2.7 and 7.3 Translation

Question 1

Define translation.

Answer 1

The synthesis of a polypeptide with an amino acid sequence determined by the base sequence of the mRNA.

Question 2

Describe the structure of a ribosome.

Answer 2

Consists of one small and one large subunit with binding sites for each of the molecules that take part in translation.

Question 3

What is the role of mRNA?

Answer 3

To carry the genetic information needed to synthesize a polypeptide; messenger RNA.

Question 4

What is the role of tRNA?

Answer 4

To decode the base sequence of mRNA into an amino acid sequence during translation; transfer RNA

Question 5

What is rRNA?

Answer 5

RNA that is part of the ribosome; ribosomal RNA.

Question 6

What is a codon?

Answer 6

A sequence of three bases.

Question 7

In what way is the genetic code degenerate?

Answer 7

Different codons can code for the same amino acid. In this context degenerate more closely means redundant.

Question 8

How do ribosomes, mRNA, and tRNA work together to produce a polypeptide.

Answer 8

• mRNA has a sequence of codons that specifies the amino acid sequence of the polypeptide
• tRNA molecules have an anticodon of three complementary bases that binds to the mRNA’s codons and they carry the corresponding amino acids to that codon
• ribosomes act as the binding site for mRNA and tRNA and also catalyze the assembly of the polypeptide

Question 9

Outline the process of translation.

Answer 9

1. The tRNA containing the matching anticodon to the start codon binds to P site of the small subunit of the ribosome
2. The small subunit binds to the 5’ end of the mRNA and moves along in a 5’→3’ direction until it reaches the start codon
3. The large subunit then binds to the smaller one
4. The next tRNA with the matching anticodon to the next codon on the mRNA binds to the A site
5. The amino acids on the two tRNA molecules form a peptide bond
6. The larger subunit moves forward over the smaller one
7. The smaller subunit rejoins the larger one, this moves the ribosome 3 nucleotides along the mRNA and moves the first tRNA to the E site to be released
8. The second tRNA is now at the P site so that another tRNA with the matching anticodon to the codon on the mRNA can bind to the A site
9. As this process continues, the polypeptide is elongated
10. Once the ribosome reaches the stop codon on the mRNA translation ends and the polypeptide is released

Question 10

Describe the function of a tRNA activating enzyme

Answer 10

Attaches an amino acid that matches the tRNA’s anticodon to the tRNA by using ATP

Question 11

Describe initiation.

Answer 11

• The small ribosomal subunit binds to the 5’ end of mRNA and moves along it until it reaches the start codon
• appropriate tRNA molecule binds to the codon via its anticodon
• large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small ribosomal subunit

Question 12

Differentiate between polypeptides produced by free ribosomes and those produced by bound ribosomes.

Answer 12

Free ribosomes: these polypeptides are destined for use in the cytoplasm, mitochondria, and chloroplasts
Bound ribosomes: Proteins to be used in the ER, Golgi Apparatus, lysosomes, plasma membrane, or outside of the cell

Question 13

Define polysomes

Answer 13

Multiple ribosomes translating a single mRNA sequence simultaneously

Question 14

Describe elongation.

Answer 14

• A second tRNA molecule pairs with the next codon in the ribosomal A-site
• The amino acid in the P-site is covalently attached via a peptide bond to the amino acid in the A-site

Question 15

Describe translocation.

Answer 15

• The ribosome moves along one codon position, causing the first tRNA to move into the E-site and be released, while the tRNA bearing the dipeptide moves into the P-site
• Another tRNA molecule attaches to the next codon in the newly emptied A-site and the process is repeated

Question 16

Describe termination.

Answer 16

• Elongation and translocation continue until the ribosome reaches a stop codon
• The polypeptide is released and the ribosome disassembles back into subunits

Question 17

How is a peptide bond formed between amino acids?

Answer 17

Amine group of one amino acid and the carboxyl group of the other combine by condensation to form a dipeptide with the waste product of one water molecule.