7.3 TRANSLATION

mRNA is convereted into amino acid sequence using tRNA

The process occurs at the Ribosome in the Cytoplasm

3 main steps within the process: 

• Initiation
• Elongation
• Termination

Components within Translation:

mRNA - Has a sequence of codons that specifies the amino acid sequence of the polypeptide

tRNA - Carry the corresponding amino acid to their codon. Has anticodon of three bases that binds to a complementary codon on mRNA

Ribosomes - Act as a binding site for mRNA and tRNA. Catalyzes the peptide bonds of the polypeptide. Small subunit binds to mRNA. Large subunit has binding sites for tRNAs and catalyzes the peptide bonds between amino acids

Small subunit: mRNA binding site

Large subunit: Contains EPA sites

Aminoacyl-tRNA: tRNA binds to its corresponding amino acid forming this complex

A = [Aminoacyl - tRNA binding site] Where incoming tRNA with its attached amino acid binds

P = [Peptidyl - tRNA binding site] Where tRNA moves after forming a peptide bond at the A site; the tRNA is holding the growing polypeptide chain

E = [Exit] Where tRNA moves to after transferring its amino acid to the growing polypeptide chain; ready to be released from the ribosome

Stage 1: Initiation

• Small ribosmal subunit binds to mRNA
• Small ribosmal subunit moves along mRNA till it reaches a start codon [AUG]
• tRNA with anticodon [AUC] complementary to codon binds to the P-site of ribosome. This tRNA carries the amino acid Methionine
• Large ribosomal subunit binds to the tRNA and small subunit

Stage 2: Elongation

• Second tRNA [with amino acid] complementary to the second mRNA codon binds to the A site of the Ribosome
• Amino acid in P site is linked to amino acid in A site by a peptide bond. Growing polypeptide increases in length. tRNA bound to polypeptide chain is now ready to be recycled
• Ribosome moves one codon along mRNA [5' to 3']. tRNA in P site is moved to E site and then released
• Another tRNA binds complementary to the next codon and binds to A site

Function: To bring amino acids from the cytoplasm to the growing polypeptide and to attach them in the correct location

Single chain of RNA with 3 loops

Acceptor Loop: Site attaches to amino acid at 3' terminal end

Anticodon: Made of 3 bases it attaches to mRNA codons

How does the tRNA hold the Amino acid

The correct amino acid is attached to the 3' terminal end of tRNA by a tRNA activating enzyme

Translation only works if the correct tRNA is loaded with the correct amino acid, corresponding to the codon on the mRNA

20 different tRNA activating enzymes correspond to the 20 different amino acids

Each enzyme attaches one specific amino acid to all tRNA molecules with anticodon corresponding to that amino acid

How is tRNA recognized: Shape & chemical properties - Enzyme-substrate specificity

Energy from ATP is used

• ATP, Amino acid and tRNA bind to activating enzyme's active site
• Pair of phosphates is released and remaining AMP binds to Amino acid raising energy levels

The reaction requires the hydrolysis of ATP molecule: ATP + H2O - AMP and PP [pyrophosphate]

• This energy allows Amino acid to bind to tRNA. Later the energy also allows the Amino acid to be linked to the growing polypeptide

tRNA is activated by a tRNA activating enzyme

Picks up new Amino acids when activated by a specific tRNA activating enzyme - uses ATP

20 enzymes which corresponds to the 20 Amino acids for which the tRNA molecule has the complementary anticodon

U6 : Translation can occur immeadiately after Transcription in Prokaryotes due to the absence of a nuclear membrane

In Prokaryotes:

• Ribsomes can be adjacent to the chromosomes
• No splicing in prokaryotes

In Eurkaryotes:

• mRNA needs to be relocated from the nucleus to the cytoplasm [through nuclear membrane]
• mRNA is modified [spliced] after Transcription after Transcription before Translation

Bound Ribosomes [attached to ER]

• Attached to membranes of Endoplasmic Reticulium
• Synthesize proteins for secretion from the cell or for use in Lysosomes

Free Ribosomes [not attached to ER]

• Synthesize proteins primarily for use within the cell

S1 : Idetification of Polysomes in electron micrographs of prokaryotes and eukaryotes

Polysome: Structure that consists of multiple ribosomes attached to a single mRNA

Multiple Ribosomes translating the mRNA together enables the cell to quickly create many copies of the required polypeptide

In Prokaryotes the chromosome may have numerous polysomes attached directly to it

Features: Polysomes show as dark side chains. The dark ball-like ribosomes on each polysome side chain are clearly visible

Polypeptides: Chains of amino acids joined by peptide bonds

20 different Amino acids which can be combined in any order

Each Amino acid has unique properties:

• Polar
• Non-polar
• Positive/Negatively charged
• Contain Sulphur

The properties of an Amino acid determines how a polypeptide folds into a protein

Protein: Molecule made up of one or more polypeptides

Primary level: Number and sequence of Amino acids Secondary level: Formation of alpha helices and beta pleated sheets stabllized by hydrogen bonds [N-H/C=O]

Tertiary level: 3D conformation of polypeptide when folded. Stabilized by intramolecular bonds between Amino acids Quaternary level: Linking two or more polypeptides to form a single protein

Stage 3: Termination

• When a stop codon is reached
• Release factor is attached to A site. Polypeptide chain is released. Ribosome dissasembles