▶️ Answer/Explanation
(a) i.
- Amylose is a linear (unbranched) polymer of glucose that forms a helical structure.
- Amylopectin is a branched polymer of glucose, with branches occurring approximately every 20–30 glucose units.
(a) ii.
- Humans do not produce cellulase, the enzyme required to break down β-1,4 glycosidic bonds in cellulose.
- As a result, cellulose passes undigested and acts as dietary fiber, aiding peristalsis and bowel movement.
(b)
(c)
| Feature | RNA | DNA |
|---|---|---|
| Strands | Usually single-stranded | Usually double-stranded |
| Nitrogenous base | Contains uracil | Contains thymine |
| Pentose sugar | Ribose | Deoxyribose |
Markscheme:
a.i. Amylose is unbranched/helical while amylopectin is branched.
a.ii. Enzymes required to digest cellulose are not present in the human gut OR undigested cellulose provides bulk/fibre.
b. Diagram requirements:
- Correct structure of two amino acids.
- H2O lost.
- C from COOH of one links to N of NH2/NH3+ from the other.
- Correct labelling of the peptide bond.
Example diagram:
c. Two structural differences between RNA and DNA:
- Number of strands (usually only one strand in RNA, two strands in DNA).
- Base composition (uracil only in RNA, thymine only in DNA).
- Type of pentose (ribose in RNA, deoxyribose in DNA).
▶️ Answer/Explanation
(a)
(b) i.
A mutation is a permanent change in the DNA sequence of an organism.
It can happen naturally (like during DNA copying) or due to external factors (radiation, chemicals, etc.).
(b) ii.
Let’s break it down easily:
- Mutations = New Variations: Mutations create new traits or variations in a population, like a longer neck in a giraffe or better eyesight in birds.
- Natural Selection = Survival of the Fittest: Nature selects the traits that help an organism survive better in its environment.
- Link: Without mutations, no new traits would appear, and natural selection would have nothing to act on.
So, evolution would not occur.
Markscheme:
a.
1. correct base sequence: T, G, A;
2. strand drawn anti-parallel;
3. correct shapes used;
b.i.
change in genetic makeup/DNA/nucleotide/base sequence
b.ii.
1. mutations cause variation among organisms of same species/population;
2. some variations/mutations make individual more suited to its environment/way of life;
3. individuals that are better adapted survive and produce offspring;
4. individuals pass on genetic characteristics/mutation/variation to offspring;
5. natural selection increases frequency of characteristics/alleles that make individuals better adapted;