6. Nucleic acids and protein synthesis
A section of Biology, 9700
Listing 10 of 441 questions
shows part of a DNA molecule. 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 G T A = adenine C = cytosine G = guanine T = thymine Key: N N N N Complete by drawing on the hydrogen bonds between the two base pairs shown. State the importance of hydrogen bonding in DNA structure. In the 1950s, Erwin Chargaff determined the relative quantities of the four bases in DNA in different organisms. His results provided important evidence for the model of DNA proposed by James Watson and Francis Crick in 1953. Some of Chargaff’s data is shown in Table 6.1. Table 6.1 organism percentage of adenine percentage of thymine percentage of guanine percentage of cytosine Escherichia coli 24.7 23.6 26.0 25.7 a yeast 31.3 32.9 18.7 17.1 wheat 27.3 27.1 22.7 22.8 octopus 33.2 31.6 17.6 17.6 sea urchin 32.8 32.1 17.7 17.3 chicken 28.0 28.4 22.0 21.6 human 29.3 30.0 20.7 20.0 With reference to , explain how the data in Table 6.1 helps to confirm the arrangement of bases in DNA. Table 6.2 shows Chargaff’s data for a virus. Table 6.2 organism percentage of adenine percentage of thymine percentage of guanine percentage of cytosine a virus 24.0 31.2 23.3 21.5 State how the result for the virus differs from the results for all the organisms given in Table 6.1. Suggest why the results for the virus are different from all the other organisms.
9700_s12_qp_21
THEORY
2012
Paper 2, Variant 1
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In order to sequence the DNA of a gene, it is first denatured to separate its two strands. Then, in the presence of a large supply of each of the four nucleotides, the single-stranded DNA is replicated by DNA polymerase. Explain what determines the sequence of nucleotides in the newly replicated strand of DNA. A low concentration of specially prepared nucleotides is also present. Once added to the chain, these nucleotides do not allow the chain to continue growing. Each special nucleotide is labelled with a fluorescent dye, using a different colour for each of the four bases. shows a replicated DNA chain ending with one of the special nucleotides. _ _ _ _ _ G _ _ _ _ G _ _ _ G _ _ _ C _ _ _ C _ _ _ special fluorescent nucleotide with C base not included here in this replication original strand of DNA new DNA strand direction of replication blue fluorescent dye With reference to and to the information given, suggest why a special nucleotide with a C base was not included by DNA polymerase at the first site requiring a C nucleotide. This method of sequencing a gene produces as many DNA fragments as there are nucleotides in the gene, each fragment differing in length by one nucleotide. shows part of a set of such fragments. A_ T_ _ C_ _ _ G_ _ _ _ A_ _ _ _ _ T_ _ _ _ _ _ These fragments are loaded onto a sequencing gel, shown in , and separated by electrophoresis. DNA fragments shown in loaded here –ve +ve detector light source tube gel In what order will the fragments shown in reach the light source and detector shown in ? Explain how gel electrophoresis separates these fragments of DNA.
9700_s12_qp_42
THEORY
2012
Paper 4, Variant 2
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Questions Discovered
441