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
subjected to a process called splicing where the introns are removed
and exons are joined in a defined order. hnRNA undergoes
additional processing called as capping and tailing. In capping an
unusual nucleotide (methyl guanosine triphosphate) is added to
the 5
'
-end of hnRNA. In tailing, adenylate residues (200-300) are
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
in the living system have assumed more importance.
6.6 GENETIC CODE
During replication and transcription a nucleic acid was copied to form
another nucleic acid. Hence, these processes are easy to conceptualise
on the basis of complementarity. The process of translation requires
transfer of genetic information from a polymer of nucleotides to synthesise
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
chemists, biochemists and geneticists. It was George Gamow, a physicist,
who argued that since there are only 4 bases and if they have to code for
20 amino acids, the code should constitute a combination of bases. He
suggested that in order to code for all the 20 amino acids, the code should
be made up of three nucleotides. This was a very bold proposition, because
a permutation combination of 4
3
(4 × 4 × 4) would generate 64 codons;
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
2022-23
MOLECULAR BASIS OF INHERITANCE
(28
S
, 18
S
, and 5.8
S
), whereas the RNA polymerase III is responsible
for transcription
tRNA
,
5srRNA
, an
d
snRNAs
(
small nuclear
RN
As
).
The RNA polymerase II transcribes precursor of mRNA, the
heterogeneous nuclear RN
A
(
hnRNA
).
A
A
(
ii
) The second complexity is that the primary transcripts contain both
the exons and the introns
and are no
n
-
functional.
Hence, it is
su
bj
ec
te
d
to
a
p
ro
ce
ss
c
al
le
d
sp
li
ci
ng
w
he
re
t
he
i
nt
ro
ns
a
re
r
em
ov
ed
ng
ge
nerati
ng
man
y
more codons than r
eq
uired.
Providing proof that the codon was a triplet, was a more daunting
task. The chemical method developed by Har Gobind Khorana was
202
2-2
3
111111
subjected to a process called
splici
ng
where the introns are removed
ng
an
d
ex
on
s
ar
e joined in a defined
or
de
r
.
h
nR
NA
u
nd
er
go
es
additional processing called as capping and tailing. In
capping
an
g
unusual nucleotide (methyl guanosine triphosphate) is added to
the 5
'
-
end of hnRNA. In
tailing
, adenylate residues (200-300) are
added at 3
'
-
end in a template independent mann
er
.
It is the
fully
processed hnRN
A
,
n
ow
c
alled mR
NA
,
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
RN
A-
wo
rld
.
I
n
recent times, the understanding of RNA and RNA-dependent processes
in the living system have assumed more importance.
6.6 G
ENET
IC
C
ODE
During replication and transcription a nucleic acid was copied to form
another nucleic acid. Hence, these processes are easy to conceptualise
on the basis of complementarity. The process of translation requires
transfer of
ge
netic information from a
p
ol
ym
er of nucleotides to s
yn
thesise
a
po
ly
mer of amino acids. Neither does a
ny
com
pl
ementari
ty
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
p
roteins. This led to the
pr
op
osition of a
g
enetic 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 disci
pl
ines
p
hy
sicists, o
rg
anic
chemists, biochemists and geneticists. It was George Gamow, a physicist,
who argued that since there are only 4 bases and if they have to code for
20 amino acids
,
the code should constitute a combination of bases. He
suggested that in order to code for all the 20 amino acids, the code should
be made
up
of three nucleotides. This was a ve
ry
bold
pr
op
osition, because
a permutation combination of 4
3
(4 × 4 × 4) would generate 64 codons;
generating many more codons than required.
112
BIOLOGY
instrumental in synthesising RNA molecules with defined combinations
of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free
system for protein synthesis finally helped the code to be deciphered.
Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful
in polymerising RNA with defined sequences in a template independent
manner (enzymatic synthesis of RNA). Finally a checker-board for genetic
code was prepared which is given in Table 6.1.
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
rule have been found in mitochondrial codons, and in some
protozoans.
(v) AUG has dual functions. It codes for Methionine (met) , and it
also act as initiator
codon.
(vi) UAA, UAG, UGA are stop terminator codons.
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-
2022-23
BIOLOGY
instrumental in
sy
nthesisi
ng
RNA molecules with defined combinations
of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free
system for protein synthesis finally helped the code to be deciphered.
Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful
in polymerising RNA with defined sequences in a template independent
manner (en
zy
matic
sy
nthesis of RNA). Final
ly
a checker
-boar
d for
ge
netic
co
de
w
as
p
re
pa
re
d
wh
ic
h
is
g
iv
en
i
n
Ta
bl
e
6.
1.
do
ce
re
an
s
e
it
he
-AUG UUU UUC UUC UUU UUU UUC-
202
2-2
3
111122
code was prepared which is given in Table 6.1.
T
able 6.1: The Codons for the V
T
T
arious Amino Acids
V
V
The salient fea
tures of genetic code are as follows
:
(i)
The codon is tri
pl
et.
61 codons code for amino acids and 3 codons do
not code for a
ny
amino acids, hence th
ey
function
as sto
p
codons.
(ii)
Some amino acids are coded b
y
more than one codon, hence
the code is
degenerate
.
(iii)
The codon is read in mRNA in a conti
gu
ous fashion. There are
no punctuations.
(iv)
The code is near
ly
universal
:
f
or example, from bacteria to human
f
f
UUU would code for
Phenylalanine (phe).
or
Some exceptions to this
rule have been found in mitochondrial codons, and in some
protozoans.
(v
)
AUG has dual functions. It codes for Methionine
(m
et
)
, and it
also act as
initiator
codon
.
r
(v
i)
UAA, UAG, UGA are stop terminator codons.
If
f
ollowing is the sequence o
f
nucleotides in mRNA, predict the
sequence of amino acid coded by it (take help of the checkerboar
d
):
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
learnt?
6.6.1 Mutations and Genetic Code
The relationships between genes and DNA are best understood by mutation
studies. You have studied about mutation and its effect in Chapter 5. Effects
of large deletions and rearrangements in a segment of DNA are easy to
comprehend. It may result in loss or gain of a gene and so a function. The
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
statement, it would read as follows:
RAM HAS
BRE DCA P
Similarly, if we now insert two letters at the same place, say BI'. Now it
would read,
RAM HAS BIR EDC AP
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
2022-23
MOLECULAR BASIS OF INHERITANCE
Now try the opposite. Following is the sequence of amino acids code
d
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
yo
u now correlate which two
pr
op
erties o
f
ge
netic code
y
ou
ha
ve
deletion of one or two bases changes the reading frame from the point of
insertion or deletion. However
, such mutations ar
e
r
eferr
ed to as
202
2-2
3
111133
le
ar
nt
?
6.6.1 Muta
ti
ons an
d
Gene
ti
c Co
de
The relationships between genes and DNA are best understood
by
mutation
studie
s.
Y
ou
Y
Y
have
stud
ied
about mutation and its ef
fect in
ff
C
hapt
er
5
. Ef
fects
ff
of large deletions and rearrangements in a segment of DNA are easy to
comprehend. It may result in loss or gain of a gene and so a function. The
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
result
s
i
n
th
e
change of amino acid residue glutamate to valine.
It results in
to
a disease
d
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
m
ade up of the following words each
havi
ng
three letters like
ge
netic
co
de
.
RAM HAS RED CAP
If we insert a letter B in between HAS and RED and rearrange the
statement, it would read as follows:
RAM HAS
B
RE DCA P
Similarly, if we now insert two letters at the same
pl
ace, say BI'. Now it
would read
,
RAM H
AS
BI
R E
DC
AP
Now we insert th
r
ee letters together
, say BIG, the statement would r
ead
RAM HAS
BIG
RED CAP
The same exercise can be r
ep
eated, b
y
deleti
ng
the letters R, E and D,
one by one and rearranging the statement to make a triplet word.
RA
M
H
AS
ED
C
A
P
RAM HAS DCA P
RAM HAS CAP
T
he conclusion from
TT
the
above exercise is very
o
bvious. Insertion or
o
o
114
BIOLOGY
frameshift insertion or deletion mutations. Insertion or deletion of
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
that point onwards.
6.6.2 tRNA– the Adapter Molecule
From the very beginning of the proposition of code, it was clear to Francis
Crick that there has to be a mechanism to read the code and also to link it
to the amino acids, because amino acids have no structural specialities to
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.
tRNA has an
anticodon loop
that has bases
complementary to
the code, and it also
has an amino acid
acceptor end to
which it binds to
amino acids.
tRNAs are specific
for each amino acid
(Figure 6.12). For
initiation, there is
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.
6.7 TRANSLATION
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
joined by a bond which is known as a peptide bond. Formation of a
peptide bond requires energy. Therefore, in the first phase itself amino
acids are activated in the presence of ATP and linked to their cognate
tRNA a process commonly called as charging of tRNA or
aminoacylation of tRNA to be more specific. If two such charged tRNAs
are brought close enough, the formation of peptide bond between them
Figure 6.12 tRNA - the adapter molecule
2022-23
BIOLOGY
frameshift insertion
or
deletion mutations
or
. Insertion or deletion of
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
that point onwards.
6.6.2 tRNA– the Adapter Molecule
A
gh gh pep
202
2-2
3
111144
From
the
om
very beginning of the proposition of code, it was clear to Francis
Crick that there has to be a mechanism to read the code and also to link it
to the amino acids, because amino acids have no structural
specialities
t
o
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
befor
e the
ge
netic code was
p
ostulated. H
owever
, its r
ole
as an adapter molecule was assigned much later
.
er
er
tRNA has an
anticodon loop
that has bases
complementary to
the code, and it also
has an
amino acid
acceptor end
t
o
which it binds to
amino acids.
tRNAs are specific
for each amino acid
(Figure 6.12). For
initiation, there is
another specific tRNA that is referred to as
initiator tR
NA
. 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 structur
e, the
tRNA is a compact molecule which looks like inverted L.
6.
7
7
7
7
7
7
7
TTTT
T
T
T
RA
RA
RA
RA
RA
RA
RA
NSLA
TION
LA
LA
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
joined by a bond which is known as a peptide bond. Formation of a
peptide bond requires energy. Therefore, in the first phase itself amino
acids are activated in the presence of ATP and linked to their cognate
tRN
A
a process commonly called as
charging of tRNA
or
aminoacylation of tRNA
to be more specific. If two such charged tRNAs
NA
are brought close enough, the formation of peptide bond between them
Fi
gu
gu
gu
re
re
re
6
6
6
.1
.1
.1
2
tR
NA
NA
NA
- the a
da
da
pt
pt
pt
er
m
m
m
ol
ol
ol
ecule