6.1. Structure of nucleic acids and replication of DNA
A subsection of Biology, 9700, through 6. Nucleic acids and protein synthesis
Listing 10 of 244 questions
Two sets of bacteria were grown using different types of nitrogen-containing growth media. One set was grown in a medium containing the ‘heavy’ isotope of nitrogen, 15N, until all the DNA was labelled. The other set were grown in a medium containing the ‘light’ isotope of nitrogen, 14N, until all the DNA was labelled. The DNA from each set of bacteria was extracted and centrifuged. The diagram shows the position in the centrifuge tubes of this DNA. DNA with 14N DNA with 15N Bacteria with 15N labelled DNA were transferred to a medium containing 14N and allowed to reproduce once. The DNA of the new generation of bacteria was extracted and centrifuged. Which tube shows the position of DNA from this new generation of bacteria?
9700_s14_qp_11
MCQ
2014
Paper 1, Variant 1
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shows the structures of four biological molecules A, B, C and D. C O O C C O O C C O O C H H H H H CH2OH H H H NH2 CH2 O P O– O– O H H H B C D A OH OH N C H H HO H3C CH2 CH3 H O CH C H OH H OH OH HO O N N O N N Give the letter, A to D, of the molecule in which: is a nucleotide can form peptide bonds contains ester bonds. Some of the molecules in can form polymers. Name a polymer which can be formed only from many molecules of C. State one way, visible in , in which the part labelled 1 of molecule A differs from the part labelled 2. Molecule D can form macromolecules with other similar monomers. These macromolecules have three dimensional shapes held in place by interactions or bonds other than those between adjacent monomers. Name two of these interactions or bonds. 1. 2.
9700_s14_qp_23
THEORY
2014
Paper 2, Variant 3
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DNA replication is an important event in the cell cycle. State when, during the cell cycle, DNA replication occurs. shows pairing between two bases, X and Y, in a DNA molecule. O H H Y X N N O N N N N N H N H H Name the type of bond shown by the dotted lines between the bases. State which base, X or Y, is a pyrimidine and explain your answer. The compound benzopyrene, found in tar from tobacco smoke, can become chemically changed in cells and interferes with DNA replication, causing gene mutations. State what is meant by the term gene mutation. Mutations that occur in dividing cells of the gas exchange system may result in lung cancer. Suggest the differences in the cell cycle of a cancer cell compared with that of a normal cell of the same type.
9700_s15_qp_23
THEORY
2015
Paper 2, Variant 3
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Meselsohn and Stahl investigated DNA in bacteria. They grew bacteria in a medium with only heavy nitrogen, 15N, until all of the bacterial DNA was heavy. These bacteria were moved from a heavy nitrogen medium and cultured in a medium with only light nitrogen, 14N. A sample of bacteria was collected from the first generation in the medium containing light nitrogen and their DNA was analysed. Hybrid DNA contains both heavy and light DNA. Which row shows the percentage of heavy DNA strands and the percentage of hybrid DNA molecules in the first generation produced in the medium containing light nitrogen?
9700_s18_qp_11
MCQ
2018
Paper 1, Variant 1
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Meselson and Stahl investigated DNA in bacteria. They grew bacteria in a medium with only heavy nitrogen, 15N, until all of the bacterial DNA was heavy. These bacteria were moved from a heavy nitrogen medium and cultured in a medium with only light nitrogen, 14N. A sample of bacteria was collected from the second generation and their DNA analysed. Hybrid DNA contains both heavy and light DNA. Which row shows the percentage of heavy DNA strands and the percentage of hybrid DNA molecules in the second generation produced in the medium containing light nitrogen?
9700_s19_qp_13
MCQ
2019
Paper 1, Variant 3
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There are many different forms of Vibrio cholerae, a bacterium that is found naturally in aquatic environments. The bacterium is motile (can move) and uses a cell structure known as a flagellum to allow it to move through water. is a drawing of four cells of one form of V. cholerae. flagellum X Y cell wall Two main forms of V. cholerae, O1 and O139, are able to colonise the small intestine and cause cholera. These two forms are able to produce a toxin, choleragen, which causes the symptoms of diarrhoeal disease. Mutant V. cholerae that lack flagella are less able to cause disease. The magnification of the diagram shown in is ×32 000. Calculate the actual width X–Y in in nanometres and give your answer to the nearest 10 nm. Complete to show the formula you will use to make your calculation. actual width = answer = nm State the term used to describe disease-causing organisms, such as the bacterium V. cholerae. Outline one way in which an uninfected person may become infected by V. cholerae. Choleragen is produced after V. cholerae has penetrated (passed through) the mucus lining and attached to intestinal epithelial cells. Choleragen is composed of two subunits: • subunit A consists of one polypeptide • subunit B consists of five identical polypeptides • the polypeptide in subunit A is different from the polypeptides in subunit B. Two genes, ctxA and ctxB, are needed to produce choleragen. Only one strand of the DNA forming gene ctxA is involved in the production of subunit A. Only one strand of the DNA forming gene ctxB is involved in the production of subunit B. Explain why only one strand of the DNA of each gene is involved in the production of the subunits. Monoclonal antibodies (mAbs) can be designed to act against components of the cell wall of V. cholerae. The cell wall has an outer membrane with lipopolysaccharide (LPS) molecules, shown in . cell wall outer membrane peptidoglycan layer fluid space lipid A core polysaccharide lipopolysaccharide (LPS) O-polysaccharide The core polysaccharide and the lipid A components of the LPS molecules are the same in V. cholerae O1 and V. cholerae O139. However they have different O-polysaccharides. There are also different types of V. cholerae O1 and these have different O-polysaccharides. Laboratory tests were carried out using two different monoclonal antibodies that had been designed and produced to act against the LPS of bacterial cultures of V. cholerae O1: • mAb 2D6 acts against the O-polysaccharide • mAb ZAC-3 acts against the core polysaccharide and lipid A components. Explain why the mAb ZAC-3 produced against the core polysaccharide and lipid A components will not act against the O-polysaccharide of the LPS molecules. The results of the tests showed that both mAbs were effective in causing agglutination of bacteria and in preventing their motility. This suggests they may be useful for preventing cholera and for treating the disease. Discuss whether mAb 2D6 and mAb ZAC-3 may be useful for preventing cholera and for treating the disease.
9700_s22_qp_23
THEORY
2022
Paper 2, Variant 3
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Questions Discovered
244