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
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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is a diagram showing the structure of part of a DNA molecule. F E adenine adenine key one hydrogen bond Identify structure E and structure F in . E F On draw a circle around one nucleotide. State the name of the covalent bond that links two nucleotides together. shows the RNA base sequence of a short length of primary transcript. Complete by writing the DNA base sequence of the template strand used to form the primary transcript. DNA base sequence used to form the primary transcript primary transcript G G U G C U A A U C U A In eukaryotic cells, the primary transcript is modified to form mRNA. Explain how the primary transcript is modified to form mRNA. The mRNA strand is translated at the ribosome to form a polypeptide. Describe how the process of translation results in the formation of a polypeptide.
9700_m22_qp_22
THEORY
2022
Paper 2, Variant 2
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Cysteine is an amino acid containing sulfur. shows the structure of the molecule formed by joining two cysteine molecules together. C C CH2 N C O O O H C CH2 SH H H H SH N H H Draw a circle around an R-group in the molecule shown in . Goblet cells in the human gas exchange system produce proteins called mucins. The ends of mucin molecules contain many cysteine residues. Mucin strands are formed by joining the ends of mucin molecules together through covalent bonds between R-groups. State the name of these covalent bonds. Mucin strands are transported out of the goblet cells and then absorb water to form mucus. Suggest and explain how mucin strands are transported out of the goblet cells. Cystic fibrosis is a genetic disease caused by a mutation in the human CFTR gene. This results in mucus that is thicker than normal. Suggest how thicker mucus interferes with the maintenance of healthy gas exchange surfaces in the lungs. Row 1 and row 2 of Table 2.1 show the DNA base sequences of part of the normal CFTR allele and the same part of a mutated CFTR allele. The base sequences shown are for the DNA strands used in the synthesis of RNA. When Table 2.1 is completed, row 3 will show the base sequence of the RNA synthesised from the same part of the mutated CFTR allele. Table 2.1 DNA base sequence of part of the normal CFTR allele T A G T A G A A A C C A DNA base sequence of part of the mutated CFTR allele T A G T A A C C A C A A RNA base sequence synthesised from the mutated CFTR allele The difference between the DNA base sequence in row 1 and the DNA base sequence in row 2 of Table 2.1 is caused by a single gene mutation. State the name of this type of gene mutation. Row 1 and row 2 in Table 2.1 show the DNA strands used in the synthesis of RNA. State the term used to describe the DNA strand that is used in the synthesis of RNA. Complete Table 2.1 to show the missing bases in row 3. The normal CFTR allele is approximately 189 000 base pairs in length. The CFTR polypeptide consists of only 1480 amino acids. Explain the reasons for this difference between the number of base pairs and the number of amino acids.
9700_m23_qp_22
THEORY
2023
Paper 2, Variant 2
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The two strands of a DNA molecule are held together by hydrogen bonds between complementary base pairs. Explain why the hydrogen bonding between the two strands of DNA is important for it to carry out its functions. Switching genes on and off allows proteins to be synthesised only when required. Processes P and Q occur when a gene is switched on, as shown in . DNA mRNA polypeptide P Q Name processes P and Q. P Q DNA is a very stable molecule. This means that it is not broken down either chemically or by enzymes during the normal life of the cell. In contrast, mRNA is described as being highly labile. This means that most mRNA molecules are broken down in the cytoplasm within a few hours of their release from the nucleus. Suggest the significance of: DNA being very stable mRNA being highly labile.
9700_s12_qp_23
THEORY
2012
Paper 2, Variant 3
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Questions Discovered
244