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
shows the two base pairs in a DNA molecule. N J K L N N N N N H N CH3 H H O O adenine N N N N N N O N H N H H H H O Name the bases labelled J and K and the bond labelled L. J K L HIV enters T-lymphocytes by a form of endocytosis. Two of the enzymes in HIV are: • reverse transcriptase, which uses viral RNA as a template to make DNA to incorporate into the chromosomes of the host’s cells • protease, which is used to break a polypeptide into smaller molecules. These molecules are used to make the protein coat of new viral particles, which will infect other cells. Various drugs have been developed to treat HIV infections. Table 4.1 gives information about some of these drugs. Table 4.1 drug enzyme inhibited mode of action zidovudine reverse transcriptase occupies active site tenofovir reverse transcriptase occupies active site efavirenz reverse transcriptase occupies sites other than the active site atazanavir protease occupies active site Explain the difference between the mode of action of zidovudine and efavirenz. People who receive drug treatment for HIV take a mixture of drugs that act in different ways. Suggest the advantage of taking a mix of the drugs shown in Table 4.1. Antibiotics are prescribed to people who have HIV/AIDS for the treatment of secondary infections, but not to treat the HIV infection. Explain why this is so.
9700_s13_qp_21
THEORY
2013
Paper 2, Variant 1
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Red blood cells are formed from cells called reticulocytes. Stem cells in the bone marrow produce reticulocytes which differentiate into red blood cells. During differentiation haemoglobin is produced. shows the structure of small sections of DNA and messenger RNA (mRNA) in the nucleus of a reticulocyte during transcription. O O O O O O O P –O O O O N N HN N N N N N N NH2 O O P –O O O O O O P –O O O O O P –O NH2 O H2N HN O O O O– O P N O O O– O P O O O O O– O P O O N N N NH N N NH2 H2N H2N O N N N N O N O O O O– O P NH P S Q R O O O O P –O O O N N N N NH2 NH2 O O P –O O O OH OH O OH O O P –O O O OH O O P –O NH2 O N O N N NH N O N O NH adenine 3' 3' 3' 5' 5' 5' Name the bases P to S. P Q R S Describe the role of the mRNA molecule shown in .
9700_s15_qp_21
THEORY
2015
Paper 2, Variant 1
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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. 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 composition of this DNA?
9700_s16_qp_12
MCQ
2016
Paper 1, Variant 2
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The bacterium Escherichia coli divides once every 50 minutes at 36 °C. E. coli were grown on a medium containing only heavy nitrogen, 15N, until all of the bacterial DNA contained heavy nitrogen (0 minutes). Some of the bacteria were moved from a heavy nitrogen medium and cultured in a medium with only light nitrogen, 14N. Some bacteria were collected after each of three generations. Their DNA was extracted and centrifuged. Hybrid DNA contains heavy and light nitrogen. The diagram shows the possible positions (upper, middle and lower) of the bands of DNA. The actual positions of bands in the first two samples are shown...
9700_s17_qp_12
MCQ
2017
Paper 1, Variant 2
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shows part of a DNA molecule. A = adenine C = cytosine Key: O O O P O O O O P O O O O P O O O P O O O O ¯ ¯ ¯ O¯ ¯ ¯ O P O O O O P O O O O O N N N N N H H H H3C N N N H H H H H O O O O O O O N N N A C N N N N Use to explain how the structure of mRNA differs from the structure of DNA. shows: • the first seven amino acids of the β chain of haemoglobin • the first amino acid in the sequence is valine (Val) • the 21 base pairs in the sequence of DNA that code for these seven amino acids. amino acid sequence Val His Leu Thr Pro Glu Glu base sequence in DNA CAC GTG GAC TGA GGA CTC CTC GTG CAC CTG ACT CCT GAG GAG Table 4.1 shows the triplets of bases that code for seven amino acids. Using and Table 4.1, state what will happen to the sequence of amino acids in the first part of the β chain of haemoglobin: if the base pair at position 6 is deleted if the three base pairs at positions 7, 8 and 9 are deleted. Table 4.1 amino acid DNA triplets cysteine (Cys) TGT TGC glutamic acid (Glu) GAA GAG histidine (His) CAT CAC leucine (Leu) CTT CTC CTA CTG proline (Pro) CCT CCC CCA CCG threonine (Thr) ACT ACC ACA ACG valine (Val) GTT GTC GTA GTG no amino acid STOP TAA TAG TGA DNA is involved in the processes of replication and transcription. Complete Table 4.2 by using a tick (3) to indicate which features apply to each of the processes. Use a cross (7) for features that do not apply. The first row has been completed for you. Table 4.2 feature replication transcription a single-stranded molecule is produced hydrogen bonds are broken both strands of DNA act as templates phosphodiester bonds are formed DNA polymerase is used Telomeres are parts of chromosomes. Describe the function of telomeres. Describe the function of ribosomes in protein synthesis.
9700_s17_qp_21
THEORY
2017
Paper 2, Variant 1
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is a diagram showing the structure of a section of a DNA molecule. X Key hydrogen bond Draw a circle around one monomer of DNA in . Name the two bases forming the base pair at X in and give a reason for your answer. bases reason The statements 1–5 describe events that occur during DNA replication. DNA polymerase forms a phosphodiester bond DNA double helix forms hydrogen bonds break hydrogen bonds form two strands of the double helix separate Write the numbers 1 to 5 in the spaces below to show the order in which these events occur. The first one has been done for you. The telomere is a region found at the end of a chromosome. Outline the function of telomeres.
9700_s19_qp_23
THEORY
2019
Paper 2, Variant 3
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Questions Discovered
244