Chapter 9 Flashcards

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1
Q

Polypeptides

A
  • 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
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2
Q

Structure of AA

A

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

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3
Q

Three regions of ribosomes

A
  • 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
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4
Q

Translation initiation in bacteria

A
  • Shine-delgarno sequence helps the translation initiation complex identify the start codon
  • The first amino acid is N-formylmethionine
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5
Q

Translation initiation in eukaryotes

A
  • Kozak sequence helps identify the start codon
  • starts with methionine
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6
Q

Translation termination

A

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

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7
Q

How does translation termination occur

A
  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
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8
Q

Polycistronic mRNA in bacteria

A
  • 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
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9
Q

The triplet code

A
  • Three consecutive nucleotides correspond to one AA
  • 64 different codons; 61 specific to AA, 3 stop codons
  • Correspond to 20 AA (synonymous codons)
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10
Q

How many codons correspond to Isoleucine

A

3

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11
Q

How many codons correspond to leucine, or serine or arginine

A

6 each

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12
Q

How many codons correspond to tryptophan

A

1

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13
Q

Synonym order in genetic code

A

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

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14
Q

Relatedness order in genetic code

A

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

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15
Q

Iso-accepting tRNAs

A

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

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16
Q

Third-base wobble

A

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

17
Q

Charging tRNA molecules

A
  • 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
18
Q

Posttranslational polypeptide processing

A
  • 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
19
Q

Cleavage of N-terminal AA

A

Removal of terminal AA

20
Q

Chemical modification of internal AA

A
  • 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
21
Q

Polypeptide Cleavage

A

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

22
Q

Peptide signal sequence

A
  • 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