Question 1
(a) List three ways in which transcription differs from translation in pro-tein synthesis.
[3]
Fig. 4.1 represents a polyribosome with several translation sites.
(b) Name the structures labelled A to C.
[3]
Fig. 4.1
(c) Name two molecules, in addition to the molecules shown in Fig. 4.1, which are required to complete translation.
[2]
(d) Describe two structural features which adapt tRNA to its role in translation.
[2]
[Total: 10] (100/P3/Q4)
Solution
(a) Three ways in which transcription differs from translation in protein synthesis are as follows:
(i) Transcription occurs in the nucleus of the cell, whereas translation occurs on the ribosomes in the cytoplasm.
(ii) Transcription results in a single strand of mRNA being formed, whereas translation results in a polypeptide chain of
amino acids being formed.
(iii)Transcription involves the pairing of bases on free nucleotides to the complementary bases on the unwound DNA section,
whereas translation involves the pairing of anticodon bases on tRNA to the complementary codon bases on the mRNA
strand.
(b) The structures are:
A - Ribosome (large sub-unit)
B- mRNA
C - Polypeptide chain
(c) Two molecules, which are req uired to complete translation are as follows:
(i) Amino acid.
(ii) Aminoacyl-tRNA synthetase
(d) Two structural features which adapt tRNA to its role in translation are as follows:
(i) It is a single-stranded polymer of nucleotides, upon which a sequence of triplet bases, called anticodons, determine the
sequence of amino acids to be formed during translation
(ii) It can form a clover shape, which has one end with a sequence of bases called an anticodon and another end with a CCA
point of attachment for an amino acid
Question 2
(a) Describe the structure of DNA.
[8]
(b) Explain how human insulin can be synthesised by bacteria.
[8]
( D 9 9 / P 3 / Q 9 )
Solution
(a) DNA is a polymer of nucleotides arranged in a double helix of two strands of nucleotides. Each nucleotide consists of a pento se
sugar, deoxyribose, a phosphate group and one of the four organic bases (adenine, guanine, cytosine or thymine). The two
strands are linked to each other by hydrogen bonding between the complementary organic bases of each strand, forming "rungs"
similar to those of a ladder. The alternating molecules of deoxyribose and phosphate form the "uprights". The two
polynucleotide strands also run antiparallel to each other, i.e. the 3' end of one strand is paired with the 5' end. In addit ion, each
organic base of one strand is paired to its complementary base by the following arrangement:
Adenine - Thymine,
Guanine - Cytosine
The DNA double helix structure comprises many millions of nucleotides and is many thousands of kilobases long. The distance
between the two strands of polynucleotides is approximately 2.0 nm and every 10 base pairs that constitute one turn of the
double helix are approximately 3.5 nm long.
(b) Human insulin can be synthesised by bacteria via the application of genetic engineering techniques, such as the process of ge ne
cloning. In the production of human insulin, genetically modified bacteria are used to manufacture the insulin. The process by
which the insulin is manufactured is as follows:
(i)
Firstly, messenger RNA (mRNA) is extracted frqm the cells of human pancreatic tissues.
(ii)
The mRNA is treated with reverse transcriptase in order to convert the mRNA coding for insulin into DNA, i.e. to produce
complementary DNA (cDNA), with the help of DNA polymerase.
(iii) Restriction endonucleases are used to cut and leave 'sticky' ends, which are single strands, to the cDNA.
(iv) A plasmid is removed from a bacterial cell and cut open at appropriate points with the help of restriction endonucleases.
The open portions of the plasmid will also have 'sticky ' ends, to which the cDNA is then inserted using DNA ligase.
(v)
The resultant plasmid now carries human DNA for insulin and is called a plasmid vector. It is re-inserted into the
plasmid-free bacterial cell.
(vi) Once back in the bacterial cell, the plasmid will replicate and use the new gene for making insulin