Transcript The genetic code

Gene expression
Translation
Genetic code
The genetic code is a set of rules defining how
the four-letter code of DNA (RNA) is translated into the
20-letter code of amino acids, which are the building
blocks of proteins.
The characteristics of the genetic code
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Triplet code
Degenerate (redundant)
Unambiguous
Nonoverlapping
Comma less
Universal
Genetic code table
Translation
Translation is the synthesis of proteins directed by a mRNA template.
Principle:
Conditions:
a template is required
 mRNA
 Ribosomes
 Amino acids
 tRNAs
 Enzymes, translation
factors
 Energy (ATP, GTP)
Aminoacyl-tRNA synthesis
a ribosome
Initiation of translation
(prokaryotic cells)
1.
2.
3.
4.
5.
6.
7.
8.
fmet-tRNA formation
30 S + IF3 → dissociation of the ribosome
+ mRNA
fmet-tRNA is brought to start codon AUG (IF2/GTP →
IF2/GDP)
fmet-tRNA recognizes AUG: the anticodon UAC (fmetтРНК) base-pairs with a complementary codon AUG on
mRNA
+IF1 which binds A site and blocks it
+ 50 S → 70 S (reassociation of the ribosome)
Dissociation of initiation factors (IF1,2,3)
fmet-tRNA is at Р site of the ribosome, А site is empty
The selection of an initiation site (AUG codon)
depends on the interaction between the 30S
subunit and the mRNA template.
Shine-Dalgarno (SD) sequence (AGGAGG) is a
ribosomal binding site in prokaryotic mRNA,
generally located around 8 bases upstream of
the start codon AUG.
SD is complementary to a pyrimidine-rich
sequence at 3’end of 16 S rRNA in a small
subunit.
Elongation of translation
(prokaryotic cells)
1.
Formation of the next aminoacyl-tRNA
2.
The aminoacyl-tRNA is brought into A site of the ribosome (Tu/GTP
→ Tu/GDP)
3.
Codon recognition: the mRNA codon in the A site of the ribosome
forms hydrogen bonds with the anticodon of an entering tRNA
4.
Peptide bond formation: an rRNA molecule catalyzes the formation
of a peptide bond between the amino acid in the P site with the new
amino acid in the A site. (23 S rRNA in the 50S ribosomal subunit has
peptidyl transferase activity)
5.
Translocation of the ribosome by one codon (G/GTP → G/GDP)
6.
Regeneration of Tu/GTP:
Tu/GDP + Ts → Tu/Ts + GDP
Tu/Ts + GTP → Tu/GTP + Ts
Termination of translation
(prokaryotic cells)
 Stop codons: UAA, UAG, UGA
 Release factors: RF1, RF2, RF3, RRF
 RF1 recognizes UAG and UAA
 RF2 recognizes UGA and UAA
These factors trigger the hydrolysis of the bond in peptidyl-tRNA and the
release of the newly synthesized protein from the ribosome.
 RF3 facilitates binding of RF-1 or RF-2 to the ribosome and their
release. It has GTPase activity.
 RRF (ribosomal recycling factor) is required for release of uncharged
tRNA from the P site, and dissociation of the ribosome from mRNA with
separation of the two ribosomal subunits.
a polysome
Several ribosomes can translate an mRNA at the same time,
forming a polysome (a polyribosome).
Translation properties
in prokaryotic cells
 Ribosomes
70S (30S, 50S)
In eukaryotic cells
80S (40S, 60S)
 The first amino acid
formylmethionine (fmet)
methionine (met)
 Initiation factors
IF1, 2, 3
eIF 1, 2, 3, 4A, 4B, 4C, 4Е, 5
etc. (12)
 Selection of
the start codon
Shine-Dalgarno
sequence
Cap (the scanning
hypothesis); Kozak
sequence; an internal
ribosome entry site in
the 5'UTR
 Assembly of
initiation complex
30S + mRNA + fmet-tRNA
or 30S + fmet-tRNA +
mRNA
40S + met-tRNA + mRNA
Translation properties
in prokaryotic cells
in eukaryotic cells
 Elongation factors
EF1 (Tu), EF2 (Ts),
EF3 (G)
eEF1, eEF2
 Release factors
RF1, RF2, RF3, RRF
eRF1, eRF3
 Location in the cell
Cytoplasm
Cytoplasm, ER, mitochondria
Transcription and translation are coupled
Transcription and translation are
spatially and temporally separated.
Transcription occurs in the nucleus
to produce a pre-mRNA molecule.
The pre-mRNA is typically
processed to produce the mature
mRNA, which exits the nucleus
and is translated in the cytoplasm.
Processing of proteins
Processing of proteins is a complex of
post-translational modifications of the
protein molecules (protein maturation)
Steps of processing
 Proteolytic cleavage (removing segments of the polypeptide
chain by enzymes called proteases)
 Chemical modification (Individual amino acids in
polypeptidemight be modified by attachment of new
chemical groups)
 Folding (a process in which a polypeptide folds into a
specific, stable, functional, three-dimensional structure;
formation of tertiary structure of the protein) by foldases
and chaperones
 Association of protein subunits