The Genetic Code

Question 1

Outline the three general steps of translation.

Answer 1

1. mRNA produced by transcription
2. Ribosome binds and recognizes a sequence on mRNA upstream of protein CDS
3. Ribosome synthesizes a polypeptide according to nucleic acid sequence on mRNA

Question 2

In what direction does the ribosome read the mRNA during translation?

Answer 2

5’ to 3’

Question 3

In which direction is the polypeptide synthesized during translation?

Answer 3

N terminus to C terminus.

Question 4

Why is the triplet code used instead of singlet, doublet, etc.?

Answer 4

It is the smallest number of nucleotides that can have unique combinations for 20 amino acids.

Question 5

What is meant by the genetic code being “comma-free”?

Answer 5

There are no nucleotides used as punctuation between the triplet nucleotides.

Question 6

What is meant by the genetic code being non-overlapping?

Answer 6

Neighbouring triplets do not overlap.

Question 7

What is meant by the genetic code being unambiguous?

Answer 7

The meaning of the nucleotide triplet never changes.

Question 8

What is meant by the genetic code being polar?

Answer 8

The code is written and read in the 5’ to 3’ direction.

Question 9

What is meant by the genetic code being degenerate?

Answer 9

Multiple different nucloetide triplets may code for the same amino acid.

Question 10

What is meant by the genetic code being nearly universal?

Answer 10

It is nearly identical across all domains of life.

Question 11

What is a codon?

Answer 11

One unit of the nucleotide triplet which specifies an amino acid.

Question 12

What is the typical start codon of a protein CDS?

Answer 12

ATG

Question 13

What are the three typical stop codons of a protein CDS?

Answer 13

TAA (ochre), TAG (amber), TGA (opal)

Question 14

What is an open reading frame?

Answer 14

The region between a start and stop codon in a gene.

Question 15

Is ATG ALWAYS a stop codon?

Answer 15

No, it codes for methionine, which can also be present within the polypeptide.

Question 16

What does it mean by there being three open reading frames?

Answer 16

Three possible ways to begin reading the mRNA. Start from first triplet/skip first nucleotide and following three are first triplet/skip first two nucleotides and following three are first triplet.

Question 17

What determines the reading frame used for translation?

Answer 17

The position of the start codon.

Question 18

How many possible frames does DNA have?

Answer 18

two strands*(3 frames/ strand) = 6 frames

Question 19

What is a benefit of the genetic code’s degeneracy?

Answer 19

Minimizes effects of mutations (ex. most mutations at 3rd codon are silent)

Question 20

What do many point mutations at the second position do?

Answer 20

Keep polarity the same

Question 21

What position do point mutations have the greatest effect?

Answer 21

First

Question 22

What is the mutation in sickle cell anemia?

Answer 22

GAA -> GUA (glutamic acid to valine)

Question 23

What is a silent mutation?

Answer 23

Point mutation that does not change the amino acid.

Question 24

What is a missense mutation?

Answer 24

Point mutation which changes the amino acid.

Question 25

What is a nonsense mutation?

Answer 25

Point mutation creating a stop codon, protein synthesis stopped prematurely.

Question 26

What is a frameshift mutation?

Answer 26

Insertion or deletion of nucleotide(s) changes the reading frame.

Question 27

Which type of mutation typically has the largest impact?

Answer 27

Frameshift (<3)