The Genetic Code Flashcards
Outline the three general steps of translation.
- mRNA produced by transcription
- Ribosome binds and recognizes a sequence on mRNA upstream of protein CDS
- Ribosome synthesizes a polypeptide according to nucleic acid sequence on mRNA
In what direction does the ribosome read the mRNA during translation?
5’ to 3’
In which direction is the polypeptide synthesized during translation?
N terminus to C terminus.
Why is the triplet code used instead of singlet, doublet, etc.?
It is the smallest number of nucleotides that can have unique combinations for 20 amino acids.
What is meant by the genetic code being “comma-free”?
There are no nucleotides used as punctuation between the triplet nucleotides.
What is meant by the genetic code being non-overlapping?
Neighbouring triplets do not overlap.
What is meant by the genetic code being unambiguous?
The meaning of the nucleotide triplet never changes.
What is meant by the genetic code being polar?
The code is written and read in the 5’ to 3’ direction.
What is meant by the genetic code being degenerate?
Multiple different nucloetide triplets may code for the same amino acid.
What is meant by the genetic code being nearly universal?
It is nearly identical across all domains of life.
What is a codon?
One unit of the nucleotide triplet which specifies an amino acid.
What is the typical start codon of a protein CDS?
ATG
What are the three typical stop codons of a protein CDS?
TAA (ochre), TAG (amber), TGA (opal)
What is an open reading frame?
The region between a start and stop codon in a gene.
Is ATG ALWAYS a stop codon?
No, it codes for methionine, which can also be present within the polypeptide.
What does it mean by there being three open reading frames?
Three possible ways to begin reading the mRNA. Start from first triplet/skip first nucleotide and following three are first triplet/skip first two nucleotides and following three are first triplet.
What determines the reading frame used for translation?
The position of the start codon.
How many possible frames does DNA have?
two strands*(3 frames/ strand) = 6 frames
What is a benefit of the genetic code’s degeneracy?
Minimizes effects of mutations (ex. most mutations at 3rd codon are silent)
What do many point mutations at the second position do?
Keep polarity the same
What position do point mutations have the greatest effect?
First
What is the mutation in sickle cell anemia?
GAA -> GUA (glutamic acid to valine)
What is a silent mutation?
Point mutation that does not change the amino acid.
What is a missense mutation?
Point mutation which changes the amino acid.
What is a nonsense mutation?
Point mutation creating a stop codon, protein synthesis stopped prematurely.
What is a frameshift mutation?
Insertion or deletion of nucleotide(s) changes the reading frame.
Which type of mutation typically has the largest impact?
Frameshift (<3)
