Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1 · 1.4
Genetic Code and the Role of DNA.
Just four letters: A, U, C and G. Put them in three places and you get 64 words, and those words spell out how every protein is built. This note teaches you to read them.
1. The genetic code and the codon
The code that carries an organism's traits from one generation to the next is the genetic code. It lies in DNA.
A particular arrangement of three nucleotides, a trinucleotide sequence, is a codon.
The nucleotides of DNA carry four kinds of base: adenine (A), guanine (G), cytosine (C) and thymine (T). Three neighbouring bases together make one genetic code.
The match between the order of nucleotides in DNA and the order of amino acids in a protein was first shown by Sarabhai and colleagues (1964) and by Yanofsky and colleagues (1964).
2. Triplet, codon and anticodon
The three complementary nucleotides that sit in mRNA opposite three nucleotides of DNA are together called a triplet.
This triplet in mRNA is the codon. Francis Crick and colleagues showed that the genetic code is a three-letter code, a triplet code.
The triplet of three nucleotides on tRNA that pairs with a codon of mRNA is the anticodon. Mixing up codon and anticodon is a classic exam slip, so tap the grid and read them side by side.
Codon and anticodon
| feature | codon | anticodon |
|---|---|---|
| location | mRNA | tRNA |
| number | 64 (61 + 3) | not in book |
| role | carries the code | pairs with the codon |
| example | AUG | UAC |
3. The 64 codons
By 1964 Nirenberg, Matthaei and colleagues had found 64 triplet codes for the 20 kinds of amino acids.
The 64 comes from plain multiplication. The codon alphabet has four letters, A, U, C and G, and a codon has three positions. So 4 × 4 × 4 = 64. Step through it.
64 codons
Of the 64 codons, UAA, UAG and UGA specify no amino acid. They stop translation instead, and are called termination codes (nonsense codes).
Each of the remaining 61 codons specifies some amino acid. Among them AUG is the starting codon, and it specifies methionine.
4. How the code reads
The language of the code is one-way: from nucleic acid to protein, never back.
When more than one code specifies a single amino acid, that is degeneracy of the genetic code. Some amino acids have one code, many have 2, 3 or even 6. Lysine has 2, for example, and valine has 4.
By 1966 the full meaning of the genetic code was known. Nirenberg and Har Gobinda Khorana won the Nobel Prize in 1968 for deciphering it.
5. The codon table
In the table, UUU and UUC code phenylalanine; UGG tryptophan; AAA and AAG lysine; GGU to GGG glycine; GUU to GUG valine.
CCU to CCG code proline; GCU to GCG alanine; UAU and UAC tyrosine; GAA and GAG glutamic acid; GAU and GAC aspartic acid.
Codons starting with U: UUA and UUG leucine; UCU, UCC, UCA and UCG serine; UAU and UAC tyrosine; UGU and UGC cysteine.
Codons starting with C: CUU to CUG leucine; CCU to CCG proline; CAU and CAC histidine; CAA and CAG glutamine; CGU to CGG arginine.
Codons starting with A: AUU, AUC and AUA isoleucine; AUG methionine (the start codon); ACU to ACG threonine (ACU, ACC, ACA, ACG); AAU and AAC asparagine; AAA and AAG lysine; AGU and AGC serine; AGA and AGG arginine.
Now look codons up yourself. Pick three letters and the answer appears below.
Codon table
The table is the book's codon table, not a measurement of you.
6. Properties of the genetic code
Several codons may code one amino acid, leucine for example. But one codon never codes more than one amino acid.
Nucleotides never overlap when codons are formed (non-overlapping); they follow one sequence.
Codons are universal: the same for all species, and unchanged through evolution.
The genetic code is always three letters long, a triplet.
The start and stop codes are fixed: a chain starts with AUG and ends with UAA, UAG or UGA.
No extra nucleotides sit between two codes. Amino acids keep being added until a stop codon arrives.
There are slight exceptions. Mitochondria, chloroplasts and prokaryotes arrange codons a little differently. In some protists UAA and UAG code glutamine instead of stopping.
Now test the properties: which are true and which are false.
True or false?
7. DNA in heredity
The basis of heredity is the hereditary material: chromosomes, DNA, RNA and so on.
DNA is the only permanent chemical of the chromosome. So DNA alone is the hereditary material and its chemical basis, and it carries traits directly from parents to offspring.
8. Check yourself
Three questions in the exam's style. Start with the first.
Question 1
Look at the figure below: [Figure: the cloverleaf structure of a tRNA molecule; the bases A, A, C are marked in the anticodon loop at the bottom.] Which amino acid will the tRNA above carry?
Read the second one slowly.
Question 2
Features of the genetic code are:
i. It is a triplet codon
ii. AUG is the starting codon
iii. UAA, UGA and UAG are termination codons
The last question.
Question 3
Which is not a termination codon?
The whole note on seven cards.
Seven cards
Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.4.