2.1 Essential ideas
2.1.6 Structure of DNA and RNA
The key understandings from this section are:
- The nucleic acids – DNA, deoxyribonucleic acid, and RNA, ribonucleic acid – are polymers of nucleotides.
- DNA differs from RNA in the number of strands present, the base composition and the type of pentose.
- DNA is a double helix made of two anti-parallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.
- Each nucleotide consists of a 5-carbon sugar (pentose) covalently bonded to a phosphate in the C-5 position and a nitrogenous base at the C-1 position.
- There are four bases in each type of nucleic acid – nucleotides differ only by their base structures.
- Nucleotides can link in any combination, giving a wide range of possible DNA or RNA sequences.
- Nucleic acids are long polymers of covalently bonded nucleotides. The sugars and phosphates alternate to form a strong backbone with the bases projecting inwards.
- In DNA, two strands of nucleic acids are arranged in opposite orientations, so that the bases project inwards towards the centre of the structure. The two strands are antiparallel.
- The two strands are held together by hydrogen bonds between bases.
- The bases bond in specific pairs: A with T and C with G. This is called complementary base pairing (see Figure 2.1.6b, right).
- The two strands spontaneously take on the configuration of a twisted ladder, or double helix.
- DNA is a very stable structure because of the strong backbone and close association of strands. Its structure is well-suited to carry and conserve genetic information.
Skill: Drawing diagrams ofand
1. Practise drawing schematic diagrams ofand . Remember:
- Use pentagons for sugars, circles for phosphates, and rectangles for bases.
- The phosphate of one nucleotide and the third carbon on the pentose sugar of the second nucleotide are linked together with a solid line representing a covalent bond.
- RNA should be drawn as a single strand with no base pairing.
- DNA should be drawn as two antiparallel strands. Base pairs are A-T and C-G.
2. Identify what is wrong with these drawings. (Click on the images for answers.)
|Figure 2.1.6e –||Figure 2.1.6f –|
Figure 2.1.6g – Twisted ladder
The double helix. How does the structure of allow efficient storage of genetic information?
Nature of science
Using models: James Watson (left) and Francis Crick (right) built actual models to help them determine the structure of DNA.