BIOL10005 Lecture Notes - Lecture 14: Open Reading Frame, Start Codon, Reading Frame
Lecture 14 - Wednesday 24 August 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
LECTURE 14
GENE EXPRESSION: TRANSLATION
CHAPTER 11 - THE GENETIC CODE
TRANSLATION AND THE GENETIC CODE
THE GENETIC CODE
•How does DNA act as a code to specify which amino acids are added into our polypeptide
sequence?
•We need to encode info for 20 different subunits of protein, but we only have 4 bases in DNA. if
1 base determined 1 amino acid, then we would only have 4 different options for amino acids.
•However if we take DNA in pairs (AC, Ag etc) then this would give 16 options to code for amino
acids. This is still not enough!
•It’s actually read as a triplet, called a codon. 3 nucleotides are read together as a unit that code
for an amino acid. This creates 64 combinations, but we only have 20 amino acids, creating
degeneracy. We have 64 possible codons that can provide info for how to make polypeptides but
only 20 amino acids, meaning that some amino acids end up having multiple codons that specify
them.
CODONS
•Translation is the process of synthesizing a protein from an mRNA. During translation, the
ribosome moves along the mRNA and allows a group of 3 nucleotides, called a codon, to specify
the addition of an amino acid to the growing polypeptide chain.
•The genetic code constitutes the precise relationship between codons and amino acids.
•More than one codon, in most cases, specifies one amino acid. Thus, the genetic code is said to be
degenerate, as changes in the third nucleotide of a codon often do not alter the specified amino
acid.
•Eg. A CU combination encodes leucine regardless of the third nucleotide.
•Codons that specify the same amino acid are termed synonymous codons.
READING FRAMES
•Not all of the sequences of an mRNA molecule are read during translation; usually sequence at
the 5’ and 3’ ends do not encode proteins and are termed the 5’ and 3’ untranslated regions.
•The start and stop signals are also themselves triplet codons. Mostly, the AUG methionine
specifying codon serves as the start codon and the most common three stop codons are UAA,
UAG and UGA.
OPEN READING FRAMES
•The region of the mRNA that is read during synthesis is the open reading frame, and consists of a
start codon followed by normal amino acid specifying codons and then a stop codon.
•Codons do not overlap; the last nucleotide of one codon is adjacent to the first of the next.
•Since they do not overlap, mRNA has 3 possible and overlapping reading frames. The frame used
is set by the AUG start codon. DNA, being double stranded, has 6.
•Usually, only one of the 6 potential reading frames in a DNA sequence actually encode a protein.
READING FRAME MUTATIONS
•Mutations arise by the addition or removal of a nucleotide in a DNA sequence during DNA
replication. This destroys the reading frame and thus the function of the gene by changing the
amino acid sequence. These are called frameshift mutations.
•Mutations that alter a codon so that is begins to specify another amino acid are missense
mutations.
•Mutations that alter a codon so that it becomes a stop codon are nonsense mutations.
Document Summary
1 base determined 1 amino acid, then we would only have 4 different options for amino acids: however if we take dna in pairs (ac, ag etc) then this would give 16 options to code for amino acids. This is still not enough: it"s actually read as a triplet, called a codon. 3 nucleotides are read together as a unit that code for an amino acid. This creates 64 combinations, but we only have 20 amino acids, creating degeneracy. We have 64 possible codons that can provide info for how to make polypeptides but only 20 amino acids, meaning that some amino acids end up having multiple codons that specify them. Codons: translation is the process of synthesizing a protein from an mrna. Thus, the genetic code is said to be degenerate, as changes in the third nucleotide of a codon often do not alter the specified amino acid: eg.
