4 minute read
105 What is the Genetic Code?
Key Idea: Each mRNA codon codes for a specific amino acid. This activity will help you practise using the amino acid table to determine the amino acids encoded by the genetic code.
The mRNA - amino acid table
The table on the right is used to ‘decode' the genetic code. It shows which amino acid each mRNA codon codes for. There are 64 different codons possible, 61 code for amino acids, and three are stop codons. Amino acid names are written as three letter abbreviations (e.g. Ser = serine). To work out which amino acid a codon codes for, carry out the following steps: i Find the first letter of the codon in the row on the left hand side of the table.
AUG is the start codon. ii Find the column that intersects that row from the top, second letter, row. iii Locate the third base in the codon by looking along the row on the right hand side that matches your codon. e.g. GAU codes for Asp (aspartic acid)
Read second letter here
UUU Phe UUC Phe UUA Leu UUG Leu CUU Leu CUC Leu CUA Leu CUG Leu
AUU Ile AUC Ile AUA Ile AUG Met GUU Val GUC Val GUA Val GUG Val UCU Ser UCC Ser UCA Ser UCG Ser CCU Pro CCC Pro CCA Pro CCG Pro
ACU Thr ACC Thr ACA Thr ACG Thr GCU Ala GCC Ala GCA Ala GCG Ala UAU Tyr UAC Tyr
UAA STOP UAG STOP
CAU His CAC His CAA Gln CAG Gln
AAU Asn AAC Asn AAA Lys AAG Lys GAU Asp GAC Asp GAA Glu GAG Glu UGU Cys UGC Cys
UGA STOP
UGG Trp CGU Arg CGC Arg CGA Arg CGG Arg AGU Ser AGC Ser AGA Arg AGG Arg GGU Gly GGC Gly GGA Gly GGG Gly
U
Read first letter here Read third letter here
1. (a) Use the base-pairing rule for to create the complementary strand for the DNA template strand shown below.
(b) For the same DNA template strand, then determine the mRNA sequence and use the mRNA - amino acid table to determine the amino acid sequence. Note that in mRNA, uracil (U) replaces thymine (T) and pairs with adenine.
Complementary strand (this is the DNA coding strand)
Template strand
Gene expression
Transcription
mRNA
Translation
Amino acids
2. What do you notice about the sequence on the DNA coding strand and the mRNA strand?
Redundancy and degeneracy are important concepts in understanding the genetic code. ` Redundancy is when several situations code for or control the actions of one specific thing. ` Degeneracy is when a particular output can be produced by several different pathways. Examples of redundancy and degeneracy are illustrated below. In modern aircraft redundant features add safety by making sure if one system fails others will ensure a smooth, safe flight. Degeneracy can be seen in proteins when different proteins have the same function.
Flight computers
Control lines Modern aircraft (left) have multiple redundant features for safety. Often there are three or four flight computers linked independently to the flight surfaces and other input/output devices. If one computer or control line fails the others can continue to fly the plane normally.
Degeneracy is seen in the production of the enzymes salivary and pancreatic amylase. Salivary amylase breaks down carbohydrates in the mouth, whereas pancreatic amylase does so in the small intestine. The enzymes are encoded by different genes (AMY1A and AMY2A) but have the same functional role (right). Salivary amylase (above) is structurally different to pancreatic amylase, but has the same function.
The genetic code shows degeneracy. This means that a number of 3 base combinations specify one amino acid. The codons for the same amino acid often differ by only a single letter (often the second or third). For example, proline is encoded by four different codons.
Pro CCU
CCC
CCA
CCG CCU
CCC
CCA
CCG
3. Explain how degeneracy adds "safety" to the coding of protein chains:
4. The genetic code shows redundancy but no ambiguity. What does this mean and why is it important?
5. Identify the following:
(a) The codons that encode valine (Val):
(b) The codons that encode aspartic acid (Asp):
6. (a) Arginine (Arg) is encoded in how many ways?
(b) Glycine (Gly) is encoded in how many ways?
(c) Which amino acid(s) are encoded in only one way?
Pro
The degeneracy of the genetic code creates redundancy, so that several codons code for the same amino acid (e.g. CCU, CCC, CCA, and CCG code for proline). Note that although there is redundancy, there is no ambiguity - none of the codons encodes any other amino acid.
