6. Nucleic acids and protein synthesis
A section of Biology, 9700
Listing 10 of 441 questions
DNA is said to replicate in a semi-conservative way. Results of Meselson and Stahl’s experiments gave overwhelming support to this theory. They used E. coli which has a generation time of 50 minutes. Here are the stages occuring in their experiment but they are in the wrong order. 14N DNA contains the ‘light’ isotope of nitrogen. 15N DNA contains the ‘heavy’ isotope. P All bacteria contain 15N DNA. Q All bacteria contain hybrid DNA (15N DNA and 14N DNA). R Bacteria contain either all 14N DNA or hybrid DNA. S Bacteria grown in a 15N medium for many generations. T Bacteria transferred to a 14N medium and sampled every 50 minutes. Which sequence of letters shows the correct order of the stages in the experiment?
9700_w14_qp_13
MCQ
2014
Paper 1, Variant 3
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Bacteria were grown in a medium containing 15N. After several generations, all of the 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. In another experiment, some bacteria from the medium with 15N were transferred into a medium of 14N. The bacteria were allowed to divide three times. The DNA of some of these bacteria was extracted and analysed. What is the composition of this DNA?
9700_w16_qp_12
MCQ
2016
Paper 1, Variant 2
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The polymerase chain reaction (PCR) is used to produce large amounts of DNA from a very small original sample. The main stages of a PCR are shown in . step 1 DNA sample heated to 95 °C step 2 cooled to 65 °C then primers added step 3 incubated at 72 °C with Taq polymerase, complementary strands of DNA synthesised step 4 heated to 95 °C again and process repeated Explain why the DNA sample is heated to 95 °C in step 1. Explain why primers are added in step 2. Explain why the enzyme Taq polymerase is used in step 3. After an organism dies, its DNA gradually breaks down. However, cells in bones that were buried hundreds of years ago may still yield small amounts of DNA that can be extracted, amplified using PCR and then analysed. Mitochondrial DNA (mtDNA) is often used because there are usually more than 100 copies of it in one cell, compared with only two copies of nuclear DNA. For example, in 1994, mtDNA from bones that had been found in a grave in Russia was analysed to confirm that these were the remains of the royal family, who were known to have been killed in 1918. The mtDNA extracted from the bones was compared with the mtDNA from a living relative of the family. The family tree of the Russian royal family and some of their relatives is shown in . A B C F G D E Olga Tatiana female Maria Anastasia Alexei Queen Victoria Key: Nicholas Alexandra male possible identities of bones found living relative Explain why there are usually more than 100 copies of mtDNA in a cell, but only two copies of nuclear DNA. All of the mitochondria in a zygote come from the egg, not the sperm. List the letters of the people in the family tree in who would be expected to have mtDNA identical to the mtDNA of the living relative, G.
9700_w16_qp_41
THEORY
2016
Paper 4, Variant 1
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The polymerase chain reaction (PCR) is used to produce large amounts of DNA from a very small original sample. The main stages of a PCR are shown in . step 1 DNA sample heated to 95 °C step 2 cooled to 65 °C then primers added step 3 incubated at 72 °C with Taq polymerase, complementary strands of DNA synthesised step 4 heated to 95 °C again and process repeated Explain why the DNA sample is heated to 95 °C in step 1. Explain why primers are added in step 2. Explain why the enzyme Taq polymerase is used in step 3. After an organism dies, its DNA gradually breaks down. However, cells in bones that were buried hundreds of years ago may still yield small amounts of DNA that can be extracted, amplified using PCR and then analysed. Mitochondrial DNA (mtDNA) is often used because there are usually more than 100 copies of it in one cell, compared with only two copies of nuclear DNA. For example, in 1994, mtDNA from bones that had been found in a grave in Russia was analysed to confirm that these were the remains of the royal family, who were known to have been killed in 1918. The mtDNA extracted from the bones was compared with the mtDNA from a living relative of the family. The family tree of the Russian royal family and some of their relatives is shown in . A B C F G D E Olga Tatiana female Maria Anastasia Alexei Queen Victoria Key: Nicholas Alexandra male possible identities of bones found living relative Explain why there are usually more than 100 copies of mtDNA in a cell, but only two copies of nuclear DNA. All of the mitochondria in a zygote come from the egg, not the sperm. List the letters of the people in the family tree in who would be expected to have mtDNA identical to the mtDNA of the living relative, G.
9700_w16_qp_42
THEORY
2016
Paper 4, Variant 2
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is a diagram of a monomer of the nucleic acid, messenger RNA. F E D Name D, E and F in . D E F State one way in which the structure of DNA differs from the structure of messenger RNA. Telomeres are repeating sequences of bases located at the ends of DNA molecules. These repeating sequences do not code for proteins. The enzyme telomerase ensures that telomeres do not shorten each time DNA is replicated. A shows the end of a DNA molecule during replication. DNA polymerase cannot attach to the region labelled X, so it cannot complete the synthesis of the new strand without the action of telomerase. Telomerase synthesises additional lengths of DNA that are added to the telomere. These additional lengths are used by DNA polymerase to complete the process of replication. B is an enlarged view of region X to show the action of the enzyme telomerase. A T 5’ 3’ 3’ 5’ A T A A T T A T A A T G C G C G C G C G C G C T A T A A T G C G C G T T A G G G T T A G X B T T A T C A A A G C G G C T C C C A U A C A U free nucleotides telomerase template RNA Telomerase contains a short length of RNA that acts as a template for the synthesis of DNA as shown in B. Explain how a molecule of telomerase synthesises additional lengths of DNA. Telomerase is not present in prokaryotic cells. Suggest why prokaryotes do not have telomerase. One of the ways to diagnose lung cancer is to determine the concentration of telomerase in cells from the lining of the bronchus. Explain why determining the activity of telomerase may be useful in the diagnosis of lung cancer.
9700_w18_qp_23
THEORY
2018
Paper 2, Variant 3
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Bacteria were grown in a medium containing 15N. After several generations, all of the 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 (14N / 15N). In another experiment, some bacteria from the medium with 15N were transferred into a medium of 14N. The bacteria were allowed to divide three times. The DNA of some of these bacteria was extracted and analysed. What is the composition of this DNA?
9700_w20_qp_13
MCQ
2020
Paper 1, Variant 3
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DNA replication in bacteria was investigated. Bacteria were grown in a medium with only heavy nitrogen, 15N, until all of the bacterial DNA was heavy. These bacteria were moved from the heavy nitrogen medium and cultured in a medium with only light nitrogen, 14N. This formed the first generation. The bacteria continued to reproduce, and a sample of bacteria was collected from the second generation and the DNA analysed. Hybrid DNA contains heavy and light DNA. Which row is correct for the second generation?
9700_w22_qp_11
MCQ
2022
Paper 1, Variant 1
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Questions Discovered
441