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O-Level Molecular Genetics
What the O-Level syllabus expects for Molecular Genetics, and how to practise it.
What the syllabus expects
- Outline how DNA, genes and chromosomes relate to one another.
- Note that DNA takes the form of a double helix of two nucleotide strands, where every nucleotide is made up of a phosphate group, a sugar and one base drawn from a set of four.
- Give the rule for complementary base pairing.
- Note that each gene is a stretch of nucleotides forming part of a DNA molecule, codes for a single polypeptide, and acts as a unit of inheritance.
- Note that DNA carries the genetic code, which is drawn on to build specific polypeptides.
Scope: How transcription and translation work in detail need not be covered. - Note that genes may be moved from one organism's cells into another's to create transgenic organisms.
- Give a brief account of how a gene governing human insulin production can be placed into bacterial DNA so that human insulin is made in medical biotechnology.
- Talk through the potential benefits and the ethical questions raised by genetic engineering in medicine and in producing commercially valuable plants and animals.
How it's examined
Questions on this topic most often ask you to describe, explain, express. About 2% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (5 marks)
Describe how DNA is copied along the leading strand.
Show the worked answer
DNA replication is semi-conservative. The two strands of the DNA double helix are unwound and separated by an enzyme (DNA helicase), breaking the hydrogen bonds between complementary bases so each strand acts as a template. On the leading strand, replication proceeds continuously in the direction of the moving replication fork. Free DNA nucleotides in the nucleoplasm pair with the exposed template bases according to complementary base pairing (A with T, C with G). The enzyme DNA polymerase moves along the template in the 5' to 3' direction, joining the aligned nucleotides together and forming the sugar-phosphate backbone by covalent (phosphodiester) bonds. This produces a new strand that is complementary to the template, giving one continuous new strand alongside the original template strand.
Example 2 (5 marks)
Explain the structure of DNA and describe what it does.
Show the worked answer
DNA is a double helix made of two strands twisted together. Each strand is built from nucleotides, and each nucleotide is made of a deoxyribose sugar, a phosphate group and a nitrogenous base. The sugars and phosphates form a backbone, and the four bases are adenine (A), thymine (T), cytosine (C) and guanine (G). The two strands are held together by hydrogen bonds between complementary base pairs: A always pairs with T and C always pairs with G. DNA's function is to store genetic information: the sequence of bases codes for the sequence of amino acids in proteins (a gene), and DNA can be copied (replicated) so the information is passed on during cell division and to offspring.
Example 3 (5 marks)
Explain how bacteria may be used to manufacture human insulin.
Show the worked answer
This uses genetic engineering. (1) The human insulin gene is cut out of human DNA using a restriction enzyme. (2) A bacterial plasmid is cut open with the same restriction enzyme, giving complementary sticky ends. (3) The insulin gene is inserted into the plasmid and joined using DNA ligase, forming recombinant DNA. (4) The recombinant plasmid (vector) is put back into a host bacterium. (5) The bacteria are grown/cultured in a fermenter where they multiply and express the gene, producing human insulin which is then extracted and purified.
More worked questions on this topic
- A further instance of a genetically modified organism is the engineering of E. coli bacteria so (5 marks)
- Canavan disease is a genetic condition brought about by a fault in the gene coding for the enzy (3 marks)
- Give two advantages of cultivating genetically modified crop plants. (2 marks)
More O-Level Pure Biology topics
Cell Structure and Organisation · Movement of Substances · Biological Molecules · Nutrition in Humans · Transport in Humans · Respiration in Humans · all of O-Level Pure Biology