Molecular biology dna, rna, rep, trancr, transl (autosaved)

DNA- RNA-REPLICATION-
MUTATION-TRANSCRIPTION-
TRANSLATION
Handouts-Molecular Biology
SP

DNA- RNA-Replication-Mutation-Transcription- Translation 1
STRUCTURE OF DNA
DNA is a polymer of deoxyribonucleotides and is found in chromosomes and mitochondria. The
nuclear DNA is found bound to basic proteins called histones. The two polydeoxyribonucleotide
chains (strands) twisted around each other on a
common axis.
The two strands are antiparallel, i.e., one strand runs in the
5’to 3’direction while the other in 3’to 5’direction.
The width (or diameter) of a double helix is 20 A° (2 nm).
Each turn (pitch) of the helix is 34 A° (3.4 nm) with 10 pairs
of nucleotides, each pair placed at a distance of about 3.4 A°.
Each strand of DNA has a hydrophilic deoxyribose phosphate
backbone (3’-5’phosphodiester bonds) on the outside
(periphery) of the molecule while the hydrophobic bases are
stacked inside (core).
The two polynucleotide chains are not identical but
complementary to each other due to base pairing.
The two strands are held together by hydrogen bonds
formed by complementary base pairs. The A-T pair has 2
hydrogen bonds while G-C pair has 3 hydrogen bonds.
The G C is stronger by about 50% than A = T.
The hydrogen bonds are formed
between a purine and a pyrimidine
only. The only base arrangement
possible in DNA structure, from spatial
considerations is A-T, T-A, G-C and C-
G.
The complementary base pairing in
DNA helix proves Chargaff’s rule.
The content of adenine equals to that
of thymine (A = T) and guanine equals
to that of cytosine (G = C).

The genetic information resides on one of the two strands known as template strand or sense strand.
The opposite strand is antisense strand. The double helix has major grooves and minor grooves along
the phosphodiester backbone. Proteins interact with DNA at these grooves, without disrupting the base
pairs and double helix.
Functions of DNA [Refer the textbooks for additional points]
• DNA is the chemical basis of heredity and may be regarded as the reserve bank of genetic
information.
• DNA is exclusively responsible for maintaining the identity of different species of organisms
over millions of years.
• Further, every aspect of cellular function is under the control of DNA.
• The DNA is organized into genes, the fundamental units of genetic information. The genes
control the protein synthesis through the mediation of RNA
DENATURATION OF DNA
The two strands of DNA helix are held together by
hydrogen bonds. Disruption of hydrogen bonds (by
change in pH or increase in temperature) results in the
separation of polynucleotide strands. This phenomenon
of loss of helical structure of DNA is known as
denaturation.

STRUCTURE OF RNA
Ribonucleic acid is a polymer of ribonucleotides of Adenine, Uracil, Guanine and Cytosine, joined together by
3‟ – 5‟ phosphodiester bonds. Thymine is absent in RNA. The RNAs are synthesized from DNA, and are
primarily involved in the process of protein biosynthesis. The three major types of RNAs.
1. Messenger RNA (mRNA)
2. Transfer RNA (tRNA)
3. Ribosomal RNA (rRNA)
Messenger RNA (mRNA)
The mRNA is synthesized in the nucleus (in eukaryotes) as heterogeneous nuclear RNA (hnRNA).
hnRNA, on processing, liberates the functional mRNA which enters the cytoplasm to participate in protein
synthesis.. The m-RNA molecules are formed with the help of DNA template strand (3’ – 5’) during the
process called transcription. The m-RNA carries a specific sequence of nucleotides in “triplets” called codons,
responsible for the synthesis of a specific protein molecule.
The eukaryotic mRNA is capped at the 5’-terminal end by 7-
methylguanosine triphosphate. It is believed that this cap helps
to prevent the hydrolysis of mRNA by 5’-exonucleases.
Further, the cap may be also involved in the recognition of mRNA
for protein synthesis.
The 3’-terminal end of mRNA contains a polymer of adenylate
residues (20-250 nucleotides) which is known as poly (A) tail.
This tail may provide stability to mRNA, besides preventing it from
the attack of 3’-exonucleases.
Transfer RNA (tRNA)
There are at least 20 species of tRNAs, corresponding to 20
amino acids present in protein structure. They remain largely in
cytoplasm. The t-RNAs are relatively small, single-stranded,
globular molecules. tRNA contains mainly four arms, each arm with
a base paired stem.
1. The acceptor arm: This arm is capped with a sequence CCA
(5’to 3’). The amino acid is attached to the acceptor arm. The 3‟ –
OH terminal of adenine may bind with the -COOH of a specific
amino acid and carry the latter as an aminoacyl-t-RNA complex to
ribosomes for protein synthesis.
2. The anticodon arm: This arm, with the three specific
nucleotide bases (anticodon), is responsible for the
recognition of triplet codon of mRNA. The codon and
anticodon are complementary to each other.
3. D arm: The third is the D-arm because it contains the
base dihydrouridine.
4. TΨC arm: Contains thymine, pseudouridine and cytosine.
5. Variable arm or extra arm: Extra arm is most variable
structure of t-RNA and it forms the basis of its classification
(a) Class I t-RNA: About 45 per cent of all t-RNA belong
to this class and have 3-5 base pairs in its extra arm, e.g.
Ala-t-RNA.
(b) Class II t-RNA: This form about 25 per cent of total t-
RNA and has 13-21 base pairs in a long chain, e.g. Phe-t-
RNA.
Ribosomal RNA (rRNA)
The ribosomes are the factories of protein synthesis. The
eukaryotic ribosomes are composed of two major
nucleoprotein complexes–60S subunit and 40S subunit.
The 60S subunit contains 28S rRNA, 5S rRNA and 5.8S
rRNA while the 40S subunit contains 18S rRNA. The
function of rRNAs in ribosomes is not clearly known. It is believed that they play a significant role in the
binding of mRNA to ribosomes and protein synthesis.
CODONS / GENETIC CODE
The three nucleotide (triplet) base
sequences in mRNA that act as code
words for amino acids in protein constitute
the genetic code or simply codons. The
codons consist of the four nucleotide bases,
the purines—adenine (A) and guanine (G),
and the pyrimidines—cytosine (C) and uracil
(U).
These four bases produce 64 different
combinations (4
3
) of three base codons. The
three codons UAA, UAG and UGA do not
code for amino acids. They act as stop
signals in protein synthesis. These three
codons are collectively known as
termination codons. The codons AUG—
and, sometimes, GUG— are the chain
initiating codons.
[Note:-See below for characteristics of
Genetic Code ]

Characteristics of genetic code
[Cont. of Genetic Code/ Codon]
The genetic code is universal, specific, non-overlapping and degenerate.
1. Universality: The same codons are used to code for the same amino acids in all the living organisms.
Thus, the genetic code has been conserved during the course of evolution. Hence genetic code is
appropriately regarded as universal.
2. Specificity: A particular codon always codes for the same amino acid, hence the genetic code is highly
specific or unambiguous e.g. UGG is the codon for tryptophan.
3. Non-overlapping: The genetic code is read from a fixed point as a continuous base sequence.
It is non-overlapping, commaless and without any punctuations. For instance, UUUCUUAGAGGG is read
as UUU/CUU/AGA/GGG. Addition or deletion of one or two bases will radically change the message
sequence in mRNA. And the protein synthesized from such mRNA will be totally different. This is
encountered in frameshift mutations which cause an alteration in the reading frame of mRNA.
4. Degenerate: Most of the amino acids have more than one codon. The codon is degenerate or
redundant, since there are 61 codons available to code for only 20 amino acids. For instance, glycine has
four codons. The codons that designate the same amino acid are called synonyms. Most of the synonyms
differ only in the third (3’end) base of the codon.
REPLICATION OF DNA
Replication is a process in which DNA copies itself to produce identical daughter molecules of DNA.
Replication is of two types:
1. Conservative
2. Semi-conservative
1. Conservative replication: In conservative replication the
parental strands never completely separate. Thus, after one
round of replication, one daughter duplex contains only parental
strands and the other only daughter strands.
2. Semiconservative replication: The process of unwinding of
the double-helical molecules, each of which is composed of a
parental strand and a newly synthesized strand formed from the
complementary strand. This is called semiconservative
replication.

SEQUENTIAL EVENTS OF DNA REPLICATION
The steps involved in DNA replication in Eukaryotes can be arbitrarily divided into five steps for better
understanding.
They are:
A. Identification of sites of origin of replication (ori) and formation of bubble
B. Unwinding of dsDNA to provide ssDNA which can act as template
C. Formation of the replication fork
D. Initiation and chain elongation
E. Ligation of the newly synthesised DNA segments.
A. Identification of Site of the Origin of Replication
There are specific sites, called origin of replication (ori); where replication starts. At the origin of replication,
there is an association of sequence-specific DNA binding proteins with a series of direct repeat DNA sequences.
Adjacent to „ori„ is A + T rich region. In eukaryotes, there are multiple sites of origin. Specific protein called
dna A binds with the site of origin for replication. This causes the double-stranded DNA to separate.
Replication bubbles
The two complementary strands of DNA
separate at the site of replication to form a
bubble. Multiple replication bubbles are formed
in eukaryotic DNA molecules, which is
essential for a rapid replication process.

B. Unwinding of DNA to Form ssDNA Which Act as Template
Main critical enzyme which helps in the unwinding is DNA helicase which allows for processive
unwinding of DNA. Single-strand binding proteins (SSB proteins) binds to each ssDNA strand and stabilise
the complex and prevents re-annealing.
Torsional strain by DNA helicase produces Nicks in one strand of unwinding double helix (dsDNA) thereby
allowing the unwinding process to proceed. The “nicks” are quickly resealed by the nick-sealing enzyme, called
DNA topoisomerases
C. Formation of the Replication Fork
RNA primer
For the synthesis of new DNA, a short fragment of RNA is required as a primer.
The enzyme primase (a specific RNA polymerase) in association with single-stranded binding proteins
forms a complex called primosome, and produces RNA primers. A constant synthesis and supply of RNA
primers should occur on the lagging strand of DNA. This is in contrast to the leading strand which has
almost a single RNA primer.
DNA synthesis is semidiscontinuous and bidirectional
The replication of DNA occurs in 5’ to 3’ direction, simultaneously, on both the strands of DNA.
On one strand, the leading (continuous strand—the DNA synthesis is continuous. On the other strand,
the lagging (discontinuous) strand—the synthesis of DNA is discontinuous.
Short pieces of DNA (15-250 nucleotides) are produced on the lagging strand.
In the replication bubble, the DNA synthesis occurs in both the directions (bidirectional) from the point of
origin.
The separation of the two strands of parent DNA results in the formation of a replication fork. The active
synthesis of DNA occurs in this region. The replication fork moves along the parent DNA as the daughter
DNA molecules are synthesized.
DNA helicases: These enzymes bind to both the DNA strands at the replication fork. Helicases move
along the DNA helix and separate the strands. Their function is comparable with a zip opener. Helicases
are dependent on ATP for energy supply.

Single-stranded DNA binding (SSB) proteins: These are also known as DNA helix-destabilizing
proteins. They possess no enzyme activity. SSB proteins bind only to single-stranded DNA (separated by
helicases), keep the two strands separate and provide the template for new DNA synthesis. It is believed
that SSB proteins also protect the single-stranded DNA degradation by nucleases.
DNA synthesis catalysed by DNA polymerase III
The synthesis of a new DNA strand, catalysed by DNA polymerase III, occurs in 5’3’ direction. This is
antiparallel to the parent template DNA strand. The presence of all the four deoxyribonucleoside
triphosphates (dATP, dGTP, dCTP and dTTP) is an essential prerequisite for replication to take place.
The synthesis of two new DNA strands, simultaneously, takes place in the opposite direction—one is in a
direction (5’3’) towards the replication fork which is continuous, the other in a direction (5’3’) away from
the replication fork which is discontinuous. The incoming deoxyribonucleotides are added one after
another, to 3’ end of the growing DNA chain.

D. Initiation and Elongation of DNA
The initiation of DNA synthesis requires priming by short
length of RNA (RNA primer).
The synthesis of new DNA strand continues till it is in close
proximity to RNA primer. Now the
DNA polymerase I come into picture. It removes the RNA
primer and takes its position. DNA polymerase
I catalyses the synthesis (5’’ direction) of a fragment of
DNA that replaces RNA primer.
The incoming deoxyribonucleotides are added one after
another, to 3’ end of the growing DNA chain. [See the
diagram below]
DNA polymerase III also has a proof-reading activity. It
checks the incoming nucleotides and allows only the
correctly matched bases (i.e. complementary bases) to be
added to the growing DNA strand. Further, DNA polymerase
edits its mistakes (if any) and removes the wrongly placed
nucleotide bases.
E. Ligation of the Newly Synthesised DNA Segments
The enzyme DNA ligase catalyses the formation of a phosphodiester linkage between the DNA
synthesized by DNA polymerase III and the small fragments of DNA produced by DNA polymerase I
[Note:]
REPLICATION IN EUKARYOTES
Replication of DNA in eukaryotes closely
resembles that of prokaryotes. Certain differences,
however, exist. Multiple origins of replication is
a characteristic feature of eukaryotic cell. Further,
at least five distinct DNA polymerases are
known in eukaryotes. Greek letters are used to
number these enzymes.

Antimetabolites and Antibiotics- interfering in replication
Antimetabolites: are drugs that interfere with one or more enzymes or their
reactions that are necessary for DNA synthesis.
• Purine antagonist: Mercaptopurine
• Pyrimidine antagonist: 5-fluorouracil
Mercaptopurine is an inhibitor of the synthesis of AMP and GMP. It acts on the
enzyme adenylsuccinase (of AMP pathway) and IMP dehydrogenase (of GMP
pathway).
• Mercaptopurine is therefore a clinically useful anticancer agent. The chemotherapeutic
effectiveness of mercaptopurine is enhanced when it is administered with allopurinol.
• 5-Fluorouracil is also effective antitumor agents. It blocks nucleotide synthesis.
• Arabinosylcytosine is being used in cancer therapy as it interferes with DNA replication
Methotrexate, hydroxyurea, fludarabine, decitabine etc.
Antibiotics
• Quinolones group of antibiotics that interfere
with DNA synthesis by inhibiting
topoisomerase II (DNA gyrase), an enzyme
involved in DNA replication. Eg. norfloxacin
and ciprofloxacin, Nalidixic acid, campthoterin, ,
amsacrime and etoposide for topoisomerase I in
the treatment of cancers
• Coumermycins and novobiocin (Anti-Bacterial)
• Adriamycin, Doxorubicin (Anticancer)

MUTATIONS
Definition: Mutation is replacement of nitrogen base with another in one or both the strands or addition or
deletion of a base pair in a DNA molecule.
Mutagens: The substances which can induce mutations are collectively known as mutagens. These can be
chemicals, radiations or viruses.
The changes that occur in DNA on mutation are reflected in replication, transcription and translation.
Types of mutations
Mutations are of two major types:
A. Point mutation
B. Frame shift mutation.
Point mutation can be Transitions or Transversions
Transversions can be:
1. Silent mutation
2. Missense mutation
3. Nonsense mutation.
A. Point mutations: The replacement of one base pair by another results in point mutation.
They are of two sub-types.
(a) Transitions: In this case, a purine (or a pyrimidine) is replaced by another.
(b) Transversions: These are characterized by replacement of a purine by a pyrimidine or vice versa.

Consequences: Single base changes in the m-RNA molecules may have one of several effects as stated below
when translated into proteins. These changes may be:
1. Silent mutation (No detectable effect).
2. Missense mutation (Missense effect).
3. Nonsense mutation (Nonsense effect).
1. Silent mutation: The codon (of mRNA) containing the changed base may code for the same amino
acid.
For instance, UCA codes for serine and change in the third base (UCU) still codes for serine. This is due to
degeneracy of the genetic code. Therefore, there are no detectable effects in silent mutation.
2. Missense mutation: In this case, the changed base may code for a different amino acid.
For example, UCA codes for serine while ACA codes for threonine. The mistaken (or missense) amino acid
may be acceptable, partially acceptable or unacceptable with regard to the function of protein molecule.
Sickle-cell anemia is a classic example of missense mutation.
3. Nonsense mutation: Sometimes, the codon with the altered base may become a termination (or
nonsense) codon.
For instance, change in the second base of serine codon (UCA) may result in UAA. The altered codon acts
as a stop signal and causes termination of protein synthesis, at that point.
B. Frame Shift Mutations
Frame shift mutations can be of two types:
1. Deletion type
2. Insertion type
These occur when one or more base pairs are inserted
in or deleted from the DNA, respectively, causing
insertion or deletion mutations.
The insertion or deletion of a base in a gene results in an altered reading frame of the mRNA (hence the
name frameshift). The machinery of mRNA (containing codons) does not recognize that a base was
missing or a new base was added. Since there are no punctuations in the reading of codons, translation
continues.
The result is that the protein synthesized will have several altered amino acids and/or prematurely
terminated protein.
REPAIR OF DNA
As already stated, damage to DNA caused by replication errors or mutations may have serious consequences.
The cell possesses an inbuilt system to repair the damaged DNA. This may be achieved by four distinct
mechanisms.

TRANSCRIPTION
Transcription is a process in which ribonucleic acid (RNA) is synthesized from DNA.
The word gene refers to the functional unit of the DNA that can be transcribed. Thus, the genetic
information stored in DNA is expressed through RNA. For this purpose, one of the two strands of DNA
serves as a template and produces working copies of RNA molecules.
The product formed in transcription is referred to as primary transcript. Most often, the primary RNA
transcripts are inactive. They undergo certain alterations (splicing, terminal additions, base modifications
etc.) commonly known as post-transcriptional modifications, to produce functionally active RNA
molecules.
The process of transcription can be divided into four stages:
1. Formation of transcription complex (of DNA and RNA polymerase)
2. Initiation
3. Elongation and
4. Termination.
In eukaryotes, a sequence of DNA bases is identified. This sequence, known as Hogness box (or TATA
box), is located on the left about 25 nucleotides away from the starting site of mRNA synthesis.
There also exists another site of recognition between 70 and 80 nucleotides upstream from the start of
transcription. This second site is referred to as CAAT box.
One of these two sites (or sometimes both) helps RNA polymerase II to recognize the requisite sequence
on DNA for transcription.
Initiation
RNA polymerase binds to DNA at Promoter region is the prerequisite for the transcription to
start.
• There are two base sequences on the coding DNA strand which the sigma factor of
RNA polymerase-I, II or III can recognize for initiation of transcription
1. Hogness box (or TATA box)
2. CAAT box

Initiation of transcription
Three stages
1. Chromatin containing the promoter sequence made accessible to the
transcription machinery.
2. Binding of transcription factors (TFs)- TFIID, TFIIA, TFIIB, TFIIF, TFIIE, TFIIH),
to DNA sequences in the promoter region.
3. Stimulation of transcription by enhancers.
• Enhancer can increase gene expression. This is made possible by binding
of enhancers to transcription factors to form activators.
• The primary mRNA transcript produced by RNA polymerase II
in eukaryotes is often referred to as heterogeneous nuclear
RNA (hnRNA). This is then processed to produce mRNA needed

Termination of Transcription
• The specific signals are recognized by a termination protein, the Rho factor.
• Attachment of Rho factor is ATP dependent process. When it attaches to the DNA,
the RNAP cannot move further. So, the enzyme dissociates from DNA and
consequently newly formed mRNA is released
• A G-C rich palindrome sequence precedes the sequence of 6-7 U residues in the RNA
chain. As a result, a stem and loop structure is formed which is crucial for
termination.

POST-TRANSCRIPTIONAL MODIFICATIONS
The RNAs produced during transcription are called primary transcripts. They undergo many alterations—
terminal base additions, base modifications, splicing.
1. The 5’ capping: The 5’ end of mRNA is capped with 7-
methylguanosine by an unusual 5’5’ triphosphate linkage.
• S-Adenosylmethionine is the donor of methyl group.
• This cap is required for translation, besides stabilizing the
structure of mRNA.
2. Poly-A tail: mRNAs possess an adenine (A) nucleotide chain at the
3’-end. This poly-A tail, as such, is not produced during transcription.
It is later added to stabilize mRNA.
3. Introns and their removal:
• Introns = do not code for proteins.
• Exons = possess genetic code.
• Spliceosome = snRNP association with hnRNA at the exon-intron
junction.
4. Modification in nucleoside: Methylferases, deaminases and
dehydrogenases may methylate, deaminate or reduce the bases into the
„minor‟ bases.
e.g. 5 methylcytosine, N6-methyladenine, hypoxanthine, dihydrouracil, etc.
Uridine may be converted into pseudouridine.

Inhibitors of Transcription
• Rifamycin: Rifampicin and streptovaricin bind with β-subunit of the
polymerase to block the initiation of transcription.
• Actinomycin D: It forms a complex with double stranded DNA and
prevents the movement of core enzyme and as a result inhibits the
process of chain elongation.
• Streptoglydigin: It binds with the β-subunit of prokaryotic
polymerase and thus inhibits the elongation.
• Heparin: It is a polyanion that binds to the β’ subunit and inhibits
transcription in vitro. The α subunit has no known role in the process.
-Amanitin : It is a toxin produced by mushroom, Amanita
phalloides.  -amanitin which tightly binds with RNA
polymerase II of eukaryotes and inhibits transcription.

TRANSLATION
The genetic information stored in DNA is passed on to RNA (through transcription), and ultimately
expressed in the language of proteins.
The biosynthesis of a protein or a polypeptide in a living cell is referred to as translation. The term
translation is used to represent the biochemical translation of four-letter language information from nucleic
acids (DNA and then RNA) to 20 letter language of proteins. The sequence of amino acids in the protein
synthesized is determined by the nucleotide base sequence of mRNA.
PROTEIN BIOSYNTHESIS
The protein synthesis which involves the translation of nucleotide
base sequence of mRNA into the language of amino acid sequence
may be divided into the following stages for the convenience of
understanding.
I. Requirement of the components
II. Activation of amino acids
III. Protein synthesis proper
IV. Chaperones and protein folding
V. Post-translational modifications.
I. REQUIREMENT OF THE COMPONENTS
1. Amino acids: Proteins are polymers of amino acids. Regular dietary
supply of essential amino acids, in sufficient quantities, is maintained,
as it is a prerequisite for protein synthesis.
2. Ribosomes: The functionally active ribosomes are the centres or factories for protein synthesis.
Ribosomes are huge complex structures (80S for eukaryotes) of proteins and ribosomal RNAs.
Each ribosome consists of two subunits—one big and one small. The functional ribosome has two sites—A
site and P site. Each site covers both the subunits. A —site is for binding of aminoacyl tRNA and P —
site is for binding peptidyl tRNA, during the course of translation. The ribosomes are located in the
cytosomal fraction of the cell. They are found in association with rough endoplasmic reticulum.
3. Messenger RNA (mRNA): The specific information required for the synthesis of a given protein is
present on the mRNA. The DNA has passed on the genetic information in the form of codons to mRNA to
translate into a protein sequence.
4. Transfer RNAs (tRNAs): They carry the amino acids, and hand them over to the growing peptide chain.
The amino acid is covalently bound to tRNA at the 3’-end. Each tRNA has a three nucleotide base
sequence—the anticodon, which is responsible to recognize the codon (complementary bases) of mRNA
5. Energy sources: Both ATP and GTP are required for the supply of energy in protein synthesis.
6. Protein factors: The process of translation involves a number of protein factors. These are needed for
initiation, elongation and termination of protein synthesis.

II. ACTIVATION OF AMINO
ACIDS
The amino acid is first attached to the
enzyme aminoacyl tRNA synthetases
utilizing ATP to form enzyme-AMP-
amino acid complex. The amino acid is
then transferred to the 3’ end of the
tRNA to form aminoacyl tRNA.
Methionine having anticodon UAC is the
first amino acid required to be involved in
the binding to the initiation codon AUG on
m-RNA.
III. PROTEIN SYNTHESIS
PROPER
The protein or polypeptide synthesis
occurs on the ribosomes. The mRNA is
read in the 5’→3’direction and the
polypeptide synthesis proceeds from N-terminal end to C-terminal end.
Translation proper is divided into three stages—initiation, elongation and termination.
A) INITIATION OF TRANSLATION
The initiation of translation in eukaryotes involves at least ten eukaryotic initiation factors (eIFs). The
process of translation initiation can be divided into four steps:-
1. Ribosomal dissociation.
2. Formation of 43S preinitiation complex.
3. Formation of 48S initiation complex.
4. Formation of 80S initiation complex.
Ribosomal dissociation
The 80S ribosome dissociates to form 40S and 60S subunits. Two initiating factors namely eIF-3 and eIF-
1A bind to the newly formed 40S subunit, and thereby block its reassociation with 60S subunit.
Formation of 43S preinitiation complex
A ternary complex containing met-tRNAi and eIF-2 bound to GTP attaches to 40S ribosomal subunit to form
43S preinitiation complex.
Formation of 48S initiation complex
The binding of mRNA to 43S preinitiation complex results in the formation of 48S initiation complex.
Formation of 80S initiation complex
48S initiation complex binds to 60S ribosomal subunit to form 80S initiation complex
ELONGATION OF TRANSLATION
Ribosomes elongate the polypeptide chain by a sequential
addition of amino acids. The amino acid sequence is determined
by the order of the codons in the specific mRNA.
Three steps:-.
1. Binding of aminoacyl t-RNA to A-site.
2. Peptide bond formation.
3. Translocation.
Binding of aminoacyl—tRNA to A-site
The 80S initiation complex contains met-tRNAi in the P-site, and
the A-site is free. Another aminoacyl-tRNA is placed in the A-site. This requires proper codon recognition
on the mRNA.
Peptide bond formation
The enzyme peptidyltransferase catalyses the formation of peptide bond. The net result of peptide bond
formation is the attachment of the growing peptide chain to the tRNA in the A-site.
Translocation

As the peptide bond formation occurs, the ribosome moves to the next codon of the mRNA (towards 3’-
end). This process called translocation, basically involves the movement of growing peptide chain from A-
site to P-site.
TERMINATION OF TRANSLATION
After several cycles of elongation, incorporating amino acids and the formation of the specific protein/
polypeptide molecule, one of the stop or termination signals (UAA, UAG and UCA) terminates the
growing polypeptide.

POST-TRANSLATIONAL MODIFICATIONS OF PROTEINS
The proteins synthesized in translation are, as such, not functional. Many changes take place in the
polypeptides after the initiation of their synthesis or, most frequently, after the protein synthesis is
completed.
1. Proteolytic degradation
Many proteins are synthesized as the precursors which are much bigger in size than the functional
proteins. Some portions of precursor molecules are removed by proteolysis to liberate active proteins. This
process is commonly referred to as trimming. The formation of insulin from preproinsulin, conversion of
zymogens (inactive digestive enzymes e.g. trypsinogen) to the active enzymes are some examples of
trimming.
2. Intein splicing
Inteins are intervening sequences in certain proteins. These are comparable to introns in mRNAs.
Inteins have to be removed, and exteins ligated in the appropriate order for the protein to become active.
3. Covalent modifications
The proteins synthesized in translation are subjected to many covalent changes. By these modifications in
the amino acids, the proteins may be converted to active form or inactive form.
modifications are described below.
1. Phosphorylation: The hydroxyl group containing amino acids of proteins,
namely serine, threonine and tyrosine are subjected to phosphorylation.
2. Hydroxylation: During the formation of collagen, the amino acids proline
and lysine are respectively converted to hydroxyproline and hydroxylysine.
3. Glycosylation: The attachment of carbohydrate moiety is essential for
some proteins to perform their functions. The complex carbohydrate moiety is attached to the amino acids,
serine and threonine or to asparagine, leading to the synthesis of glycoproteins.
INHIBITORS OF PROTEIN SYNTHESIS
1. Streptomycin:
It interferes with the binding of f-met- t-RNA to ribosomes and thereby inhibits the initiation process.
2. Puromycin: This inhibits protein synthesis by releasing nascent polypeptide chains before their synthesis is
complete. It binds to the A site on ribosome and inhibits the entry of aminoacyl-t RNA.
3. Tetracycline: It binds to the 30S subunit and inhibits binding of aminoacyl t-RNA, thus inhibits the initiation
process.
4. Chloramphenicol: It inhibits the peptidyl transferase activity of 50S subunit. Thus it inhibits the process of
elongation.
5. Cycloheximide: This inhibits peptidyl transferase activity of 60S ribosomal subunit in eukaryotes. It also
inhibits elongation.
6. Erythromycin: It binds to the 50S subunit and inhibits translocation.
7. Sparsomycin: This inhibits peptidyl transferase and release factor-dependent termination.

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