3.9 DNA replication and the genetic code

Question 1

Describe the semi conservative replication of DNA. (5 marks)

Answer 1

1. Double helix DNA structure unwinds and separates into two strands (hydrogen bonds broken).
2. Free DNA nucleotides pair with their complementary bases.
3. Hydrogen bonds form between them.
   4.The new nucleotides join to their adjacent nucleotides with phosphodiester bonds.

Question 2

When the double helix DNA structure unwinds, what bonds must be broken? (1 mark)

Answer 2

Hydrogen bonds.

Question 3

What bonds are between nucleotides? (1 mark)

Answer 3

Phosphodiester bonds

Question 4

DNA strands are semi-conservative, what does this mean? (2 marks)

Answer 4

One strand is used as a template, and the other is a new strand formed from this.

Question 5

What enzyme separates the two strand of DNA double helix? (1 mark)

Answer 5

DNA helicase

Question 6

What does DNA helicase do? (2 marks)

Answer 6

Travel along the DNA backbone, catalysing reactions that break the hydrogen bonds between complementary base pairs as it reaches them. ‘Unzipping’.

Question 7

What enzyme catalyses the formation of phosphodiester bonds between nucleotides when the new strand is forming? (1 mark)

Answer 7

DNA polymerase

Question 8

What does DNA polymerase do? (1 mark)

Answer 8

Catalyses the formation of phosphodiester bonds between nucleotides when the new strand is forming

Question 9

What is the leading strand? (1 mark)

Answer 9

The strand that is unzipped from the 3’ (OH) end.

Question 10

Which strand of DNA is continuously replicated? (1 mark)

Answer 10

The leading strand.
The strand that is unzipped from the 3’ (OH) end.

Question 11

Which strand of DNA is discontinuously replicated? (! mark)

Answer 11

The lagging strand
The strand of DNA that is unzipped from the 5’ end.

Question 12

Why is one strand of DNA replicated continuously and the other discontinuously? (5 marks)

Answer 12

DNA polymerase always moves along the template strand in the same direction.
It can only bind to the 3’ end.
Because DNA only unwinds and unzips in one direction, DNA polymerase has to replicate each of the template strands in opposite directions.
The strand that is unzipped from the 3’ end can be continuously replicated as the strand unzips,
however, the other strand is unzipped from the 5’ end so DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.

Question 13

What is the lagging strand? (1 mark)

Answer 13

The strand of DNA that is unzipped from the 5’ end.

Question 14

Describe discontinuous replication. (4 marks)

Answer 14

Because DNA only unwinds and unzips in one direction, DNA polymerase has to replicate each of the template strands in opposite directions, from 3’ to 5’.
Because one of the strands is unzipped from the 5’ end, DNA polymerase has to wait until a section of the strand has unzipped and then work back along the strand.

Question 15

What happens if an error spontaneously occurs in the sequence of DNA nucleotides? (1 mark)

Answer 15

A mutation

Question 16

DNA codes for a sequence of amino acids. This is called the _____________________. (1 mark)

Answer 16

DNA codes for a sequence of amino acids that makes a protein. This is called the genetic code.

Question 17

What is a codon? (1 mark)

Answer 17

A sequence of three bases.

Question 18

What does a codon do? (! mark)

Answer 18

Each codon codes for an amino acid.

Question 19

What is a gene? (1 mark)

Answer 19

A section of DNA that contains the complete sequence of bases to code for an entire protein.

Question 20

‘Genetic code is universal.’ What does this statement mean? (1 mark)

Answer 20

All organisms use the same code.

Question 21

How many different codons possible are there? (1 mark)

Answer 21

4 x 4 x 4
64

Question 22

What is start codon? (1 mark)

Answer 22

The codon that comes at the beginning of a gene to signal the start of a sequence that codes for a protein.

Question 23

What is a stop codon? (1 mark)

Answer 23

They signal the end of the sequence.

Question 24

How many different amino acids regularly occur in biological processes? (1 mark)

Answer 24

20

Question 25

Many amino acids can e coded for by more than one codon, what does this make the genetic code? (1 mark)

Answer 25

Degenerate (same function)

Question 26

The genetic code is degenerate, what does this mean? (1 mark)

Answer 26

That different combinations of bases can code for the same amino acid.