A1.2 Nucleic acids. Practice questions with markscheme.

• each backbone is built of alternating phosphates (circles) and sugars (pentagons), so each strand has a direction set by the order of these groups;
• the right-hand strand is drawn inverted relative to the left, so its sugars and phosphates run in the opposite order down the molecule;
• the strand ends are therefore labelled 5′ and 3′ at opposite ends of the molecule / one strand runs 5′ to 3′ downwards while the other runs 3′ to 5′ downwards;
• the strands are joined by hydrogen bonds (dashed lines) between bases on opposite strands;
• the pairing is complementary: cytosine with guanine and thymine with adenine, as shown;
• hydrogen bonds are individually weak, so the strands can be separated, but together they hold the double helix stably, OWTTE;

Accept 'the two strands run in opposite directions' for the second point if the candidate refers to the diagram. Students are not required to state the numbers of hydrogen bonds (A1.2.6), so accept any number of dashed lines per pair.

• DNA is the genetic material / stores hereditary information (in all cells);
• RNA carries/transfers information for protein synthesis (mRNA) / has roles in expressing genes (tRNA/rRNA), OWTTE;

• both contain adenine, guanine and cytosine;
• DNA contains thymine, whereas RNA contains uracil in its place;
• in both, adenine pairs with its complementary pyrimidine (thymine in DNA, uracil in RNA) and guanine pairs with cytosine;

• the two strands are complementary;
• every A on one strand pairs with a T on the other (by hydrogen bonding);
• (so) A and T occur in equal numbers (and likewise G and C), OWTTE;

• pentose sugar (shown as a pentagon) labelled deoxyribose/sugar;
• phosphate (group) attached to the sugar, labelled;
• (nitrogenous) base attached to the sugar (on a different corner from the phosphate), labelled;

Phosphate and base must be attached to the sugar, not to each other. Accept circle for phosphate and rectangle for base.

• (a) [1]
  • P and R;
• (b) [2]
  • 48 000 base pairs = 96 000 bases;
  • 20 % of 96 000 = 19 200 (guanine bases);

  Award [2] for 19 200 with or without working. Award [1] for 9 600 (20 % of 48 000).

• (c) [2]
  • DNA;
  • (all four) genomes contain thymine, which occurs only in DNA / RNA would contain uracil in place of thymine, OWTTE;
• (d) [2 max]
  • genetic information is carried by the sequence/order of the bases, not by their proportions;
  • the same percentages of A, T, G and C can be arranged in an enormous number of different sequences;
  • (so) the genomes can contain different genes / code for different proteins, OWTTE;

• A: almost every codon (such as UUU and GGC) has the same meaning in all four systems, which is best explained by one shared origin; the few differences are single codons reassigned later in particular lineages, so the near-universal code remains evidence of common ancestry;

• A: DNA has deoxyribose and thymine; RNA has ribose and uracil;

• B: the bead holds the nucleotide at the 3′ end (A) and each later nucleotide joins the 5′ end, so the strand is built from its 3′ end to its 5′ end, the reverse of the order in which a cell adds nucleotides; A is the order a cell would use (5′ to 3′), and B and D list complementary bases, which would make a different strand;

• C: DNA has four bases, so one base gives 4 combinations and two give 4² = 16, too few for 20 amino acids, whereas three give 4³ = 64, which is enough;

• D: nucleosome = DNA wound around an octamer of histones (with an H1 linker histone), packaging DNA in eukaryotes;

• (a) [3 max]
  • a purine paired with a pyrimidine gives every base pair the same length / A–T and C–G pairs are equally long;
  • (so) the double helix has the same width / three-dimensional shape along its whole length, whatever the base sequence;
  • the bases lie inside the helix, with the sugar–phosphate backbones on the outside;
  • the backbone is the same repeating chain of sugars and phosphates whatever the base sequence;
  • histone proteins bind to the outside of the helix / to the backbone rather than to the bases, so their binding does not depend on the base sequence, OWTTE;
  • (so) any stretch of DNA can be wound around a core of histones in the same way, OWTTE;

  Award marks for links between levels; restating the table gains no mark. Accept attraction between negatively charged phosphate groups and positively charged amino acids of the histones for the fifth point.

• (b) [3 max]
  • the DNA is far longer than the nucleus is wide / over 300 000 times as long, so it must be packed tightly to fit;
  • wrapping the DNA around histone cores shortens it, and chains of nucleosomes can be coiled further / supercoiled;
  • a stretch that could not form nucleosomes would stay unpacked / loose, so it could tangle or fail to fit, OWTTE;
  • a further histone attached to the linker DNA holds the DNA in place / links nucleosomes into a compact, stable chain;

  Beyond this subtopic. Award if given; the marks above can be earned without it.

  • D2.2 packing and unpacking nucleosomes controls which genes are accessible for transcription / helps regulate gene expression, OWTTE;
  • D2.1 (further condensation into) chromosomes lets the DNA be separated during mitosis without tangling;

• (condensation reactions form) covalent bonds between nucleotides;
• the phosphate (group) of one nucleotide bonds to the sugar/pentose of the next;
• forming a (continuous) sugar–phosphate backbone (with bases projecting from it);

Accept phosphodiester bond. Do not accept hydrogen bonds.

• 1: phosphate (group);
• 2: pentose sugar (deoxyribose, if DNA);
• 3: nitrogenous base;

• the base sequence stores/encodes (genetic) information;
• (any) sequence and length are possible, giving unlimited capacity/diversity;
• complementary base pairing (A–T, G–C) allows each strand to act as a template;
• (so) DNA can be replicated accurately (and information passed on);
• hydrogen bonds between strands are (individually) weak, so the strands can be separated (for replication/transcription);
• the covalent sugar–phosphate backbone makes the molecule stable, protecting the information;