1. Gene expression Translation

2. The characteristics of the 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
Triplet code
Degenerate (redundant)
Unambiguous
Nonoverlapping
Comma less
Universal

3. Genetic code table

4. Translation Translation is the synthesis of proteins directed by a mRNA template.
Principle:
a template is required
Conditions:
mRNA
Ribosomes
Amino acids
tRNAs
Enzymes, translation factors
Energy (ATP, GTP)

5. Aminoacyl-tRNA synthesis

7. a ribosome

8. Initiation of translation (prokaryotic cells)
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 (fmet-тРНК) 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

9. 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.

10. Elongation of translation (prokaryotic cells)
Formation of the next aminoacyl-tRNA
The aminoacyl-tRNA is brought into A site of the ribosome (Tu/GTP → Tu/GDP)
Codon recognition: the mRNA codon in the A site of the ribosome forms hydrogen bonds with the anticodon of an entering tRNA
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)
Translocation of the ribosome by one codon (G/GTP → G/GDP)
Regeneration of Tu/GTP:
Tu/GDP + Ts → Tu/Ts + GDP
Tu/Ts + GTP → Tu/GTP + Ts

11. 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.

12. a polysome
Several ribosomes can translate an mRNA at the same time, forming a polysome (a polyribosome).

13. Translation properties
In eukaryotic cells
80S (40S, 60S)
methionine (met)
eIF 1, 2, 3, 4A, 4B, 4C, 4Е, 5 etc. (12)
Cap (the scanning hypothesis); Kozak sequence; an internal ribosome entry site in the 5'UTR
40S + met-tRNA + mRNA
in prokaryotic cells
Ribosomes S (30S, 50S)
The first amino acid formylmethionine (fmet)
Initiation factors IF1, 2, 3
Selection of Shine-Dalgarno
the start codon sequence
Assembly of S + mRNA + fmet-tRNA
initiation complex or 30S + fmet-tRNA +
mRNA

14. Translation properties
in prokaryotic cells
Elongation factors EF1 (Tu), EF2 (Ts),
EF3 (G)
Release factors RF1, RF2, RF3, RRF
Location in the cell Cytoplasm
Transcription and translation are coupled
in eukaryotic cells
eEF1, eEF2
eRF1, eRF3
Cytoplasm, ER, mitochondria
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.

15. Processing of proteins
Processing of proteins is a complex of post-translational modifications of the protein molecules (protein maturation)

16. 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