1. FUNCTION OF GENETIC MATERIAL
 A gene is a section of the DNA molecule
which extends the length of the
chromosome, more or less at its centre, and
hence forms the core of a chromosome.
 The ultimate function of a gene in an
individual is to control and influence its
phenotype.

2.  However between the gene and the
ultimate phenotype of the individual
there occur many complex events.

3.  The main ideas to explain the mode by which
genes are able to exercise their control on
phenotypic expression has been
hypothesized as follows:
 All biochemical processes in all organisms
are controlled by genes.
 The biochemical processes proceed in series
of individual stepwise reactions.

4.  Each single reaction is controlled in a primary
fashion by a single gene, so that a one-to-
one correspondence exists between genes
and biochemical reactions.
 Mutation of a single gene results only in an
alteration in the ability of the cell to carry out
a single primary chemical reaction.

5.  Since all biochemical reactions are catalysed
by enzymes then the ultimate product of a
metabolic process may be considered to be
affected by a stepwise succession of
enzymes, each produced by a particular
gene.
 This hypothesis is strongly supported by
findings from various studies that certain
hereditary human defects are associated
with certain biochemical defects.
 The first known case of the genetic control of
a specific chemical reaction was found in a
rare metabolic disease in man called
alcaptonuria.

6.  The disease is characterised by the hardening
and blackening of the cartilage of the bones and
the blackening of urine on exposure to air.
 The blackening of urine is due to an
accumulation of alkapton or homogentisic acid,
which is an intermediate product of
phenylalanine or tyrosine metabolism.
 The metabolic pathway of phenylalanine
involves various intermediate steps each
controlled by a specific enzyme, and hence by a
specific gene.

7.  In a normal person an enzyme is present that
changes homogentisic acid to aceto-acetic acid,
and pyruvic acid which are clear in urine.
 An alcaptonuric individual, however, lacks this
enzyme, hence homogentisic acid accumulates
in abnormal amounts in urine.

9.  Apparently alcaptonuria is inherited as
a recessive trait, but the condition is
particularly manifested when
alcaptonuric compounds such as
phenylalanine and tyrosine are fed.

10.  In phenyl-ketonuria (an imbecility
disease) the affected individual lacks
the enzyme phenylalanine hydrolase
which is necessary for normal
metabolism of phenylalanine to
tyrosine.
 This leads to the accumulation of
abnormal metabolites (including
phenyl-pyruvic acid) in body tissues
leading to idiocy.

12.  Albinism is due to the lack of the enzyme
tyrosinase which converts tyrosine into
melanin.
 Thus when tyrosinase is not active, no
pigment is formed in the individual.

14.  The sickle-cell condition is due to the
substitution of glutamic acid by valine in one
position on the ß-polypeptide chain of
haemoglobin protein molecule.

15. α
β

16.  Similar findings have been demonstrated in
many organisms.
 On the basis of findings from numerous studies
it has been established that a mutation in any of
the genes controlling a metabolic pathway would
lead to a blockade of the pathway.

17.  The cause of the block is usually
localized to an impaired function of the
enzyme that is usually active at that
particular metabolic step.
 However, all other subsequent steps
may also be affected.

18. Proteins and Protein Derivatives
 Proteins are large molecules consisting of amino
acids linked together by peptide bonds that
connect the carboxyl group of one amino acid
with the amino group of another through loss of
a water molecule.

19.  Not only do different proteins differ with
respect to their molecular size, but also with
respect to the kinds of amino acids
composing the proteins as well as the
sequence of the amino acids, their
relationship and frequency.
 Thus although only about twenty amino
acids constitute most proteins the number of
different kinds of proteins that can be formed
is immensely large.

20. - COOH

21. •

28.  The amino acids are added one at a time in
the polymerization of proteins. i.e. the
assembly of proteins proceeds step by step
and in only one chemical direction.
 Protein synthesis begins at the amino (-NH2)
terminus and continues through to the
carbocyl (-COOH) terminus.

30.  In this way polypeptide chains
consisting of many amino acid units
are formed.

33.  One or several polypeptide chains may
constitute a protein molecule.

35.  Although the theoretical number of
amino acids is limitless only about 20
different acids are used in making
proteins.

36. Function of Proteins and their Derivatives
 Proteins are the active working components of
cellular machinery.
 As a matter of fact, except for its water
content, the major part of animal tissue is
composed of protein
 The skeleton of animal body is composed of a
protein matrix on which mineral compounds
are deposited.

37.  Muscles, cartilage, and many other body
tissues are composed largely of proteins.
 Furthermore, the enzymes which play a
major role in various metabolic processes
including digestion, cell respiration,
synthesis, etc, are essentially proteins.

38.  Haemoglobin, a constituent of red blood cells
which is responsible for oxygen supply to
body cells and carbon dioxide transport to
the lungs, is a protein.

40.  The various antibodies responsible for body
defense are essentially proteins.
 The hormones which play regulatory roles in
various physiological processes in the
animal body are all either proteins or protein
derivatives.

41. GENETIC CONTROL OF PROTEINS
The DNA molecule and Protein Synthesis
 It is well accepted now that the DNA controls
the expression of characters in organisms
through determination of the sequence of
amino acids in polypeptide chains, and hence
through determination of the secondary, tertiary
and quarternally structures of the chains, which
in turn determine the biochemical properties of
the proteins.

42.  i.e. the nucleotide sequence in the DNA
molecule forms a sequence of codes which
determine the order in which the amino acids
will be linked together to form a protein.
 In turn the the linear order of amino acids in
a protein molecule determines the function of
the protein.

43.  This proposition is especially appealing
since both the DNA and the polypeptide
chains are linear structures.
 While the DNA is composed of a linear
sequence of nucleotides, the polypeptide
chain consists of a linear sequence of amino
acids
 Therefore it should be expected that a linear
sequence of nucleotides should determine a
specific linear sequence of amino acids.

44.  Furthermore, a mutational change in a
particular position of the nucleotide
sequence should produce a change in a
corresponding linear position in the amino
acid sequence.
 Findings from various studies have shown
that most gene mutations cause single amino
acid substitutions.

45.  Amino acid substitution on a polypeptide chain
may have far-reaching effects on the organism
carrying the mutant gene because the
secondary and tertiary structure of the protein
in which amino-acid substitution has occurred,
and hence its biochemical properties may be
completely altered.
 The function of some proteins is so sensitive
that any change in primary amino acid structure
of the molecules leads to observable
phenotypic effects.

46.  However such sensitivity does not exist for
all proteins.
 i.e. some proteins seem to be completely
functional despite significant changes in
their amino acid sequence.

47.  Since the linear order of amino acids in a
protein molecule determines the function of
the protein it is important that the
mechanism for ensuring the order be very
accurate and precise.
 The design and function of the protein
synthesizing apparatus is similar in all cells
of an organism and in all organisms.

48.  Not only do different proteins differ with
respect to their molecular size, but also with
respect to the kinds of amino acids
composing the proteins as well as the
sequence of the amino acids, relationship
and frequency.
 Thus although only about twenty amino
acids constitute most proteins the number of
different kinds of proteins that can be formed
is immensely large.

49. TRANSFER OF GENETIC INFORMATION
 By the early forties it had been established that
while DNA was always confined only within the
nucleus RNA existed both in the nucleus and in
the cytoplasm.

50.  These findings led to the proposition that
RNA might be responsible for protein
synthesis.
 It was further observed that RNA occurred in
much larger amounts in high protein
producing cells (e.g. liver and pancreas cells)
than in low protein producing cells (e.g.
kidney, heart, and lung cells).

51.  The high protein producing cells had specific
cytoplasmic areas that stained densely with
basic dyes and absorbed ultraviolent
radiation at a wavelength similar to nucleic
acids.
 Also the enzyme that breaks down RNA (i.e.
ribonuclease) caused a termination in protein
synthesis and also removed the dark staining
areas in the cytoplasm.

52.  Later when methods of separating cellular
contents (or organelles) by lysis of the cells
followed by centrifussion, had been
developed it was shown that most of the RNA
was contained in the microsomes.
 Using labelling techniques with radioactive
material it was shown that the labelled amino
acids were rapidly assimilated in the
microsomes, and that the acids were
connected together by peptide bonds and
incorporated into proteins.

53.  Later the microsomal fraction was shown to
consist of granules called ribosomes, associated
with larger membranes called endoplasmic
reticulae.
 The ribosomes were shown to contain most of the
RNA and to perform protein synthesis.
 Further studies demonstrated that the ribosomes
were a protein factory in themselves.

54.  Today it is ribosomes are known to be complex
intra-cellular structures composed of individual
RNA molecules (rRNA) and more than 50 types
of proteins, all organized into two sub-units, a
large sub-unit and a smaller one.
 A ribosomal unit consists 40-60% ribosomal
RNA (rRNA) and the rest is protein.
 Both the proteins and RNA molecules differ in
the two sub-units, the large sub-unit
possessing a large rRNA molecule, and the
smaller sub-unit possessing a small rRNA
molecule.

56.  Further work pointed to a special form of RNA
(i.e. the messenger RNA or mRNA) as carrier of
genetic message from the gene located inside
the nucleus of the cell to the surrounding
cytoplasm where many of the proteins are
synthesized.

57.  As a result of numerous experimental
findings the process of protein synthesis has
now been well elucidated.
 The process is known to involve three kids of
RNA which play cooperative roles in linking
amino acids together in the correct linear
arrangement.

58.  Messenger RNA (mRNA) encodes genetic
information that is copied from DNA.
 The copying (transcription) of mRNA from a
DNA strand is achieved through the enzymatic
action of RNA polymerase.
 The information is in the form of a sequence of
bases that specifies a sequence of amino acids.
 The messenger RNA (mRNA) arranges itself on
an unoccupied ribosome.

65.  Another form of RNA, i.e. the ribosomal RNA
(rRNA) combines with many different
proteins to form ribosomes which provide
binding sites for all the interacting molecules
necessary for protein synthesis.
 Yet another form of RNA, i.e. the transfer
RNA (tRNA) decodes (translates) the base
sequence of the mRNA into the amino acid
sequence of a protein.

66.  The message for incorporation of amino
acids into proteins resides solely in the
nucleotide configuration of tRNA.
 tRNA molecules are short molecules about
70-80 nucleotides long and are of different
types.
 Each type is able to recognize one or more
of the several codons that can specify the
same amino acid.

67.  The transfer RNA (tRNA) seems to have a large
portion of its structure in the form of a double
helix, and also contains a number of rare bases
such as pseudouridine and inosine, as well as
some normal bases to which methyl groups have
been added.

68.  Some of the unusual nucleotides are
unable to form hydrogen bonds with
other bases and therefore produce
looped sections in which the double
helical structure of tRNA is interrupted.
 This gives the backbone of the tRNA
structure a stem-loop appearance
resembling a clover leaf.

70.  The tRNA performs its function by:
 Picking a specific amino acid from the medium
and carrying it to the mRNA;
 Attaching itself to the ribosome in accord with
the sequence of nucleotide bases specified by
mRNA.
 Protein formation then proceeds by linking the
amino acids carried by neighbouring tRNA
molecules.

71.  The translation of nucleotide sequence on
mRNA into a particular amino acid sequence
is achieved with the help of ribosomes.

72. The Protein Synthesis Process
 First the amino acids are activated
through their attachment to adenosine
tryphosphate (ATP), to form highly
reactive amino-acyl-phosphate-adenyl
groups.
 The enzymes involved in the formation
of these groups are usually highly
specific to particular amino acids.

73.  Thus each of the twenty amino acids has its
own activating enzyme or enzymes.
 Secondly free-floating transfer RNA
molecules (tRNA) become attached to the
amino acids and then transfer them to the
ribosomes.

77.  Again here there is a high degree of
specificity between the tRNA molecules and
the amino acids so that a certain type of
tRNA would attach to only a particular amino
acid.
 After the attachment of an amino acid
molecule to a tRNA molecule the adenyl
group is freed and the amino-acyl-tRNA
travels to the ribosome, where a messenger
RNA (mRNA) has been attached.

79.  The messenger RNA serves as a template for
the interconnection of different amino acids
that are carried to the template by transfer
RNA (tRNA) molecules to form a polypeptide
chain which, either singly or together with
other similar chains would constitute protein.

83.  After the formation of a polypeptide
chain has been completed both the
polypeptide chain and the messenger
RNA are detached from the ribosome
which then becomes free to pick up a
new messenger RNA.

87.  The clover leaf-like structure consists of
four base-paired stems and three loops, i.e.
the didryuridine (D-loop), the anti-codon
loop, and the TψGG loop.

88.  The anti-codon loop contains three
nucleotides that can form base pairs with the
nucleotides of a specific codon of the mRNA.
 The three nucleotides in tRNA are called the
anti-codon.
 They are complementary (not identical) to the
three nucleotides in the mRNA codon.

89.  Part of the ribonucleotide sequence of tRNA
is added after it comes off the DNA molecule
template.

90.  The addition consists of an identical
sequence of three nucleotides (A-C-C) which
are attached to all the different tRNA
molecules by a set of specific enzymes.
 One of the nucleotides in this terminal
sequence (i.e. adenine) serves as the point of
attachment to which a single amino acid is
covalently bonded by a particular amino-acid
activating enzyme.

91. Activation of tRNA
 There are at least 20 amino acid-specific
enzymes that recognize amino acids and
their compatible (or cognate) tRNAs.
 Each enzyme can attach one amino acid
molecule to the end of a cognate
(appropriate) tRNA.
 A given enzyme is capable of recognizing
different tRNAs for the same amino acid.

92.  These enzymes are called amino acyl-tRNA
synthetases.
 The amino acid is linked to the free 3'
hydroxyl group of the ribose of the terminal
nucleotide of the tRNA (adenosine).

93.  The reaction is:
1. Enzyme + amino acid + ATP ──── >
enzyme-amino-acyl-AMP +
Inorganic phosphate
2. tRNA + enzyme – amino-acyl-AMP ──>
amino-acyl-tRNA+AMP+enzyme.

94. Summary
 Amino acid + tRNA + enzyme + ATP ────>
Aminoacyl-tRNA + Enzyme + AMP +
InorgPhos
 AA + tRNA+ATP ────> AA-tRNA+AMP +
Inorganic Pyrophosphate.

95.  The amino acid residue is said to have
become activated and the tRNA is said to
have become amino-acylated.
 The overall process releases AMP and
inorganic pyrophosphate.

96.  The basis of the specificity between a tRNA
molecule and its cognate tRNA synthetase is
probably due to their three dimensional
structures.
 The fact that one enzyme can add the same
amino acid to different tRNAs with different
anti-codons suggests that the respective
tRNAs must contain similar binding sites for
the synthetase.

97. • Three nucleotides of each tRNA molecule are
used for coding purposes to pair with the
triplet sequences of messenger RNA
(mRNA).

98.  The set of three nucleotides which pair with a
particular triplet on the mRNA is called an
anticodon.
 The location of the anti-codon is probably in
one of the exposed positions on tRNA
molecule, probably in one of the unpaired
loops.

99.  A tRNA molecule carrying a specific amino
acid aligns itself on the ribosome, pairing its
anticodon with the codon of mRNA.
 Next, a new mRNA codon is brought into
position and a new tRNA molecule bearing
an amino acid is positioned next to the
previous tRNA.

100.  Then the amino acid carried by the previous
tRNA molecule is removed and linked
through a peptide bond to the amino acid
carried by the second tRNA molecule with
the help of some enzymes and the energy
rich molecule guanosine triphsphate (GTP).

101.  Having lost its amino acid the previous tRNA
molecule is released from the ribosome.
 In subsequent steps the linked amino acid
chain is transferred to new tRNA molecules
which have been attached to the ribosome.
 When the peptide chain is completed the
ribosome detaches from the mRNA and the
polypeptide chain is released from the last
tRNA.

102.  Also the last tRNA molecule is released
from the ribosome.

103.  The attachment of amino acids to their
cognate tRNAs is a very critical stage in
protein synthesis because once the tRNAs
are loaded with the correct amino acids the
accuracy of protein synthesis depends only
on the base pairing between anti-codons on
the tRNAs and the codons on the mRNA.
 A tRNA specific to a particular amino acid is
designated as tRNA-AA where AA is the
amino acid concerned.

104.  If an amino acid residue which is already
attached to its cognate tRNA is chemically
changed into some other amino acid residue,
the altered amino acid will still be added to
the growing chain at the position where the
cognate tRNA for the original amino acid
would add it.

105.  The ribosome is important for proper
pairing between the three nucleotides
constituting the anti-codon of the tRNA
and the three nucleotides constituting
the codon of the mRNA, and hence for
the stabilization of trinucleotide
attachment between mRNA and tNRA,
otherwise such attachment would not
be sufficiently strong or stable to
permit the amino acids carried by tRNA
to become linked together in peptide
formation.

106. Role of Ribosomes
 The critical function of protein synthesis
would be very slow if the interacting
components had to react in free solution
since simultaneous collisions between the
necessary components of the reaction would
be rare.

107.  Instead the mRNA with its encoded
information and the individual tRNAs already
loaded with their correct amino acids are
brought together by their mutual binding to
ribosomes.
 Thus the most important role played by the
ribosome is to bind reversibly with both
mRNA and tRNA.

109. Sequence and rate of protein synthesis
 Experimental evidence indicates that protein
synthesis occurs sequentially from one
particular end of the chain to the other.
 It appears that growth starts at the amino end
(N-terminal) of the polypeptide chain and
continues towards the carboxyl end (C-
terminal).

110.  There is evidence suggesting that the
sequential order of polypeptide synthesis
follows the order of synthesis of the mRNA
molecule itself.
 i.e. protein synthesis may begin even before
synthesis of the mRNA is completed through
the attachment of ribosomes to mRNA chain
as it is coming off the DNA template.

112.  The length of a polypeptide chain translated
on a particular mRNA may not necessarily
correspond with the nucleotide length of the
mRNA molecule.
 Instead, several polypeptide chains may be
translated on different sections of a mRNA
molecule.

113. SYNTHESIS OF PROTEINS
Rules for synthesis of proteins
Proteins are made up of a limited number of
different amino acids.
Although the theoretical number of amino acids
is limitless only about 20 different acids are used
in making proteins.

114. Protein Synthesis
 While DNA directs the synthesis of RNA,
which in turn directs the synthesis of protein,
special proteins catalyse the synthesis of
both RNA and DNA.
 i.e. there is a cyclic flow of information in the
cell.
• DNA────>RNA ───>Protein
• │ │ │
• └───────┴─────────┘

115.  Of the 64 possible codons under the 3-base
code model only 3 do not specify amino
acids.
 Since there are 61 codons for 20 amino acids
many amino acids are coded by more than
one codon.

117.  Occasionally the DNA sequence may contain
overlapping information still in a triplet code.
 Since it is possible to shift the reading frame
for any set of triplets by moving the starting
point for translation either one or two bases
in either direction, two or three different
amino acid sequences can be encoded by
the same region of the nucleic acid chain.

118.  Overlapping triplets read in two different
frames - although the mRNA is the same
sequence in both lines the sequence of
amino acids coded in the region are very
different.

119.  The different codons for a given amino acid
are said to be synonymous and the code
itself is said to be degenerate - meaning that
it contains redundancies.
 Since each triplet codes for only one amino
acid there is no ambiguity in the translation
of amino acids, except for GUG which apart
from coding for the amino acid valine, may
occasionally also code for methiomine.

120.  AUG is the most common initiator or
start codon specifying the amino acid
methionine, while UAA, UAG and UGA
act as termination codons.
 All protein chains in prokaryotic and
eukaryotic cells begin with methionine.

121.  The three codons UAA, UAG, and UGA do
not specify any amino acids, and hence
constitute termination (top) signals at the
ends of protein chains.
 Therefore a precise linear arrangement of
nucleotides grouped into triplets in the
mRNA specifies, not only the linear sequence
of amino acids in a protein, but also signals
to ribosomes where to start and stop
synthesis of a protein chain.

122. Summary of degeneracy of codes.
Amino acids Coded Codes per amino acid No. of codes
Arg, Leu and Ser 6 codes each x 3 = 18
Ala, Gly, Pro, Thr and 4 codes each x 5 = 20
Val
Ile 3 codes x 1 = 3
Asn, Asp, Cys, Gln, 2 codes each x 9 = 18
Glu, His, Lys, Phe, and
Tyr
Meth and Trp 1 code each x 2 = 2
Total number of codes 61
for amino acides
Number of codes not 3
coding for amino acids

123.  In the synthesis of a polypeptide chain the
protein synthesizing system uses the tRNA
to translate or adapt the information in each
mRNA code word so that the appropriate
amino acid is added to the chain.
 The adaptor molecule must recognize
 First, a codon in mRNA
 Second, an amino acid matching the codon

124.  The adaptor function is performed by a tRNA
molecule to which an amino acid molecule is
attached at one end to form an aminoacyl-
tRNA complex.
 The correct amino acyl-tRNA molecule binds
to the codon on the mRNA strand and
transfers its attached amino acid to the
polypeptide chain growing there.

125.  The structure of a tRNA molecule always
ends in CCA. The amino acid is attached to
the 3' hydroxyl group of the terminal
nucleotide (i.e. adenosine).

126.  In solution the tRNA molecules are folded
into three dimensional structures.
 The backbone of the structure is a stem-loop
structure resembling a clover leaf.
 The four stems are stabilized by base
pairing.

127.  Three of the four stems end in loops.
 The stem-loop structure is then folded into
an L-shaped three-dimensional form.
 Hydrogen bonds help to maintain the
molecule's shape.

128.  The tRNA bases are highly modified after
tRNA is synthesized.
 The most frequent modification is the
addition of a methyl group to specific bases.

129.  Most tRNA molecules are synthesized with a
four-base sequence of UψCG near the middle
of the molecule.
 The first U-nucleotide is methylated to
become a thymine (thymidine) nucleotide
while the (uridine) U-nucleotide is rearranged
into a pseudo uracil nucleotide in which the
sugar is attached to a carbon instead of to a
nitrogen.

130.  These modifications produce a characteristic
TψCG segment which is localed in an
unpaired region at about the same position in
nearly all tRNAS.
 A clover leaf-like structure consisting of four-
base-paired stems and 3 loops - the
didrydrouridine loop (D-loop), the anti-codon
loop, and the TψCG loop.

131.  Although the exact role of the tRNA
modifications is not yet well understood the
fact that certain sites on the tRNA structure
are frequently modified in similar ways
suggests that these sites have a common
role in protein synthesis.
 The constant features are the D loop, the
TψCG loop, and the anti-codon loop.

132.  If perfect base pairing was required for codon-anti-
codon pairing 61 different tRNA types (one for each
codon) would have been necessary.
 But this is not the case.
 Rather, tRNA molecules with same anti-codon
sequence are capable of recognizing more than one
codon corresponding to a particular amino acid.
 This is possible due to wobble (or non-standard)
base pairing between the third position of the codon
and its partner in the anticodon. Certain
combinations of two bases form interactions

133.  This is possible due to wobble (or non-
standard) base pairing between the third
position of the codon and its partner in the
anti-codon.
 Certain combinations of two bases form
interactions
 For example A-U, G-C and several other
combinations of two bases form interactions
that are stable enough to allow codon
recognition in the wobble position.

134.  e.g. whereas the condon (5')UUU(3') in mRNA
calls for phenylalanine-tRNA (Phe-tRNA Phe)
the anti-codon in the Phe-tRNA-Phe could be
either (3') AAA(5'), (3')AAG)5') or (3') AAI(5').
 Inosine modified nucleoside (base) in which
amine group of guanine has been substituted
by a hydrogen atom) - a guanosine analogue
that lacks an amino group at the No. 2 carbon
position.

135.  This is because bonds between U and G or
between U and I in the wobble position No. 1
tRNA with inosine in the wobble position can
decode three different codons.
 It is hyphothesized that the effect of the
wobble in the third position is to speed up
protein synthesis by the use of alternative
tRNAs.

138. Protein Synthesis
 The process of protein synthesis may be looked
at in three stages: i.e.
 Initiation;
 Elongation and
 Termination.
 Each of these processes involves distinct
biochemical events.

139. Initiation
 It seems that the AUG codon of the mRNA is
the initiation signal for polypeptide growth.
 This codon codes for methionine.
 Thus the first event of the initiation stage in
the synthesis of any protein is the
attachment of a free methionine molecule to
the end of a tRNA met with the help of
methionly-tRNA met synthetase.

140.  The Met-tRNA Met so formed together with a
molecule of GTP and the smaller ribosomal
submit bind to the mRNA (with the help of
initiation proteins- initiation factors) at a
specific site near the AUG initiation code.
 Note that although there may be AUG codons
in other places along the mRNA molecule,
protein synthesis always begins at the
correct AUG near the ribosomal binding
sites.
 Translation then proceeds in the 5' ── 3'
direction along the mRNA.

141.  It seems that the recognition of AUG
initiation sites is due to the high affinity for
ribosomes by the mRNA base sequences just
preceding the codons.
 An initiation factor first binds a GTP
molecule and a molecule of Met-tRNA Met to
form a complex which then binds to mRNA
and the small ribosomal submit.

142.  Other initiation factors then joint to make an
initiation complex.
 These processes position the Met-tRNA met
correctly at the AUG initiation code.

143. Summary
 Met-tRNA +GTP+Ribosome + mRNA initiation
factor ────>
Met-tRNA-Ribosome-mRNA + initiation
factor + inorganic phosphate

144.  After the complex of Met-tRNAMet, GTP and
the small ribosomal submit is correctly
bound to the mRNA at the initiation site the
large ribosomal submit joins the complex.
 This is followed by the hydrolysis of the GTP
to GDP and inorganic phosphate, and the
detachment of the initiation factors from the
complex, leaving the Met-tRNAMet bound at
the P site of the large ribosomal submit.

145. Elongation
 A second amino acid that is correctly bound to
its cognate tRNA is then brought into the
second binding site (the A site) on the ribosome
which positions the second tRNA at the
appropriate codon of the mRNA.
 A peptide bond is then formed between the
carboxyl group of the Met-tRNA Met and the
amino group of the incoming aminoacyl-tRNA-
AA2.

146.  The tRNA-Met then vacates the P binding site
of the ribosome into the medium, leaving
behind the methionyl-aminoacyl-tRNA-AA2
(the peptidyl-tRNA-AA2) on the ribosome.
 In the meantime the peptidyl-tRNA-AA2
vacates the A site to the P site.
 The cycle is repeated for the addition of each
amino acid, until all the amino acids encoded
by the mRNA have been added.

147.  In each translation step the ribosome and its
attached peptidyl-tRNA move three
nucleotides closer to the 3' end of the mRNA.
 i.e. advance one colon on the mRNA.
 This movement is probably achieved through
the change in the configuration of some
proteins of ribosome or in the configuration
of RNA thus propelling the mRNA through
the ribosome.
 The energy of GTP is probably used in the
propulsion.

148.  Since some of the hydrogen bonds existing
in rRNA are between distant nucleotides the
breakage and restitution of these bonds
might be responsible for contraction and
relaxation cycles which cause the folding of
the ribosome to change, thus causing
translation of the ribosome to occur.
 Note: The major role of the ribosome is to
offer binding sites to amino-acyl- tRNA in
such a way that the correct codon-anti-codon
match is made.

149. Termination
 When the UAG or UGA or UAA codon is
encountered on the mRNA the protein
termination factors cause the peptidly-tRNA
complex to be hydrolysed and released from
the ribosome and the complex splits
instantaneously into an uncharged tRNA
molecule and newly completed protein chain.
 After releasing its peptidyl-tRNA the
ribosome disintegrates from the mRNA and
divides into its two submits.

150.  Experimental evidence has shown that a
segment of about 35 amino acids long of the
protein chain being synthesized is embedded
within the ribosome structure at any time
before the synthesis of the chain is
completed.
 Therefore the chain starts to emerge from the
ribosome only after it has grown more than
35 amino acids long.
 The protein secreted from the cell may go
directly through the cell membrane,
suggesting that the exit site on the ribosome
may be bound to the cell membrane.

151. Suppression of non-sense mutations
 Since UGA, UAA and UAG normally code for
chain termination a mutation in a gene could
produce an abnormal termination signals,
causing the translation apparatus to stop too
soon.
 This type of mutation is called a non-sense
mutation.
 It is to be distinguished from a mis-sense
mutation which would cause an amino acid
to be substituted for another.

152. • The chain terminating mutations on the mRNA
are correctable by other mutations on the tRNA.
 These are called suppression mutations.
 Suppressor mutations cause the reading of the
chain-terminating codon on the mRNA to be as a
codon for an amino acid.
 This is brought by a mutation in the anti-codon
of a tRNA leading to the production of a low
frequency of misinterpretation of stop signals.

153.  This would allow chain synthesis to
continue.
 Due to the existence of suppressor tRNAs
the 3' ends of coding regions in mRNA often
contain more than one stop codons within a
short stretch, giving the protein synthesis a
fail-safe mechanism.

154.  Each chain has a specific starting point, and
growth proceeds in one direction to a fixed
terminus.
 There are elaborate cellular mechanisms for
starting and stopping the process correctly.
 The primary synthetic product is usually
modified.
 The functional form of a protein molecule is
rarely the same length as the initially
synthesised form.

155.  For example methyl groups can be added to
specific sites of proteins.
 Also phosphate groups and a wide variety of
polysaccharides can be added to proteins.

156.  The original chain is often inactive or
incomplete.
 Through the action of enzymes the original
chain is trimmed down, linked to another
chain, or even cut apart and reassembled
from selected pieces to make a fully active
chain.
 Primary chains may also undergo certain
chemical additions either during their
formation or after synthesis is complete.

157.  Findings from studies with lower forms of life
have indicated the existence of a close
linkage of genes controlling the production
of enzymes for a particular metabolic
pathway.
 However such a correspondence between
the sequence of genes and that of enzymes
catalysing steps of a metabolic pathway is
lacking in higher forms of life.