Protein synthesis involves two main steps: transcription and translation. In transcription, DNA is used as a template to make messenger RNA (mRNA) which carries the genetic code out of the nucleus. In translation, transfer RNA (tRNA) molecules match amino acids to the mRNA code on ribosomes to produce proteins according to the DNA's instructions. Quality control mechanisms like codon-anticodon interactions and elongation factors ensure the correct amino acids are joined in the proper sequence.

1. Protein Synthesis
Iswar Hazarika, Date:18/09/2012
1st
yr. M.Pharm, Pharmacology

2. • Proteins are composed of amino acids – there are 20
different amino acids
• Different proteins are made by combining these 20
amino acids in different combinations

3. • Proteins are manufactured (made) by the ribosomes

4. •Function of proteins:
1.Help fight disease
2.Build new body tissue
3.Enzymes used for digestion and other chemical reactions
are proteins
(Enzymes speed up the rate of a reaction)
4. Component of all cell membranes

5. Making a Protein—Transcription
•First Step: Copying of genetic information from DNA to RNA called
Transcription
Why? DNA has the genetic code for the protein that needs
to be made, but proteins are made by the ribosomes—ribosomes
are outside the nucleus in the cytoplasm.
DNA is too large to leave the nucleus (double stranded), but
RNA can leave the nucleus (single stranded).

6. • Part of DNA temporarily unzips and is used as a
template to assemble complementary nucleotides
into messenger RNA (mRNA).

7. • mRNA then goes through the pores of the nucleus with
the DNA code and attaches to the ribosome.

8. Making a Protein—Translation
•Second Step: Decoding of mRNA into a protein is called
Translation.
•Transfer RNA (tRNA) carries amino acids from the
cytoplasm to the ribosome.

9. These amino acids come from the food we eat. Proteins
we eat are broken down into individual amino acids and
then simply rearranged into new proteins according to the
needs and directions of our DNA.

10. •A series of three adjacent bases
in an mRNA molecule codes for
a specific amino acid—called a
codon.
•A triplet of nucleotides in tRNA
that is complementary to the
codon in mRNA—called an
anticodon.
•Each tRNA codes for a different
amino acid.
Amino acid
Anticodon

11. • mRNA carrying the DNA instructions and tRNA carrying
amino acids meet in the ribosomes.

12. • Amino acids are joined together to make a protein.
Polypeptide = Protein

13. 5’-ATGCCTAGGTACCTATGA-3’
3’-TACGGATCCATGGATACT-5’
5’-AUGCCUAGGUACCUAUGA-3’
5’-AUG CCU AGG UAC CUA UGA-3’
N-MET-PRO-ARG-TYR-LEU-C
DNA
Transcription
decoded as
Translation
mRNA
Protein

14. Generalized tRNA

15. = UH2
Modified Bases
Found in tRNAs

18. Alanine tRNA

19. tRNAs are activated by amino-acyl tRNA synthetases
Amino-acyl tRNA
synthetase

21. Amino-acyl tRNA synthetases:
One synthetase for each amino acid
a single synthetase may recognize multiple tRNAs
for the same amino acid
Two classes of synthetase.
Different 3-dimensional structures
Differ in which side of the tRNA they recognize
and how they bind ATP
Class I - monomeric, acylates the 2’OH on the terminal ribose
Arg, Cys , Gln, Glu, Ile, Leu, Met, Trp Tyr, Val
Class II - dimeric, acylate the 3’OH on the terminal ribose
Ala, Asn, Asp, Gly, His, Lys, Phe, Ser, Pro, Thr

22. Two levels of control to ensure that the proper amino acid
is incorporated into protein: 1) Charging of the proper tRNA

23. 2) Matching the
cognate tRNA to the
messenger RNA

24. and mitochondria

25. The association of the large and small subunits creates the
structural features on the ribosome that are essential for
protein synthesis
Three tRNA binding
sites:
A site = amino-acyl
tRNA binding site
P site = peptidyl-tRNA
binding site
E site = exit site

26. In addition to the APE sites there is an mRNA binding groove
that holds onto the message being translated

29. Incorporation of the correct amino acyl-tRNA is determined
by base-pairing interactions between the anticodon of the
tRNA and the messenger RNA
STEPS OF TRANSLATION:
1. Initiation
2. Elongation
3. Termination

30. Initiation of Translation
Initiation is controlled differently in prokaryotic and
eukaryotic ribosomes
In prokaryotes a single transcript can give rise to multiple proteins

31. In prokaryotes, specific
sequences in the mRNA
around the AUG codon, called
Shine-Delgarno sequences,
are recognized by an intiation
complex consisting of a Met
amino-acyl tRNA, Initiation
Factors (IFs) and the small
ribosomal subunit

32. GTP hydrolysis by
IF2 coincident with
release of the IFs and
binding of the large
ribosomal subunit leads
to formation of a complete
ribosome,on the mRNA
and ready to translate.

33. Eukaryotic mRNAs have a distinct structure at the 5’ end

34. In contrast, Eukaryotes
use a scanning mechanism
to intiate translation.
Recognition of the AUG
triggers GTP hydrolysis
by eIF-2

35. GTP hydrolysis by
eIF2 is a signal for
binding of the large
subunit and beginning
of translation

36. Proper reading of the
anticodon is the second
important quality control
step ensuring accurate
protein synthesis
=EF-1
Elongation factors
Introduce a two-step
“Kinetic proofreading”
STEP 2:
Elongation

37. A second elongation factor
EF-G or EF-2, drives the
translocation of the ribosome
along the mRNA
Together GTP hydrolysis
by EF-1 and EF-2 help drive
protein synthesis forward

38. STEP 3: Termination of
translation is triggered by
stop codons
Release factor enters
the A site and triggers
Hydrolysis of the peptidyl-tRNA
bond leading to release of
the protein.

39. Release of the protein causes
the disassociation of the
ribosome into its constituent
subunits.

40. THANK
YOU