Chapter 12. The Genetic Code
Since there are 20 different kinds of amino acids in proteins and only four kinds of nucleotides in
DNA, the relationship between the gene and its most elementary functional product, i.e., between DNA
and protein, can hardly be interpreted through a code of one nucleotide = one amino acid. A coding
sequence of two nucleotides for one amino acid, or a doublet code, would produce only 16 possible
coding combinations, or codons. Codons are group of nucleotides that specifies one amino acid. By the
genetic code (George Gamov, 1954), we mean, a collection of base sequences (codons) that correspond
to each amino acid and to translation signals. A codon size of three nucleotides for one amino acid are
triplet codon seems more likely, since it produces 64 possible codons, however, only 20 amino acids need
to be coded, 44 codons in a triplet code seem to be superfluous. To account for the excess of codons
beyond the necessary 20, we can suppose that more than one codon can code for a particular amino acid.
For example, if each kind of amino acid were coded by three different possible codons, 60 possible
codons would be accounted for. A code in which there is more than one codon for the same amino acid, is
called degenerate. It is also possible that some or all of the codons in excess of 20 do not code for any
amino acid and are therefore nonsense codons.
Codon: 1 2 3 4 5 6 7 8 9
DNA template sequence: 3' TAC CCG ATA TCA GCC AAG GGG ATC GAT 5'
mRNA codon sequence: 5' AUG GGC UAU AGU CGG UUC CCC UAG CUA 3'
Amino acid: Met Gly Tyr Ser Arg Leu Pro STOP Leu
CHARACTERISTICS OF GENETIC CODE
Fig. 12.1. Amino acids and their messenger RNA codons.
The genetic code has following general properties, mostly applicable to the genes of all the organisms:
Genetic code is triplet. As discussed earlier, singlet and doublet codons cannot form 20 combinations,
which is the minimum requirement, therefore triplet codon is a necessity, so that all the amino acids must
Genetic code is non-overlapping. During translation, the codons are read one after another, in a
sequence. One base of a codon is not used by the other codons. Therefore, if there are six bases, they will
code for two amino acids only. e.g.; in case of non-overlapping, a gene sequence of UUUCCC only two
amino acids will be coded, phenylalanine (UUU) and proline (CCC), whereas for an overlapping code,
more than two amino acids could be coded, phenylalanine (UUU), serine (UCC) and proline (CCC).
Further, non-overlapping reduces the effects caused by mutation. If a base is altered due to mutation, it
will only affect one codon, to which it belong and thus, only one amino acid will be affected. However, if
we presume the genetic code to be overlapping, alteration of one base will affect at least two codons and
thus, two amino acids will be changed in the particular protein.
Fig. 12.2. Overlapping and non-overlapping genetic code.
Genetic code is commaless. The bases are read one after the other in the codons, i.e., no bases or
codons are reserved for punctuation or comma. When the first amino acid is coded, the second will be
coded by the next three bases immediately, and no base will be wasted to serve as a comma. Once the
translation begins, the codons are read one after the other with no break or demarcating signals in between
Genetic code is non-ambiguous. Each codon has a particular amino acid for coding, and it will code
for that amino acid only. There is one to one relationship between codon to amino acid. However, there is
an exception, AUG codes for methionine and GUG codes for valine, but if AUG is absent, then GUG
codes for methionine, as starting codon for protein synthesis. In an ambiguous code, one codon can code
for more than one amino acid.
In certain rare cases, the genetic code is found to be ambiguous i.e, some codons codes for
different amino acids under different conditions, e.g. in streptomycin sensitive strain of E.coli the codon
UUU normally codes for phenylalanine, but it may also code for isoleucine, serine when treated with
streptomycin. This ambiguity is enhanced at high Mg ion concentration, low temperature and the presence
of ethyl alcohol.
Genetic code is degenerate. Since there are more codons than the amino acids, more than one codon
may specify the same amino acid. Such different codons that specify the same amino acid are called as
synonymous codons. e.g., UUU = UUC = phenylalanine.
Genetic code has start/stop signals. Some codons are specially meant for initiation and termination
of protein synthesis. e.g.: AUG codes for methionine, serves as initiation codon in eukaryotes and GUG
in case of prokaryotes. Three codons UAG (amber), UAA (ochre), UGA (opal) are called as termination
codons, because they terminate protein synthesis. Earlier, they were called as nonsense codons, because
they do not code for any amino acid, but, since they are involved in termination of protein synthesis, they
are called as termination codons. The initiation and termination codons are known as signals and this
phenomenon is known as punctuation.
Genetic code is polar. It means that the genetic code has a fixed start and termination ends, and is
always read in a fixed direction, i.e. in 5′→ 3′ direction and the polypeptide chain is synthesized in N→C
direction i.e., from amino group (NH2) to carboxylic group (COOH).
Genetic code is universal. The same genetic code is applicable to all organisms, from bacteria to man,
i.e.; the codons have the same meaning in all the organisms. e.g., UUU = phenylalanine in bacteria,
mouse, man and tobacco. The universality of the genetic code, however, does not mean that DNA base
ratios must be similar in different species for genes specifying similar proteins. The fact that the code is
degenerate enables many bases to be changed by mutation in a sequence of mRNA, but this mRNA could
still produce the same amino acid sequence.
In 1979 investigators began started DNA sequencing of mitochondrial DNA in humans, cattle and
mice. During their experiments, they were surprised to learn that the genetic code used by the
mitochondrial DNA was not the same as the universal genetic code. e.g. UGA, which is a non-sense
codon, but it codes for tryptophan in mtDNA, AGG which codes for arginine is a non-sense codon in
mtDNA. So, extra chromosomal DNA such as mtDNA and ctDNA do not come under the universality of
the genetic code.
Wobble Hypothesis. Out of the 64 codons, three are involved in termination process. So, there are only
61 codons specifying the amino acids, and the cell should have 61 different types of tRNAs, each having
a different anticodon for the recognition of codons. However, the actual number of tRNA is found to be
much less than 61. This means that the anticodons of tRNA read more than one codon on the mRNA.
Crick (1966) proposed a hypothesis to explain the degeneracy of the genetic code; the hypothesis is
known as Wobble hypothesis.
According to this hypothesis, the major degeneracy occurs at the third position, i.e., the third
codon is not important in base pairing, and the actual pairing occurs only in the first two codon-anticodon
pairs. The base at the 5′ end of the anticodon and the base at the 3′ end of the codon form hydrogen bonds
without any specificity. The third base is called as the wobble base. This wobble base of codon lacks
specificity and the base in the first position of the anticodon is usually abnormal e.g., inosine, tyrosine,
etc. These abnormal bases are able to pair up with more than one nitrogen base at the same position e.g.,
inosine (I) can pair up with A, C and U. The pairing between unusual base of tRNA and wobble base of
mRNA is called wobble pairing.
COMPETITION DESK # 12
1. Which of the following length of a cistron in (b) code for initiator amino acid which is
base pair is capable of synthesizing a later removed.
polypeptide of 67 amino acids ? (c) are never present on the mRNA.
(a) 200 (b) 210 (d) do not code for any amino acid.
(c) 198 (d) 212
4. Point out the wrong statement about an
2. Wobbling is generally restricted to anticodon
(a) 1st N base (b) 2nd N base (a) it consists of three nucleotides.
(c) 3 N base (d) all N bases (b) it is the basic unit of genetic code.
(c) it extends from one end of a tRNA
3. Non sense codons are so called because they molecule.
(a) code for non essential amino acids. (d) it may pair with more than one codons.
13. Which of the following is initiation codon ?
5. How many amino acids can be coded from a (CBSE 2003)
gene of 600 nucleotide length ? (a) AUG (b) AUU
(a) 100 (b) 200 (c) UAG (d) UVU
(c) 300 (d) 400
14. Which amino acid can be used for the
6. The genetic code in which the amino acids synthesis of haemoglobin ? (EAMCET
are represented by more than one codons, is 2001)
called (a) Glutamic acid (b) Valine
(a) deaminate (b) commaless (c) Serine (d) all of these
(c) degenerate (d) overlapping
15. In the genetic code dictionary, how many
7. Triplet codon means codons are used to code for all the 20
(a) a sequence of three nitrogen bases in essential amino acids ? (CBSE 2003)
mRNA (a) 20 (b) 64
(b) a sequence of three nitrogen bases in (c) 61 (d) 60
(c) a sequence of three nitrogen bases in 16. Which one of the following codes, triplet is
rRNA correctly matched with its specificity for an
(d) presence of only three bases in mRNA amino acid in protein synthesis or as start or
stop codons ? (CBSE 2003)
8. The known number of amino acids used in (a) UCG – start (b) UUU – stop
protein synthesis is (c) UGU – leucine (d) UAC – tyrosine
(a) 20 (b) 35
(c) more than 50 (d) more than 100 17. Out of the total 64 codons, 61 codes for 20
amino acids, this suggests (CBSE 2002)
9. The average weight of 20 amino acids is (a) degeneracy of codons
128. What is the approximate weight of a (b) overlapping of codons
protein with 100 amino acids? (MANIPAL (c) redundancy of codons
2002) (d) ambiguous nature of codons
(a) 12,800 (b) 640
(c) 60,000 (d) 1,28,000 18. Which of the following has function similar
to UGA and UAA ? (Manipal 2002)
10. In a genetic code the process of degeneracy (a) UAG (b) AUG
is that (AMU 2003) (c) GUG (d) AGG
(a) one codon has many meanings
(b) many codons have same meaning 19. Degeneracy of genetic code is due to
(c) only two codons share many amino acids (MHCET 2005)
(d) third base is not stable (a) functional 61 codons and 20 amino acids.
(b) functional 64 codons and 20 amino
11. Which one of the following codons codes acids.
for the same information as UGC? (AIIMS (c) functional 20 codons and 20 amino acids.
2003) (d) functional 20 codons and 61 amino
(a) UGA (b) UGU acids.
(c) UAG (d) UGG
20. UGA, UAG and UAA are called as
12. Degeneration of genetic code is attributed to termination codons because (MHCET
the (CBSE 2003) 2005)
(a) first member of the codon (a) they terminate anticodons.
(b) second member of the codon (b) they are present at the beginning of
(c) third member of the codon mRNA.
(d) entire codon (c) they do not specify any amino acid.
(d) they indicate initiation of translation.
ANSWERS # 12
1. b 2. c 3. d 4. b 5. b 6. c 7. a 8. a 9. b 10. b
11. b 12. c 13. a 14. d 15. c 16. d 17. a 18. a 19. a 20. c
EXPLANATION # 12
2. (c): According to Wobble hypothesis, the major degeneracy occurs at the third position, i.e., the third
codon is not important in base pairing, and the actual pairing occurs only in the first two codon-anticodon
3. (d): UAG (amber), UAA (ochre), UGA (opal) were earlier called as nonsense codons, because they do
not code for any amino acid, but, since they are involved in termination of protein synthesis, they are
called as termination codons.
6. (c): In a genetic code, there are 64 codons and the amino acids coded are 20 only, so more than one
codon may specify the same amino acid. This phenomenon is called as degeneracy.
15. (c): There are total 64 codons out of which 3 codons do not specify any amino acid. The actual
number of codons specifying 20 amino acids are 64 – 3 = 61.