Higher Biology Extended Response Question Worth 9 marks
Describe DNA under the following headings.
(i) Structure of DNA
(ii) Replication of DNA
Marks are numbered in red
Structure of DNA
DNA is an abbreviation of Deoxy-ribonucleic acid. Which is a double helix structure(1), found in the
nucleus of a cell, carrying genetic codes.
The DNA is made up of subunits called nucleotides. These are composed of a phosphate group, a
deoxyribose sugar and one of 4 bases(2).The bases found in DNA are Thymine, Adenine, cytosine
and guanine. The two stands of DNA are linked through hydrogen bonds – Adenine always pairs with
Thymine and Cytosine always pairs with Guanine.(4 +5)
DNA has a 5’ end and a 3’ end. The strands run in opposite directions, they are antiparallel. (6)
= 5 marks in total for pt (i)
Replication of DNA.
DNA must replicate for a cell to divide effectively.
For DNA replication the cell requires energy, enzymes a DNA template and DNA nucleotides.
The DNA unwinds and then unzips.
Free nucleotides line up with the exposed bases and form hydrogen bonds, holding them in place.
The ‘back bone’ of the new strand forms bond through the sugar and phosphates.
The replication happens from the 3 towards the 5 end, this means one stand produces a continuous
piece of DNA while the other forms small sections (12) which must have the gaps filled in and then
be joined together. In this way the DNA replicates and is semiconservative.
= 1 mark for pt (ii)
This extended question is an example of what might appear in the CfE Higher
Biology Exam. It is worth 9 marks and this essay would score 6.
To improve the second section the answers should contain more detail about how
the DNA unwinds and unzips and all of the enzymes involved.
Mark Scheme
1 double strand of nucleotides/double helix
2 deoxyribose sugar, phosphate and base
3 sugar phosphate backbone
4 complementary bases pair or A-T and C-G
5 H bonds between bases
6 antiparallel structure with deoxyribose and phosphate at 3´ and 5´ends
7 proteins/histones
8 DNA unwinds into 2 strands
9 primer needed to start replication
10 DNA polymerase adds nucleotides to 3´(deoxyribose) end of strand
11 DNA polymerase adds nucleotides in one direction
12 one strand replicated continuously, the other in fragments
13 fragments joined by ligase
 1 mark should be allocated for each correct description. No more than 5
marks should be awarded from points 1 – 7.
 No more than 4 marks should be awarded from points 8 – 13. Check any
diagram(s) for relevant information not present in text and award
accordingly.