Page 149 - SUBSEC October 2017_Neat

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02107020/CAPE/SPEC/KMS 2017
BIOLOGY
UNIT 1 – PAPER 02
KEY AND MARK SCHEME

Question 2 Specific Objectives: 2.6, 4.1, 4.2, 4.4, 5.4, 5.4

(a) (i) D → C → A → B – 1 mark [1 mark]

(ii) Labelling of Stage B
Labels should show
- cell plate (center)
- cell wall (outer)
- chromosomes

1 mark each [3 marks]

(iii) Crossing over can occur at Stage D (prophase) [1 mark]

Crossing over and variation

- Homologous chromosomes pair and wind around each other.
- Where two chromatids touch each other, the DNA breaks and
sections are exchanged between non-sister chromatids. This is
called crossing over.
- Crossing over results in chromosomes that are part maternal in
origin and part paternal
- This gives different combinations of alleles of different genes
- variation.

1 mark each = 4 marks
[5 marks]

(b) R-DNA process

1. Isolation of genes: DNA is removed from a host source. (The
insulin gene (INS) coding for insulin production is extracted
from a sample of human DNA).

2. Enzyme restriction: circular plasmid DNA from a bacterium is
“cut” via an endonuclease enzyme that breaks the DNA phosphate
backbone making it linear.

3. Ligation: the “pasting together” of (different source) fragments
of DNA forming the recombinant DNA molecule… using the ligase
enzyme, the human insulin gene is then combined with the linear
plasmid DNA and is re-circularized.
4. Uptake of plasmid: uptake of DNA by an organism (bacterium)...
the new recombinant DNA is taken-up by other bacteria to
facilitate replication of the plasmid.
5. Cloning: the subsequent growth (ideal conditions) allows for
high production of multiple copies of the new plasmid (R-DNA
molecule) and production of the gene product ... insulin.

1 mark each = 5 marks