7.3 Translation

Question 1

Ribosome Structure

Answer 1

proteins and rRNA molecules
2 sub-units one large one small
3 binding sites for tRNA on the surface of the ribosome (2 tRNA molecules can bind at the same time to the ribosome) - “E” site (exit), “P” site (peptidyl), “A” site (aminoacyl)
there is a binding site for mRNA on the surface of the ribosome

Question 2

tRNA Structure

Answer 2

sections that become double-stranded by base pairing, creating loops
a triplet of bases called the anticodon which is part of a loop of 7 unpaired bases
2 other loops
the base sequence CCA at the 3’ end which forms a site for attaching an amino acid

Question 3

tRNA-activating enzymes

Answer 3

attaches an amino acid to the 3’ terminal of the tRNA
20 different tRNA activating enzymes
needs ATP
enzyme specific t both the correct amino acid and the correct tRNA
once ATP and amino acid are attached to the active site of the enzyme the amino acid is activated by the formation of a bond between the enzyme and adenosine monophosphate (AMP) then he activated amino acid is covalently attached to the tRNA energy from this bond is later used to link the amino acid to the growing polypeptide chain during translation

Question 4

Initiation of Translation

Answer 4

mRNA molecule binds to the small ribosomal sub-unit at an mRNA binding site
an initiator tRNA molecule carrying ethionine then binds at the start codon “AUG”
the large ribosomal sub-unit binds to the small one
initiator tRNA is in the P site
the next codon signals another to bind, it occupies the A site
a peptide bond is formed between the amino acids in the P and A site

Question 5

Elongation of the Polypeptide

Answer 5

series of repeated steps
the ribosome translocates 3 bases along the mRNA moving the tRNA in the P site to the E site freeing it and allowing a tRNA with the appropriate anticodon to bind to the next codon and occupy the vacant A site

Question 6

Termination of Translation

Answer 6

the process continues until a stop codon is reached when the free polypeptide is released

Question 7

Free Ribosomes

Answer 7

located in the cytoplasm

synthesize proteins for use primarily within the cell (cytoplasm, mitochondria, chloroplast)

Question 8

Bound Ribosomes

Answer 8

bound to ER
synthesizes proteins destined for use in the ER, GA, lysosomes, plasma membrane, or outside the cell
whether the ribosome is free or bound depends on the presence of a signal sequence on the polypeptide being translated it is the first part of the polypeptide
as the signal sequence is created it becomes bound to a signal recognition protein that stops the translation until it can bind to a receptor on the surface of the ER once this happens translation begins again with the polypeptide moving into the lumen of the ER as it is created

Question 9

Primary Structure

Answer 9

the sequence and number of amino acids in the polypeptide

Question 10

Secondary Structure

Answer 10

the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding

Question 11

Tertiary Structure

Answer 11

the further folding of the polypeptide stabilized by interactions between R groups

• hydrophobic interaction
• disulphide bridge
• ionic bond

Question 12

Quaternary Structure

Answer 12

proteins with more than one polypeptide chain