The document discusses different types of RNA and the process of translation. It describes the four main types of RNA - messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), and small nuclear RNA (snRNA). It provides details about the structure and function of each RNA type. The document also explains translation, including the three stages of initiation, elongation, and termination. It discusses the components required for translation and the binding sites on the ribosome. Finally, it covers post-translational modification and the wobble hypothesis.

1. TYPES OF
RNA &
TRANSLATIO
N
Dr. Ifat Ara Begum
Assistant Professor
Dept. of Biochemistry
Dhaka Medical College,
Dhaka

2. CENTRAL DOGMA OF
MOLECULAR BIOLOGY

3. INTRODUCTION TO RNA
one of the three major biological
macromolecules that are
essential for all known forms of
life
Other two macromolecules are
DNA and proteins

4. CONTD
For many years RNA was believed
to have only three major roles in
the cell:
as a DNA photocopy (mRNA)
as a coupler between the genetic
code and amino acid, the protein
building blocks (tRNA)
And
as a structural component of
ribosomes (rRNA)

5. CONTD
In recent years, however, we have
begun to realize that the roles
adopted by RNA are much broader
and much more interesting
RNA has role as enzymes
(called ribozymes) to speed
chemical reactions.
In a number of clinically important
viruses RNA (rather than DNA)
carries the viral genetic
information.

6. CONTD
RNA has important role in
regulating cellular processes
(from cell division, differentiation
and growth to cell aging and
death)
Defects in certain RNAs or the
regulation of RNAs have been
implicated in a number of
important human diseases,
including heart disease, some
cancers, stroke and many others.

9. DIFFERENT
TYPES OF
RNA

11. REMEMBER, THREE MAIN
TYPES OF RNA FOR
TRANSLATION

13. 1. MESSENGER RNA
A large family of RNA molecules that
convey genetic information from DNA
to the ribosome, where they specify
the amino acid sequence of the
protein products of gene expression
Single stranded RNA
Represent 2-5% of cellular RNA
Are found in nucleus & cytoplasm
Are produced by post transcriptional
modification of primary transcript of
a gene (hnRNA)

14. CONTD
It contains only the coding
information of the coding region of a
gene flanked by 5’ UTR & 3’ UTR

15. CONTD
Monocistronic (one mRNA contains
one gene)
Half life: short (hours to days)
Most heterogenous in size (depends
on size of gene)
Function: Conveys genetic
information from DNA to ribosome &
then acts as a template to faithfully
translate that genetic information for
protein synthesis

16. 2. TRANSFER RNA
An adaptor molecule that serves as
the physical link between
the mRNA and the amino
acid sequence of proteins.
It does this by carrying an amino acid
to ribosome as directed by a codon in
a mRNA.
As such, tRNAs are a necessary
component of translation

18. CONTD
It represents 10-20% of cellular RNA
Smallest RNA found in cytoplasm
The structure of tRNA can be
decomposed into its primary
structure, its secondary
structure (usually visualized as
the cloverleaf structure), and
its tertiary structure

19. CLOVER LEAF STRUCTURE BY
INTERNAL BASE PAIRING

20. CONTD
Attaches with specific AA at its 3’
end
Carry anticodon at its anticodon loop
Anticodon: A unit made up of
three nucleotides that correspond to
the three bases of the codon on
the mRNA to which it specifically
attaches

21. CONTD
D arm is a 4- to 6-bp stem ending in a
loop that often
contains dihydrouridine
T arm is a 4- to 5- bp stem containing
the sequence TΨC where Ψ
is pseudouridine, a modified uridine

22. CONTD
Function: tRNA acts as an adaptor
molecule to recognize a definite
codon on one hand & a specific AA on
other hand. Thus it carries AA to
ribosome for protein synthesis

23. 3. RIBOSOMAL RNA
The RNA component of the ribosome
In other words, it associates with
protein to form ribosome (Structure,
which is approximately 60% rRNA
and 40% protein by weight. They
contain two major rRNAs and 50 or
more proteins)
It is essential for protein synthesis in
all living organisms
Represent 70-80% of cellular RNA
Found in ribosome & nucleolus

24. CONTD
The ribosomal RNAs form two
subunits of ribosome, the large
subunit (LSU) and small subunit (SSU)
mRNA is sandwiched between the
small and large subunits
The LSU rRNA acts as a ribozyme,
catalyzing peptide bond formation

25. CONTD
Depending on their sedimentation
velocity coefficient measured in
Svedberg (S) unit, there are 4 types
of rRNA
I. 28S rRNA
II.18S rRNA
III.5.8S rRNA
IV.5S rRNA
Function: rRNA constitutes ribosome
which acts as platform where mRNA
& tRNA interact for protein synthesis

26. 4. SMALL NUCLEAR RNA
Less than 1% of cellular RNA
30 different types of snRNA exist
Function:
 Facilitates post transcriptional
modification of RNA
 Helps in gene regulation

27. Summary
about RNA

28. Point mRNA tRNA rRNA
Conten
t 2-5% 10-20% 70-
80%
Site Nucleus,
Cytoplasm
Cytoplasm Ribosome,
Nucleolus
Size Heterogenous Homogenous Heterogen-
ous

29. Poin
t
mRNA tRNA rRNA
Binds
with
40S
ribosome
AA tRNA &
mRNA
Func
ti-on
Acts as
template for
protein
synthesis
Carries AA
to site of
protein
synthesis
Acts as
platform
for
mRNA &
tRNA for
protein
synthesis

30. RIBOSOME

31. WHAT IS RIBOSOME
A complex molecular machine found
within all living cells, that serves as
the site of biological protein
synthesis (translation)

32. CONTD
Ribosome links amino acids together
in the order specified by mRNA
molecules
It is made from complexes of RNAs
and proteins and is therefore called
a ribonucleoprotein
Have 2 major components: small
ribosomal subunit & large ribosomal
subunit .
Each subunit is composed of one or
more rRNA molecules & a variety
of proteins.

33. CONTD
 The small ribosomal subunit: It reads
the mRNA
 The large subunit: It joins amino
acids to form a polypeptide chain
The ribosomes and associated
molecules are also known as
the translational apparatus.
Both prokaryotic (E. coli)
and eukaryotic (human)
ribosomes can be broken down into
these two subunits

34. Type Size LSU (rRNA) SSU
(rRNA)
Prokaryoti
c
70
S
50S (5S :
120
nt, 23S :
2906 nt)
30S
(16S :
1542 nt)
Eukaryotic 80
S
60S (5S :
121
nt, 5.8S :
156
nt, 28S :
40S
(18S :
1869 nt)

35. CONTD
Please note,
S: Svedberg units
nt= length in nucleotides of the
respective rRNAs

36. BINDING SITES OF
RIBOSOME

38. CONTD
 Aminoacyl-tRNA : A tRNA bound to an
amino acid
 Peptidyl-tRNA: A tRNA containing
last AA of the growing peptide chain
 [The amino (NH2) group of the
aminoacyl-tRNA attacks the ester
linkage of peptidyl-tRNA to form a
new peptide bond. This reaction is
catalyzed by peptidyl transferase]

40. CONTD
 E site : The empty tRNA (that
previously was holding onto the last
amino acid of peptide chain) is moved
to the E site (and what used to be the
aminoacyl-tRNA is now the peptidyl-
tRNA)
Remember:
A single mRNA can be translated
simultaneously by multiple
ribosomes.

42. TRANSLATIO
N

43. DEFINITION
The process in which cellular
ribosomes create proteins
Or
Synthesis of protein according to
the base sequence of mRNA
Or
mRNA directed protein synthesis
where genetic message coded by
mRNA is translated in to protein
structure

45. REQUIREMENTS
mRNA with initiating codon (AUG
coding for methionine) & termination
codon (any of 3 stop codons)
tRNA
AA
Ribosome
Protein factor: IF, EF, TF (termination
factor)
ATP & GTP : 2 of each needed for
each peptide bond synthesis
Amino acyl-tRNA synthetase

46. AMINO ACYL-TRNA
SYNTHETASE
Enzyme needed for synthesis of
amino acyl-tRNA
Catalyzes the attachment of AA with
tRNA to form amino acyl-tRNA
[In amino acyl-tRNA, the AA is called
activated AA & the tRNA is called
charged tRNA]
Highly selective for a specific AA &
its tRNA
Has proof reading & editing function

50. 1. INITIATION
Dissociation of 80S ribosome in to
40S and 60S subunit
PIC (preinitiation complex)
formation:
Met-tRNA, IF & GTP binds with 40S
ribosome to form PIC

51. CONTD
PIC binds with mRNA
Searching for initiating codon (AUG):
40S ribosome scans mRNA from 5’
end towards 3’ end to recognize AUG
Synthesis of 80S initiation complex:
By PIC+ mRNA+ 60S ribosome

52. CONTD
60S ribosome containing “P” site
and “A” site binds with PIC placing
initiator aminoacyl tRNA (met-
tRNA) in the “P” site
i. P site: Is positioned against
initiating codon (AUG) which
contains met-tRNA now
ii. A site: Is positioned against C1 (1st
codon/ the codon that is next to
AUG) that is still empty now

53. CONTD
Remember, it is the 60S ribosome on
which amino acids are assembled to
synthesize protein

54. 2. ELONGATION OF CHAIN
It means simply the ribosome
travelling down the message (mRNA)
reading codons and bringing in the
proper aminoacyl tRNA's to translate
the message out to protein.
The incoming aminoacyl tRNA is
brought into the ribosome A site,
where it is matched with the codon
being presented

55. CONTD
Done by EF via repeated cycles
Successful completion of one cycle
translates one codon by recruiting
the specified AA of that codon in the
process of protein synthesis

56. CONTD
Each cycle can be described under 3
headings:
A. Codon recognition
B. Peptide bond formation
C.Translocation

58. A) CODON RECOGNITION
 Attachment of appropriate amino
acyl-tRNA with the empty “A” site
positioned against first codon (C1)
 Lets think, this amino acid is A1

59. B) PEPTIDE BOND
FORMATION
 We know, “P” site positioned
against initiating codon (AUG) is
already attached with met-tRNA
 Methionine (Met) leaves tRNA of
“P” site & goes to “A” site
 It is followed by formation of
peptide bond with the appropriate
amino acid (A1) of “A” site
 The tRNA of “A” site is now known
as peptidyl tRNA (met-A1-tRNA)

60. C) TRANSLOCATION
 Removal of tRNA from “P” site (as
this tRNA is empty now) to make
the “P” site empty
 Then peptidyl tRNA (met-A1-tRNA)
moves from “A” site to “P” site to
make the “A” site empty
 Ribosome moves to next codon
down towards 3’ end of mRNA, so
that, the loaded “P” site & empty
“A” site can be positioned against
1st
codon (C1) & 2nd
codon (C2)
respectively

61. CONTD
Now 2nd
cycle of chain elongation
begins
2nd
cycle translates the 2nd
codon
(C2) by adding the specific amino
acid (A2) of that codon in the
growing peptide chain
The chain elongation cycle repeats
again and again till all the sense
codons are translated with the
synthesis of a long polypeptide
chain

64. 3. TERMINATION
Once a stop codon is reached, the
“A” site is positioned against it
leading to termination of protein
synthesis.

66. CONTD
Facilitated by release factor (RF) /
termination factor (TF)
The polypeptide is released from
the tRNA
The tRNA is released from the
ribosome
and
The two ribosomal subunits separate
from the mRNA

68. Post
translational
modification

69. INTRODUCTION
Chemical modification of proteins
after their biosynthesis
Objective: To make the protein
functionally active
Occurs mostly in ER & golgi
apparatus

70. Protein/PP
Foldin
g
Removal of
N- terminal
methionine
Limited
proteolysis
e.g.
Pepsinogen
Intein
splicing
Chemical/
covalent
modificatio
n

71. PROTEIN FOLDING

72. INTEIN SPLICING
An intein is a segment of
a protein that is able to excise itself
and join the remaining portions
(the exteins) with a peptide bond in
a process termed protein splicing
or
Inteins are intervening sequences in
certain proteins, comparable to
introns in mRNAs (protein introns)

73. CONTD
lnteins have to be removed &
exteins ligated in the appropriate
order for the protein to become
active.

74. COVALENT MODIFICATION
Hydroxylation: Lysine and proline of
collagen are hydroxylated after
synthesis of collagen
Gamma carboxylation: Clotting
factors
Glycosylation: the addition of
a glycosyl group to
either arginine, asparagine, cys
teine, hydroxylysine, serine, thr
eonine, tyrosine,
or tryptophan resulting in
a glycoprotein.

75. CONTD
Phosphorylation:
 Addition of phosphate group to a
protein (esp. on serine, threonine &
tyrosine residue)
 Also called “phospho regulation”

77. WoBBlE
HYPotHEsis

78. INTRODUCTION
It explains why multiple codons can
code for a single amino acid
(degeneracy of genetic code)
or
It is the phenomenon in which a
single tRNA can recognize more than
one codon
Crick postulated the wobble
hypothesis

79. CONTD
One tRNA molecule (with one amino
acid attached) can recognize and
bind to more than one codon, due to
the less-precise base pairs that can
arise between the 3rd base of the
codon and the base at the 1st
position on the anticodon.

80. CONTD
i.e. The pairing between codon
(mRNA) and anticodon (tRNA) at the
1st
two base position always follows
the usual base pairing rule but
wobbles (means move a bit) occur at
3rd
position

81. CONTD
It reduces the number of tRNA
required
It accounts for the degeneracy of
genetic code (the reason why there
is 64 codons but only 40-50 tRNAs)