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
State the name of the causative organism of measles. Describe how the measles pathogen is transmitted. The measles pathogen contains a single stranded RNA molecule and no DNA. The cells that the measles pathogen infects contain double stranded DNA molecules. State two other ways in which the RNA in the measles pathogen differs from the DNA in the infected cells. 1. 2. The measles pathogen must carry out RNA replication to make new RNA molecules for the new pathogens. This happens inside the infected cell. The pathogen carries its own enzyme for RNA replication, but no other enzymes. Explain why the measles pathogen cannot use an enzyme from the cell to carry out RNA replication. The outer part of the measles pathogen contains an antigen called the H-protein. This antigen appears on the surface of cells infected with the measles pathogen. Describe the role played by T-lymphocytes in a primary immune response to an infection by the measles pathogen.
9700_w15_qp_21
THEORY
2015
Paper 2, Variant 1
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The treatment for people with active tuberculosis (TB) lasts six months and involves a combination of antibiotics. This is usually very effective if the person has a susceptible (non-resistant) strain of Mycobacterium tuberculosis, the causative organism of TB. Table 2.1 summarises one recommended treatment strategy that involves a combination of antibiotics. Table 2.1 antibiotic length of treatment mode of action of antibiotic rifampicin (R) 6 months enters bacterial cells and inhibits protein synthesis isoniazid (H) 6 months prevents the synthesis of cell wall components known as mycolic acids ethambutol (E) first two months prevents mycolic acids from being added to the cell wall pyrazinamide (Z) first two months prevents the synthesis of fatty acids Susceptible strains of M. tuberculosis will be killed using any one of the antibiotics listed in Table 2.1. However, combination treatment is preferred as it is one method that can be used to reduce the impact to society of antibiotic resistance. With reference to Table 2.1, explain how combination treatment for TB can help to reduce the impact of antibiotic resistance compared to single antibiotic treatment. Rifampicin binds tightly to an RNA polymerase molecule close to its active site. This affects the activity of the enzyme. RNA polymerase catalyses the formation of messenger RNA (mRNA) from DNA. State the term for this process. During the formation of RNA, a number of events occur that involve the action of RNA polymerase. Suggest ways in which rifampicin can affect the activity of RNA polymerase. RNA polymerase is composed of five different polypeptides. Gene rpoB codes for one of these polypeptides known as the β-subunit. One or more mutations in a specific region of rpoB result in strains of M. tuberculosis that are resistant to rifampicin. In these strains, mutations often occur in two DNA triplets in this region, in positions 526 and 531. Table 2.2 summarises the results of an investigation into seven rifampicin-resistant strains, A to G, that have amino acid changes for positions 526 and 531. Table 2.2 includes: • the change in the mRNA codon for position 526 or position 531 • the amino acid change that has occurred as a result of the mutation • the minimum concentration of rifampicin required to inhibit growth of the bacterial strain (MIC) • the number of other mutations occurring within the specific region of rpoB. Table 2.2 Key . approximately H greater than or equal to G less than or equal to strain codon involved mRNA codon change amino acid change MIC / µg cm–3 number of other mutations in the specific region A CAC UAC His Tyr G 50 B CAC AAC His Asn H 100 C CAC CGC His Arg . 50–75 D CAC CGC His Arg H 100 E CAC CGC His Arg . 50 F CAC UUC His H 100 UCG UUG Ser Leu G CAC UAC His H 100 UCG UUC Ser Phe Complete Table 2.2 to show the amino acid changes that have occurred in strains F and G. With reference to Table 2.2, list the strains of M. tuberculosis that show the greatest resistance to rifampicin. Suggest reasons to explain why strains C, D and E show: • resistance to rifampicin • different levels of resistances to rifampicin.
9700_w20_qp_22
THEORY
2020
Paper 2, Variant 2
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Bacteria were grown in a medium containing 15N. After several generations, all of the bacterial DNA contained 15N. Some of these bacteria were transferred to a medium containing the common isotope of nitrogen, 14N. The bacteria were allowed to divide once. The DNA of some of these bacteria was extracted and analysed. This DNA was all hybrid DNA containing equal amounts of 14N and 15N. The remaining bacteria were left in the medium with 14N and allowed to divide one more time. The DNA of some of these bacteria was extracted and analysed. What is the percentage of hybrid DNA?
9700_w23_qp_11
MCQ
2023
Paper 1, Variant 1
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Questions Discovered
244