Chapter 9

Question 1

Polypeptides

Answer 1

• made of 20 different amino acids
• linked by covalent peptide bonds
• sequence of AA determines the properties of a polypeptide: shape, hydrophobicity, specificity, interactions with other proteins and function

Question 2

Structure of AA

Answer 2

central carbon, amino group, carboxyl group and an R chain

Question 3

Three regions of ribosomes

Answer 3

• A-site (aminoacyl site): entry for charged tRNAs
• P-site (peptidyl site): holds the tRNA to which the polypeptide is attached
• E-site (exit site): exit of uncharged tRNA

Question 4

Translation initiation in bacteria

Answer 4

• Shine-delgarno sequence helps the translation initiation complex identify the start codon
• The first amino acid is N-formylmethionine

Question 5

Translation initiation in eukaryotes

Answer 5

• Kozak sequence helps identify the start codon
• starts with methionine

Question 6

Translation termination

Answer 6

Happens when any one of the tree stop codons UAA, UAG, UGA enters the A-site of the ribosome

Question 7

How does translation termination occur

Answer 7

1. Release factor recruitment and binding to stop codon in the A-site
2. polypeptide release occurs, eRF1 fills A-site to trigger release by hydrolysis of GTP
3. Ribosome dissociation, RF ejection and mRNA release

Question 8

Polycistronic mRNA in bacteria

Answer 8

• Multiple polypeptide-producing segments separated by an intercistronic spacer
• each has its own shine-Dalgarno site that participates in identification of the start of translation

Question 9

The triplet code

Answer 9

• Three consecutive nucleotides correspond to one AA
• 64 different codons; 61 specific to AA, 3 stop codons
• Correspond to 20 AA (synonymous codons)

Question 10

How many codons correspond to Isoleucine

Answer 10

3

Question 11

How many codons correspond to leucine, or serine or arginine

Answer 11

6 each

Question 12

How many codons correspond to tryptophan

Answer 12

1

Question 13

Synonym order in genetic code

Answer 13

Codons assigned to the same amino acid are generally mutationally close to each other

Question 14

Relatedness order in genetic code

Answer 14

Codons assigned to different AA with the same chemical properties are generally mutationally close to one another

Question 15

Iso-accepting tRNAs

Answer 15

tRNAs that are charged with the same AA but that have different anticodons - degenerate code

Question 16

Third-base wobble

Answer 16

Some tRNAs can recognize multiple mRNA codons that differ only in their 3rd position - degenerate code

Question 17

Charging tRNA molecules

Answer 17

• Aminoacyl-tRNA synthetases catalyze the addition of AA to tRNAs
• Mutations in the tRNA including the anticodon positions prevent the tRNA synthetase from recognizing a tRNA

Question 18

Posttranslational polypeptide processing

Answer 18

• Removal of amino acids after translation (often N-terminus leader region)
• Modification of AA: Phosphorylation by kinases, dephosphorylation by phosphorylases, Methylation, acetylation, hydroxyl addition, addition of carbohydrate side chain to form glycoprotein, proteolysis, lipidation, ubiquitination
• All influence folding, stability/degradation, localization, and activity

Question 19

Cleavage of N-terminal AA

Answer 19

Removal of terminal AA

Question 20

Chemical modification of internal AA

Answer 20

• one of the most common AA modifications is phosphorylation - serves to activate or inactivate a protein
• can also add methyl, hydroxyl or acetyl groups to AA
• Carbohydrate side chains are also added to some proteins to make glycoproteins

Question 21

Polypeptide Cleavage

Answer 21

Cleavage of the polypeptide after translation generates a pro-peptide, which is the active form

Question 22

Peptide signal sequence

Answer 22

• 15 to 20 AA at the N-terminus of some proteins directs them to their cellular destinations
• Translation on the surface of the ER shuttles proteins to the Golgi apparatus where they can be exported in vesicles