Lecture 16: Genteic Code and Translation

Question 1

What causes autosomal dominant ICA

Answer 1

A nonsense, a frameshift, and 5 missense mutations

Question 2

One gene one enzyme hypothesis

Answer 2

Genes function by encoding enzymes, and each gene encodes a separate enzyme

Question 3

Auxotroph

Answer 3

An organism with a mutation that prevents synthesis of a nutritional component (i.e. an amino acid) necessary for growth

Question 4

How did one gene one enzyme hypothesis get proven

Answer 4

Mutation could be identified by the spores ability to grow in minimal medium supplemented y the substance that the mutant could not synthesize
-when they place arginine in minimal media, auxotrophic organism was able to grow

Question 5

How did they identify the amino acid specified by a homopolymer

Answer 5

They placed a homopolymer (UUU) to a test tube with 1 radioactive AA and 19 unlabeled AAs
-The test tube with radioactive phenylalanine lit up so they knew UUU encoded phenyalanine

Question 6

How did they determine AAs specified by heteropolymers

Answer 6

They added mRNA with a heteropolymer (GUU) and also added tRNAs. They washed out the tRNAs that did not bind and the ones left were bound to valine- showing that GUU corresponds to valine

Question 7

Stop codons include

Answer 7

UAA, UAG, UGA

Question 8

Degenerate code

Answer 8

Amino acid may be specified by more than one codon

Question 9

Synonymous codons

Answer 9

Codons that specify the same amino acid

Question 10

Isoaccepting tRNAs

Answer 10

Different tRNAs that accept the same amino acid but have different anticodons

Question 11

Wobble position

Answer 11

3rd codon pair is less important

A single anticodon can pair with more than one codon

Question 12

Nonoverlapping

Answer 12

A single nucleotide may not be included in more than one codon

Question 13

Amino acid attaches to the __’ end of tRNA

Answer 13

3’ end

carboxyl group of AA attaches to hydroxyl of 3’ end of tRNA

Question 14

The base on the 3’ end of the tRNA is always

Answer 14

Adenine

Question 15

T or F: ATP is used to attach tRNA to AA

Answer 15

True

Question 16

Initiation of translation

Answer 16

Initiation factor IF-2, initiator tRNA with N-formylmethionine attached to form fmet-tRNA
GTP used for initiation**

Question 17

Shine-Dalgarno sequence

Answer 17

In prokaryotes- this sequence in mRNA is required for attachment of small unit of ribosome

Question 18

Kozak sequence

Answer 18

This sequence in mRNA signals the beginning of the coding sequence in eukaryotes
(AUG)

Question 19

IF3 and initiation start

Answer 19

IF-3 binds small subunit so large cannot bind

• small unit can then attach to mRNA
• tRNA charged with fmet forms a complex with IF-2 and GTP, and binds initiation codon while IF-1 joins small subunit
• All IFs dissociate, GTP hydrolyzed to GDP, large subunit joins

Question 20

What role does poly A tail play in initiation

Answer 20

Proteins attach to poly A tail and interact with cap-binding proteins, which enhance the binding of the ribosome to the 5’ end of mRNA

Question 21

Elongation steps

Answer 21

fmet-tRNA occupies P site
EF-Tu, GTP, charged tRNA enter A site
GTP cleaved, EF-Tu-GDP complex released
Peptide bond forms between AAs in A and p site
Ribosome moves down mRNA to next codon

Question 22

What is required for the ribosome to move down the mRNA to the next codon (translocation)

Answer 22

EF-G and GTP

Question 23

Release factors

Answer 23

Proteins that are triggered by presence of stop codon to bind ribosome and trigger release of polypeptide
GTP is required

Question 24

T or F: An mRNA molecule may be transcribed simultaneously by several ribosomes

Answer 24

True