HKDSE Biology – a modern approach 4
Answers to Coursebook Exercises
23.1 What is the structure of DNA?
True or false
1.
T
2. F
3. F
23.2 How does DNA determine the characteristics of an organism?
Fill in the blanks
1. (a) Genes
(b) 46
2. (c) triplet
(d) proteins
3. (e) Structural
(f) functional
23.3 DNA – the genetic code
Multiple choice
1. C
2. D
23.4 How are proteins synthesised?
Fill in the blanks
1. (a) transcription
2. (b) nuclear pore
(c) ribosome
3. (d) tRNA
© 2010 Aristo Educational Press Ltd.
(1 mark each)
( Total: 3 marks )
(1 mark each)
( Total: 6 marks )
(1 mark each)
( Total: 2 marks )
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HKDSE Biology – a modern approach 4
Answers to Coursebook Exercises
(e) codons
(f) amino acids
23.5 Mutations – changes in the genetic material
True or false
1. F
2. F
3. T
4. T
5. F
23.6 Applied genetics
(1 mark each)
( Total: 6 marks )
(1 mark each)
( Total: 5 marks )
Multiple choice
1. C
2. D
3. C
A.
Multiple choice
(1 mark each)
( Total: 3 marks )
1. D
2. B
3. C
4. C
5. B
6. A
7. A
B.
Short question
(1 mark each)
( Total: 7 marks )
1. (a) DNA contains two nucleotide chains, while RNA contains one nucleotide chain.
© 2010 Aristo Educational Press Ltd. 2

HKDSE Biology – a modern approach 4
Answers to Coursebook Exercises
(1 mark)
Thymine can only be found in DNA while uracil can only be found in RNA. (1 mark)
DNA contains deoxyribose while RNA contains ribose. (1 mark)
(b) During transcription, the DNA triplet codes are transcribed into complementary three-base sequences in RNA, which are called codons.
(1 mark)
(1 mark)
These RNA is called mRNA (or messenger RNA) which can transfer DNA’s information from the nucleus to the cytoplasm where proteins are synthesised. (1 mark)
( Total: 6 marks )
C.
Structured questions
1. (a) Gene mutations are a kind of mutations that occur within a single gene and involve changes in the sequences of bases. (1 mark)
It usually leads to a change in the amino acid sequence in the polypeptide formed from translation. (1 mark)
(b) Down syndrome:
It is caused by a chromosomal mutation.
People with Down syndrome have three chromosome 21.
OR
Sickle cell anaemia:
It is caused by a gene mutation, in which the gene that codes for haemoglobin polypeptides has a base substituted, leading to a change in the shape and properties of haemoglobin molecules. (3 marks)
(c) Mutagens are agents that induce changes in DNA and increase the rate of mutation.
(1 mark)
Examples of mutagens: (Any two of the following)
- dioxin / cigarette smoke / ionising radiation / other reasonable answers
(2 marks)
( Total: 8 marks )
2. (a) TACCGGAGCTATTGCGAC…
(b) 6
(1 mark)
(1 mark)
(c) (i) rough endoplasmic reticulum / ribosomes (1 mark)
(ii) mRNA carries the copy of DNA and attach onto a ribosome, which moves along the mRNA strand to ‘read’ the information of the codons. (1 mark)
At the ribosome, the anticodon of the tRNA binds with the complementary codon on the mRNA.
Each tRNA carries a particular amino acid.
Peptide bonds are formed between adjacent amino acids,
(1 mark)
(1 mark)
(1 mark)
© 2010 Aristo Educational Press Ltd. 3

HKDSE Biology – a modern approach 4
Answers to Coursebook Exercises forming a polypeptide chain which is released from the ribosome. The polypeptide chain is transformed into protein molecule. (1 mark)
( Total: 8 marks )
3. (a) (i) Sugar-phosphate backbone
(ii) L: thymine
(b) (i)
M: uracil
N: guanine
(1 mark)
(1 mark)
(1 mark)
(1 mark)
The fourth DNA base A cannot be transcribed and the original fourth RNA base
U is deleted in the mRNA strand. (1 mark)
This causes all of the codons occurring after the deletion to be read incorrectly during translation, (1 mark) producing an altered or nonfunctional protein.
(ii) base analog / analog of uracil
(1 mark)
(1 mark)
(iii) The disease can be treated by gene therapy, in which the defective genes are replaced with normal genes.
(1 mark)
(1 mark)
( Total: 10 marks )
© 2010 Aristo Educational Press Ltd. 4