111

MOLECULAR BASIS OF INHERITANCE

(28S, 18S, and 5.8S), whereas the RNA polymerase III is responsible

for transcription of tRNA, 5srRNA, and snRNAs (small nuclear

RNAs). The RNA polymerase II transcribes precursor of mRNA, the

heterogeneous nuclear RNA (hnRNA).

(ii) The second complexity is that the primary transcripts contain both

the exons and the introns and are non-functional. Hence, it is

added at 3

'

-end in a template independent manner. It is the fully

processed hnRNA,

now called mRNA, that is transported out of the

nucleus for translation (Figure 6.11).

The significance of such complexities is now beginning to be

understood. The split-gene arrangements represent probably an ancient

feature of the genome. The presence of introns is reminiscent of antiquity,

and the process of splicing represents the dominance of RNA-world. In

recent times, the understanding of RNA and RNA-dependent processes

a polymer of amino acids. Neither does any complementarity exist between

nucleotides and amino acids, nor could any be drawn theoretically. There

existed ample evidences, though, to support the notion that change in

nucleic acids (genetic material) were responsible for change in amino acids

in proteins. This led to the proposition of a genetic code that could direct

the sequence of amino acids during synthesis of proteins.

If determining the biochemical nature of genetic material and the

structure of DNA was very exciting, the proposition and deciphering of

genetic code were most challenging. In a very true sense, it required

involvement of scientists from several disciplines – physicists, organic

3

generating many more codons than required.

Providing proof that the codon was a triplet, was a more daunting

task. The chemical method developed by Har Gobind Khorana was

for transcription

small nuclear

The RNA polymerase II transcribes precursor of mRNA, the

) The second complexity is that the primary transcripts contain both

the exons and the introns

and are no

n

functional.

Hence, it is

subjected to a process called

additional processing called as capping and tailing. In

-

, adenylate residues (200-300) are

112

Table 6.1: The Codons for the Various Amino Acids

The salient features of genetic code are as follows:

(i) The codon is triplet. 61 codons code for amino acids and 3 codons do

not code for any amino acids, hence they function as stop codons.

(ii) Some amino acids are coded by more than one codon, hence

the code is degenerate.

(iii) The codon is read in mRNA in a contiguous fashion. There are

no punctuations.

(iv) The code is nearly universal: for example, from bacteria to human

UUU would code for Phenylalanine (phe). Some exceptions to this

If following is the sequence of nucleotides in mRNA, predict the

sequence of amino acid coded by it (take help of the checkerboard):

-AUG UUU UUC UUC UUU UUU UUC-

of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free

system for protein synthesis finally helped the code to be deciphered.

ey

113

MOLECULAR BASIS OF INHERITANCE

Now try the opposite. Following is the sequence of amino acids coded

by an mRNA. Predict the nucleotide sequence in the RNA:

Met-Phe-Phe-Phe-Phe-Phe-Phe

Do you face any difficulty in predicting the opposite?

Can you now correlate which two properties of genetic code you have

effect of point mutations will be explained here. A classical example of

point mutation is a change of single base pair in the gene for beta globin

chain that results in the change of amino acid residue glutamate to valine.

It results into a diseased condition called as sickle cell anemia. Effect of

point mutations that inserts or deletes a base in structural gene can be

better understood by following simple example.

Consider a statement that is made up of the following words each

having three letters like genetic code.

RAM HAS RED CAP

If we insert a letter B in between HAS and RED and rearrange the

Now we insert three letters together, say BIG, the statement would read

RAM HAS BIG RED CAP

The same exercise can be repeated, by deleting the letters R, E and D,

one by one and rearranging the statement to make a triplet word.

RAM HAS EDC AP

RAM HAS DCA P

RAM HAS CAP

The conclusion from the above exercise is very obvious. Insertion or

deletion of one or two bases changes the reading frame from the point of

insertion or deletion. However, such mutations are referred to as

Now we insert th

The same exercise can be r

the letters R, E and D,

one by one and rearranging the statement to make a triplet word.

114

frameshift insertion or deletion mutations. Insertion or deletion of

three or its multiple bases insert or delete in one or multiple codon hence

read the code uniquely. He postulated the presence of an adapter molecule

that would on one hand read the code and on other hand would bind

to specific amino acids. The tRNA, then called sRNA (soluble RNA),

was known before the genetic code was postulated. However, its role

as an adapter molecule was assigned much later.

another specific tRNA that is referred to as initiator tRNA. There are no

tRNAs for stop codons. In figure 6.12, the secondary structure of tRNA

has been depicted that looks like a clover-leaf. In actual structure, the

tRNA is a compact molecule which looks like inverted L.

Translation refers to the process of polymerisation of amino acids to

form a polypeptide (Figure 6.13). The order and sequence of amino acids

are defined by the sequence of bases in the mRNA. The amino acids are

three or its multiple bases insert or delete in one or multiple codon hence

one or multiple amino acids, and reading frame remains unaltered from

read the code uniquely. He postulated

e the

ostulated. H