2. • Structure of DNA , RNA , tRNA
• Replication
• Repair of DNA
• Mutation
• Transcription
• Translation
• Drugs affecting replication , transcription and
translation
( antitumours,antibiotics)
3. Among all the properties of living organisms, one is
absolutely important for continuance of life. A living
system must be able to replicate itself. To do so an
organism must possess a complete description of
itself. In living organisms this description is stored in
the substances called nucleic acids.
Nucleic acids are non protein nitrogenous substances,
transmitting hereditary characters and directing
protein synthesis. They are polymers of purine and
pyrimidine nucleotides.
Nucleotide is a non-protein nitrogenous compound
made of a nitrogenous base, a pentose sugar and one
or more phosphate group.
4. The nucleotides forming nucleic acids contain either a
purine or a pyrimidine as the nitrogenous base and are
accordingly called purine nucleotides and pyrimidine
nucleotides respectively.
Both types of nucleotides may contain either ribose or
2-deoxyribose as the pentose residue and are
accordingly known as ribonucleotides and
deoxyribonucleotides respectively.
DNA or deoxyribonucleic acids are polymers of purine
and pyrimidine deoxyribonucleotides while RNA or
ribonucleic acids are polymers of purine and
pyrimidine ribonucleotides.
8. DNA(Deoxyribonucleicacid)
DNA are macromolecular polymers of purine and
pyrimidine deoxyribonucleotides and occur in
chromosomes, mitochondria etc. Chromosomal DNA
consists of very long double- stranded DNA molecules (DNA
duplex).
• Primary structure
Each of the two strands of a DNA duplex is an unbranched
linear polydeoxyribonucleotide strand, made of thousands
to billions of purine and pyrimidine deoxyribonucleotides
joined serially in a specific order. Its primary structure is
constituted by the sequence of purine and pyrimidine
deoxyribonucleotides in its strand. By carrying the genetic
information in the form of the sequences of bases of these
polynucleotide strands, DNA forms the chemical basis of
heredity.
9.
10. • The nucleotides are joined to one another in a chain
by covalent bonds between the sugar of one nucleotide
and the phosphate of the next, resulting in an
alternating sugar-phosphate backbone.
11. • The nitrogenous bases of the two separate polynucleotide strands
are bound together, according to base pairing rules (A with T and C
with G), with hydrogen bonds to make double-stranded DNA.
• DNA stores biological information. The DNA backbone is resistant
to cleavage, and both strands of the double-stranded structure
store the same biological information. This information is
replicated as and when the two strands separate.
• The two strands of DNA run in opposite directions to each other
and are thus antiparallel.
12.
13. • Within eukaryotic cells DNA is organized into long structures
called chromosomes. During cell division these chromosomes are
duplicated in the process of DNA replication, providing each cell its
own complete set of chromosomes.
• Eukaryotic organisms(animals, plants, fungi and protists ) store
most of their DNA inside the cell nucleus and some of their DNA
in organelles, such as mitochondria or chloroplasts.
14.
15.
16.
17. • Secondary Structure
The secondary structure of the DNA molecule consists of a
double-stranded helix formed by the coiling of two linear
polydeoxyribonucleotide strands around an identical central
axis. This is known as the Watson - Crick Model of DNA
structure after James Watson and Francis Crick who first
formulated it.
• Supercoiled DNA
This is the tertiary structure of a circular double-helical DNA
molecule. Because each strand of a circular DNA duplex is
covalently closed, the circular double-helix may remain
either in a relaxed form without any twist, or as a supercoil
with varying degrees of twists or turns. Supercoiling may in
one form twist the DNA double-helix around itself to change
it into an interwound super helix with two stretches of the
same duplex wound around one another.
18.
19. • RNA (Ribonucleic acid)
RNA is a polymer of ribonucleotides of Adenine, Uracil,
Guanine and Cytosine, joined together by 3’-5’
phosphodiester bonds. Thymine is absent in RNA. The
RNA is found in nucleolus, ribosomes, mitochondria and
cytoplasm. The pentose sugar of the nucleotide is D-
ribose.
Structure of RNA
Primary structure of RNA
The primary structure of RNA is defined as the number
and sequence of ribonucleotides in the chain. Each
linear strand is held together by the ribonucleotides
bound to each other by 3’, 5’ phosphodiester bonds
joining 3’-OH of one nucleotide with 5’-OH of the next.
20. Secondary structure of RNA
The secondary structure of RNA
involves various coil formation of the
polyribonucleotide chain. These coil
structures are stabilized by
hydrophobic interactions between
the purine and pyrimidine bases.
There are intra-chain hydrogen bonds
between G-C and A-U.
21. • Tertiary structure of RNA
The tertiary structure RNA involves the folding of the
molecule into three dimensional structure. The cross-
linking also occurs at various sites stabilized by
hydrophobic and hydrogen bonds producing a
compactly coiled globular structure.
• Types of RNA
There are mainly three types of RNA found in human
beings.
• Messenger RNA or m-RNA
• Transfer RNA or t-RNA
• Ribosomal RNA or r-RNA
22. • The functional form of single-stranded RNA molecules, just
like proteins, frequently requires a specific tertiary structure.
The scaffold for this structure is provided by secondary
structural elements that are hydrogen bonds within the
molecule. This leads to several recognizable "domains" of
secondary structure like hairpin loops, bulges, and internal
loops. Since RNA is charged, metal ions such as Mg2+ are
needed to stabilize many secondary and tertiary structures.
23. • 1. t-RNA
They remain largely in cytoplasm. The t-RNAs are
relatively small, single-stranded, globular
molecules with molecular weight of 2 to 3 X 104.
There are at least 20 different t-RNA molecules.
These binds with a specific amino acid, brings it
to a particular codon or base-triplet of an m-RNA
and transfers it to the C-terminal end of a
growing peptide chain, elongating the latter
during peptide synthesis.
24.
25. • The structure of tRNA can be decomposed into its primary
structure, its secondary structure (usually visualized as
the cloverleaf structure), and its tertiary structure (all tRNAs have
a similar L-shaped 3D structure that allows them to fit into
the P and A sites of the ribosome). The cloverleaf structure
becomes the 3D L-shaped structure through coaxial stacking of the
helices, which is a common RNA tertiary structure
26.
27. Primary structure of t-RNA
• The primary structure of t-RNA is constituted by the
sequence of purine and pyrimidine nucleotides in its
polynucleotide strand. While the majority of the nucleotides
consist of those of adenine, guanine, cytosine and uracil, up
to 25% of the nucleotides may be those of modified or
minor bases such as dihydrouracil, methylguanines, inosine,
methylinosine and pseudouridine. Each t-RNA molecule
invariably carries one thymine nucleotide.
28. Secondary structure of t-RNA
• The secondary structure of t-RNA assumes a cloverleaf
form due to hydrogen bonding of complementary bases
in different stretches of the same polynucleotide strand.
All t-RNA molecules contain 4 main arms or loops.
1. Acceptor arm.
• This consists of unpaired sequences of cytosine-cytosine-
adenine at 3’ end also called as acceptor end. The 3’-OH
terminal of adenine may bind with the α-COOH of a
specific amino acid and carry the latter as an aminoacyl-
tRNA complex to ribosomes for protein synthesis.
29. 2. D- arm.
The D-arm will bear one or more dihydrouracil (D)
residues.
3. Anticodon arm.
This is another unpaired and non-bonded loop
carrying specific sequences of three bases constituting
the anticodon. The bases of anticodon are hydrogen
bonded with three complementary bases of codon of m-
RNA.
30. 4. TΨC arm.
This arm of RNA bears 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 like class 1 t-RNA, class 2 t-
RNA.
31. Tertiary structure t-RNA
The t-RNA assumes an L- shaped tertiary structure in
which the base pairs are arranged into two double helical
columns. The tertiary structure is held and stabilized by a
large number of stacking interactions between the bases of
both double-helical stems and non -helical stretches of the
molecule and a number of cross links by tertiary H-bonds.
32. • 2. m-RNA
The m-RNA conveys messages from the genes of DNA
to the protein-synthetic machinery and determines
thereby the amino acid sequence of the peptide being
translated. m-RNA molecules are large, single
stranded, and very heterogeneous in size ranging from
about 30-2000KD in molecular weight. The m-RNA
molecules are formed with the help of DNA template
(3’- 5’) during the process called Transcription. They
carry a specific sequence of nucleotides in triplets
called Codons, responsible for the synthesis of a
specific protein molecule
33. • The 3’-OH end of most m-RNA molecules carries a
polymer of adenylate ribonucleotides consisting of
20 to 250 residues in length. This is called as ‘Poly A
tail’, the function of which is not yet fully understood
but it seems to maintain the intracellular stability of
the specific m-RNA by preventing the attack of 3’-
exonuclease. On the other hand, the 5’-OH end of
the m-RNA carries a cap structure consisting of 7
methyl guanosine triphosphate. The cap is probably
involved in the recognition of protein biosynthetic
machinery and it helps in stabilizing the m-RNA by
preventing the attack of 5’- exonuclease.
34.
35. • 3. r-RNA
The r-RNA forms about 60- 80% of the cellular RNA
and mostly remains combined with proteins in
macromolecular aggregates called ribosomes. It is on
the ribosomes that the mRNA and r-RNA interact
during the process of protein synthesis.
• Comparison between DNA and RNA
(a) Similarities
• 1. Both have adenine, guanine, and cytosine.
• 2. The nucleotides are linked together by
phosphodiester bonds.
• 3. The bonding is in 3’- 5’ direction.
• 4. Main function involves protein biosynthesis.
36. DNA RNA
1. In addition to adenine,
guanine, cytosine the
fourth base thymine is
present. Uracil is absent.
2. Pentose sugar is
deoxyribose.
3. Present in nucleus,
mitochondria but never in
cytoplasm.
1. In addition to adenine,
guanine, cytosine the fourth
base uracil is present.
Thymine is absent.
2. Pentose sugar is ribose.
3. In addition to nucleus,
RNA is found in cytoplasm.
(b) Differences
37. 4. They consist of two
strands.
5. There are A, B, C, D and E
forms of DNA.
6. These are large
molecules.
7. DNA can form RNA by the
process of transcription.
8. Purine and Pyrimidine
contents are almost equal
4. These are single stranded.
5. There are t-RNA, m-RNA
and r-RNA.
6. Only m-RNA and r-RNA are
large molecules.
7. RNA cannot give rise to
DNA under normal condition
but it can under special
experimental conditions using
reverse transcriptase.
8. Purine and Pyrimidine
contents are not equal
38. Ribosomes
• These are particulate aggregates of r-RNA and
proteins. The ribosomal proteins are of various sizes
and diverse amino acid sequences, but most are basic
proteins with many Lysine and Arginine residues and
few Phenylalanine, Tyrosine and Tryptophan residues.
The eukaryotic cytoplasmic ribosomes are 80S
ribosomes made of 40S and 60S subunit. The 3-
dimensional form of a ribosome gives rise to its
peptidyl or P site and aminoacyl or A site, where any
tRNA molecule may fit in. During peptide synthesis,
several ribosomes are serially joined by an mRNA
molecule to constitute a linear aggregate called a
polyribosome or polysome.
39. DNA Replication
Replication of DNA is accomplished by the
polymerization of the deoxyribonucleotides in to new
DNA strand, each synthesized on the template of one
of the strands of a pre-existing DNA duplex.
• Cellular proofreading and error-checking mechanisms
ensure near perfect fidelity for DNA replication.
40. • Semiconservative replication: During cell division, each
daughter cell gets an exact copy of the genetic information of
the mother cell. This process of copying the DNA is known as
DNA replication.
• In the daughter cell, one strand is derived from the mother
cell; while the other strand is newly synthesized. This is
called semi-conservative type of DNA replication.
• Each strand serves as a template or mold, over which a new
complementary strand is synthesized
41.
42.
43. Biological significance
• Extreme accuracy of DNA replication is necessary in order to
preserve the integrity of the genome in successive generations.
• In eukaryotes , replication only occurs during the S phase of the
cell cycle.
• Replication rate in eukaryotes is slower resulting in a higher
fidelity/accuracy of replication in eukaryotes.
46. • Sequential process of replication
1. Initiation of DNA replication
• Replication begins at a specific initiation point. This is
a unique sequence of bases called ori for each
organism. It is normally a site where the DNA comes
into close proximity with an infolding of the cell
membrane during cell fission.
• The process of replication is synchronized with
events preparing for cell fission. A short RNA primer
strand is laid down against a specific segment of the
DNA template strand.
47. 2. Unwinding of parental DNA
After the initiation point is located before the
replication forks are formed where nascent
synthesis of DNA occurs, the parental DNA
unwinds. The unwinding of the DNA takes place
once every 10 nucleotide pairs. This allows the
strand separation. This is facilitated by specific
enzymes that introduce ‘nicks’ (cuts) in one of
the strand of the unwinding double helix
thereby allowing the unwinding to proceed. The
nicks are quickly sealed by nicking-sealing
enzyme called as DNA topoisomerases.
48. The unwinding and separation of the
strands of a pre-existing DNA duplex,
preparatory to their role as template
strands, require the concerted actions of
several proteins at the replication fork.
The unwinding of DNA duplex is brought
about by Rep protein, DNA helicase-II and
Helix-destabilizing or single stranded
binding protein (HD protein or SSB protein)
at the replication fork.
49.
50. The Rep protein binds to the leading strand
template of the DNA duplex ahead of the
replication fork. DNA helicase-II binds to the
lagging strand template of the duplex just
beside the site of binding of the Rep protein to
the leading strand template. Both Rep protein
and DNA helicase-II act as helicases to bring
about the unwinding and separation of the two
strands of a pre-existing duplex ahead of the
replication fork.
51. They move side by side in the same direction
along the duplex to proceed progressively away
from the replication fork and towards the
helically coiled part of the duplex. But because
the two strands the two strands are antiparallel
to each other, the Rep protein traverses the
leading strand template in its upstream or
3’→5’ direction while the helicase–II
simultaneously shifts along the lagging strand
template in the downstream or 5’→3’ direction
of the latter
52.
53. • During their movement along the strand, the
interaction between each base pair of the two
strands is destabilized at the cost of hydrolysis of
two ATP molecules, thus separating the strands and
extending the replication fork over one base-pair.
Repetition of such action advances the replication
bubble.
• Immediately behind the advancing helicases, a
number of molecules of single-stranded binding
protein (SSB protein) bind stably to the single
stranded segments, just unwound from the DNA
duplex by helicases, in order to cover the single
strands and prevent their renewed base-pairing and
rewinding.
54. • As the parting of the two strands
advances further with the movements of
helicases, the SSB protein molecules
leaves the sites of their earlier binding
and move ahead to fresh single-strand
sites just behind the helicases. The part
of each single strand, thus freed from
SSB, is then exposed to the binding and
action of DNA polymerases.
55.
56. • Each strand acts as template – Complementary base
pairing ensures that T signals addition of A on new strand,
and G signals addition of C – Two daughter helices
produced after replication
57. • DNA polymerases
These enzymes lay down a new DNA strand on the
unwound segment of each template strand of a DNA
duplex. They extend the growing DNA strand in its
5’→3’ direction by the successive addition of new
deoxyribonucleotides to its 3’end. Because the new
DNA strand and its template are antiparallel to each
other with the 3’ end of each facing the 5’ end of the
other and vice-versa, DNA polymerases move along
and copy the template strand in its 3’→5’ direction
only. They cannot act in reverse direction.
62. • DNA ligase is an enzyme that repairs irregularities or breaks in the
backbone of double-stranded DNA molecules. It has three
general functions: It seals repairs in the DNA, it seals
recombination fragments, and it connects Okazaki fragments
(small DNA fragments formed during the replication of double-
stranded DNA).
• Exonuclease When an incorrect base pair is recognized, DNA
polymerase moves backwards by one base pair of DNA. The 3'-
5' exonuclease activity of the enzyme allows the incorrect base pair
to be excised (this activity is known as proofreading). A
hydrolyzing reaction that breaks phosphodiester bonds at either
the 3' or the 5' end occurs.
63. Replication Termination
A small portion (about 300 nucleotides) in the 3′ ends of
the parent strands could not be replicated. This end piece
of the chromosome is called telomere.
Therefore another enzyme telomerase takes up this job of
replication of the end piece of chromosomes.
The telomeres are noncoding repetitive sequences.
After the normal replication, there is only single
strand in this region; so this portion is degraded by
exonucleases.
66. Exonucleolytic Proofreading
• The DNA polymerase has 3′ to 5′ exonuclease activity. Hence any
mispaired nucleotide added is immediately removed.
67. Nucleotide Excision Repair
• This takes place along with the replication process
(proofreading). The original template DNA contains
methylated residues (N6-methyl adenine and 5-methyl
cytosine). The newly synthesized strand will not have
methylated bases.
• So enzymes can recognize the original (correct) DNA
strand. The mismatched base is identified and
removed along with a few bases around that area. The
wrong base is removed by the endonuclease activity. It
removes 24 – 32 nucleotides around the wrong base.
73. 1. Silent mutation: There may be no detectable effect due to the
degeneracy of the code. This is likely to occur if the changed base
in the m-RNA molecule occur at the third nucleotide of a codon.
Example
Hb-A: If valine at B67is replaced by alanine, no change occurs in
the function of Hb (Hb sydney).
2. Missense mutation (missense effect): A missense effect
occurs when a different amino acid is incorporated at the
corresponding site in the protein molecule. This mistaken amino
acid or missense, depending upon its location in the specific
protein molecule might be acceptable, partially acceptable or
unacceptable to functioning of that protein molecule.
74. The replacement of a specific lysine at 61 position in
Beta-chain by Asparagine does not alter the normal
function of Hb in these individuals, hence it is
acceptable missense mutation.
75. This missense mutation producing HbS interferes with
normal function of Hb and results in sickle cell
anaemia in homozygous. But it is classified as partially
acceptable because HbS can bind and release O2
although abnormally.
76. An unacceptable missense mutation in a Hb
molecule produces a non-functional Hb.
3. A nonsense Mutation or Effect
Sometimes the codons with the altered base may become one
of the three termination codon called as “nonsense codon”.
This altered codon acts as a stop signal and causes
termination of the protein synthesis at that point, results in a
non-functioning protein molecule.
77. B. Frame Shift Mutations
Frame shift mutations can be of two types:
• 1. Deletion type
• 2. Insertion type
1. Deletion type: Effects of deletion
i. The deletion of a single nucleotide from the coding
strand of a gene results in an altered reading frame in
the m-RNA (hence called as frame-shift-mutation).
The machinery translating the m-RNA does not
recognise that a base is missing since there is no
punctuation in the reading of codons. Thus a major
alteration in the sequence of the amino acids in the
protein molecules occur. Such an alteration in the
reading frame results in a garbled translation of the m-
RNA distal to the single nucleotide deletion.
78. ii. Not only the sequence of amino acids distal to the deletion is
garbled, there may appear a nonsense (chain terminating)
codon on the way terminating the protein synthesis. Thus in
such a situation, the polypeptide chain produced is not
only garbled but prematurely terminated non-functional
protein is produced.
79. 2. Insertions Type: Effects
Insertions of one or two nucleotides into a gene results in a m-
RNA in which the reading frame is distorted and same type of
effects as noted with deletion can occur which may result in
garbled amino acid sequences distal to the insertion or
generation of a nonsense (chain terminating) codon at or distal to
insertion can lead to termination of the polypeptide chain,
which may be non-functional prematurely terminated protein.
80. TRANSCRIPTION
• Transcription is the process by which the synthesis of RNA
molecules is initiated and terminated representing one strand of
DNA duplex. By ‘representing’ means that the RNA is identical
in sequence with one strand of the DNA, it is complementary to
the other strand, which provides the template for its synthesis. It
takes place by the usual process of complementary base
pairing, catalysed by the enzyme RNA polymerase.
• The genetic information contained in the base sequence
of the gene is transcribed thereby into the
complementary base sequence of the RNA. The RNA
molecule, initially synthesized is called a primary RNA
transcript which undergoes various transcriptional
modifications to change into its final active form.
81. RNA polymerase: RNA polymerase being the key enzyme in
transcription, it is worthwhile to study the details of its structure
and mode of its action.
A single type of RNA polymerase is responsible for synthesis of
m-RNA, r-RNA and t-RNA in bacteria.
However, in eukaryotes several different enzymes are required to
synthesise the different types of RNA. They are called as:
• • RNA polymerase I,
• • RNA polymerase II, and
• • RNA polymerase III.
82. Stages of Transcription
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.
1. Formation of transcription complex: The enzyme RNA
polymerase needs to bind with specific sequences on a DNA.
These sequences recognised by RNA polymerase are called as
promoter.
83. The core enzyme cannot recognise the promoter region, sigma
factor is required for recognition and formation of the complex.
Following four steps occur:
i. Sigma factor recognises the promoter sequences.
ii. RNA polymerase attaches to promoter region.
iii. RNA polymerase melts the helical structure and separates 2
strands of DNA locally.
iv. RNA polymerase initiates RNA synthesis. The site at which the
first nucleotide is incorporated is called the start site or start
point.
84.
85.
86. 2. Initiation
• Core enzyme starts transcription at the separated
DNA strands of an initiation complex. As the enzyme
moves along, the unwound region moves
with it.
• The first base copied is always within six to nine
bases of the conserved T of the Pribnow box on the
unwound portion of 3’-5’ strand of DNA.
87.
88.
89. • Formation of hydrogen bonds is always as per the base-pairing
rules. The first incoming NTP binds to RNA polymerase at the
start point of initiation site and H-bonds to the complementary
base on the DNA within the complex. This site binds only purine
NTP—either A or G. The binding is with 3’ end of the NTP
leaving 5’ end to be free.
• The second incoming NTP binds to the elongation site on the
polymerase. The NTP is selected as per base-pair rule which can
H-bond with complementary base on DNA.
• After this phosphodiester bond formation the sigma- factor is
released. First base is then dissociated from initiation site and
that marks the completion of initiation.
90. 3. Elongation
• The core enzyme polymerase moves in 3’-5’ direction
of the coding strand and it adds successive NTPs at
the 3’-OH end of the ribonucleotide chain already laid
down in 5’-3’ direction.
• The incoming NTP forms a phosphodiester bond with
3’-OH end of the preceding ribonucleotide.
• The bases are determined by the sense strand by
base-pair rules.
91. 4. Termination: Specific sequences on the DNA molecule
function as the signal for termination of the transcription process.
• The specific signals are recognized by a termination protein, the
Rho factor.
• 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 upstream which is crucial for termination.
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.
99. Spliceosomes
• SnRNPs associated with hnRNA at the exon-intron junction form
spliceosomes. This is taking place inside the nucleus. Cuts are
made at both ends of intron; it is removed; and exon-exon ends
are ligated at G-G residues.
100. TRANSLATION OF m-RNA (PROTEIN SYNTHESIS)
• Protein is a polymer of amino acids joined together by
peptide bonds. In the process of protein synthesis also
known as translation of m-RNA, the amino acids are
added sequentially in a specific number and sequence,
determined by the sequence of codons in the genetic
code of the relevant m-RNA.
101. The amino acids need to be activated before they can
be incorporated into the peptide chain. The key
enzyme in this process is aminoacyl t-RNA
synthetase.
102. • Obviously there are at least 20 different t-RNAs and 20
different aminoacyl t-RNA synthetases in a protein
synthesising system.
In the aminoacyl t-RNA, the -carboxyl group of the
amino acid remains esterified with the 3’-OH of the
3’-terminal adenosine on acceptor arm of t -RNA.
103.
104. 2. Initiation
Formation of 80S Initiation Complex
• The 80S Ribosome complex has two receptor sites:
‘P’ site or peptidyl site: At this point, the met t-RNA
is on the ‘P’ site. On this site, the growing peptide
chain will grow.
‘A’ site or aminoacyl site: At this point it is free, the
new incoming t-RNA with the amino acid to be added
next is taken up, at this site.
105. 3. Elongation
• Elongation is a cyclic process on the ribosome in which one
amino acid is added to the nascent peptide chain.
• The peptide sequence is determined by the codons present in
the m-RNA.
• Steps involved in elongation:
The steps are mainly three:
A. The binding of new aminoacyl-tRNA to ‘A’ site
B. Peptide bond formation
C. Translocation process.
121. 4. Termination Process
• After multiple cycles of elongation process, it
results to formation of polypeptide chain. When
the desired protein molecule is synthesised, a
stop codon or terminating codon appears in
the A site of m-RNA. The stop codons are:
UAA, UAG, or UGA. There is no t-RNA with an
anticodon capable of recognising such a
termination signal.
122. POSTTRANSLATIONAL MODIFICATION OF
POLYPEPTIDE CHAINS
A. Trimming
Many proteins destined for secretion from the cell
are initially made as large, precursor molecules
that are not functionally active.
Portions of the protein chain must be removed by
specialized endoproteases, resulting in the
release of an active molecule. The cellular site of
the cleavage reaction depends on the protein to
be modified. Example; Insulin
123.
124. B. Covalent attachments
Proteins may be activated or inactivated by the covalent
attachment of a variety of chemical groups. Examples
include:
130. Inhibitors of Protein Synthesis
The modern medical practice is heavily dependent on the
use of antibiotics. They generally act only on bacteria and
are nontoxic to human beings. This is because
mammalian cells have 80S ribosomes, while bacteria have
70S ribosomes.
• Reversible Inhibitors in Bacteria
These antibiotics are bacteriostatic. Tetracyclines bind
to the 30S subunit of bacterial ribosome and so inhibit
attachment of aminoacyl tRNA to the A site of ribosomes.
Chloramphenicol inhibits the peptidyl transferase activity
of bacterial ribosomes.
Erythromycin and clindamycin prevent the translocation
process.
131. • Irreversible Inhibitors in Bacteria
These antibiotics are bactericidal. Streptomycin and
all other aminoglycoside antibiotics bind to 30S subunit
of bacterial ribosomes. They cause misreading of mRNA
and at high concentrations, they completely inhibit the
initiation complex formation and totally inhibit protein
synthesis.
