The structure of DNA/Protein synthesis T2

Question 1

DNA is called a polynucleotide. Explain why.

Answer 1

It is a polymer: of nucleotides.

Question 2

What is the structure of a DNA nucleotide?

Answer 2

A phosphate group, linked to a 5 carbon deoxyribose, linked to a nitrogenous base.
Joined through condensation reactions.

Question 3

What is the bond between nucleotides?

Answer 3

Phosphodiester bonds

Question 4

What are the organic complementary bases that make up DNA?

Answer 4

Adenine and thymine

Cytosine and guanine

Question 5

Why do the bases only form these pairs?

Answer 5

A and G both have a two ring structure, whereas C and T have only one ring.
The bases pair to make three rings at each of the ‘rungs’.
There are only 2 hydrogen bonds between A and T,
but 3 hydrogen bonds between C and G.

Question 6

Name the place in a eukaryotic cell where mRNA will be synthesised.
(1)

Answer 6

Nucleus

Question 7

Phenylalanine is an amino acid found in many proteins in the human diet.
Converted to tyrosine using enzyme.
Phenylketonuria is the result of a gene mutation.
People with phenylketonuria cannot convert phenylalanine to tyrosine.
Explain why people with this gene mutation cannot convert phenylalanine to tyrosine.
(4)

Answer 7

• Change in DNA sequence/base sequence of gene.
• Change in amino acid, change in primary structure of protein/enzyme.
• Different R groups.
• Leading to different type/position bonds.
• Change in shape of active site/properties.
• Phenylalanine/substrate does not fit in enzymes active site.

Question 8

Explain why a gene mutation involving the replacement of one base with another has less effect than the loss of a base.
(2)

Answer 8

• Loss causes whole amino acid sequence (beyond mutation) to change/frame shift.
• Replacement only changes one codon/amino acid. May not change amino acid if third base is eq.
• Number of amino acids remains the same after replacement.

Question 9

One function of DNA is to act as a template strand.
State what is meant by the term template for the synthesis of mRNA.
(1)

Answer 9

• RNA nucleotides attach to this strand

- Nucleotide/base sequence that directs the synthesis of complementary mRNA.

Question 10

Describe how transcription is involved with the synthesis of an enzyme.
(4)

Answer 10

• DNA unwinds and hydrogen bonds between complementary strands are broken.
• The antisense/template/coding strand used for mRNA synthesis.
• RNA polymerase used to join RNA nucleotides.
• Complementary base pairing of A with U, not T.

Question 11

Explain triplet code.

Answer 11

-Each amino acid is coded for by 3 bases.

Question 12

Explain non overlapping genetic code.

Answer 12

• Each base/nucleotide only used once/for one codon.

- Triplet is discrete.

Question 13

Explain degenerate genetic code.

Answer 13

-More then one codon can be used for a specific amino acid.

Question 14

Describe how translation of this mRNA synthesises part of a polypeptide molecule.
(5)

Answer 14

• Ribosome is involved.
• tRNA attached to specific amino acid.
• Anticodons on tRNA bind/line up with codons on mRNA.
• Hydrogen bonds between bases of tRNA and mRNA.
• Formation of peptide bonds between adjacent amino acids.

Question 15

Explain nature of genetic code.

2

Answer 15

• Triplet codes/ 3 bases to each code.
• Each triplet of bases codes for one specific amino acid.
• Code is non over-lapping.
• Code is universal.
• Code is degenerate.

Question 16

Describe process of DNA replication.

5

Answer 16

-Semi-conservative replication.
-DNA strand unwinds.
-Nucleotides line up across both strands.
-Complementary base pairing.
-Hydrogen bonds formed between bases.
-Phosphodiester bonds formed between adjacent nucleotides, with DNA polymerase linking.
-Through condensation reactions.
-Using DNA polymerase, DNA helicase and ligase.
Two identical daughter strands are created

Question 17

Why is DNA rep. said to be semi-conservative?

Answer 17

When the DNA unwinds, and the new strand is built, two DNA molecules will contain an original ad newly synthesised strand.

Question 18

Meselson and Stahl:
Regarding N14 and N15 experiment:
Where would you expect the bands of DNA to be after one round of replication according to each DNA rep. theory.

Answer 18

Conservative replication:
Light band at top and heavy band at bottom.
Fragmentary:
Middle band.
Semi-conservative:
Middle band.

Question 19

Where would the bands in the tube be if the fragmentary replication was correct, after a second replication.

Answer 19

Middle band

Question 20

What did their experiment show?

Answer 20

After the first rep:
There was a middle band: ruling out conservative.
After the second:
There was one medium and one light band proving semi-conservative.

Question 21

On which type of RNA would you find an a) anti-codon, b) codon.

Answer 21

a) tRNA

b) mRNA

Question 22

The sequence bases AGT form a triplet code on the sense (coding) strand. What is:

a) its triplet code on the antisense strand?
b) its codon?
c) its anticodon?

Answer 22

a) TCA
b) AGU
c) UCA