ancestral RNA molecule was the precursor to current life
What evidence supports the RNA world hypo?
The fact that RNA molecules play such important roles in turning our genetic info from DNA into proteins
