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Nucleotide chemistry explains how a DNA molecule can hold information and be copied. Keep the covalent backbone bonds distinct from the hydrogen bonds between complementary bases.
What you'll be able to do
A nucleotide contains a pentose sugar, phosphate group and nitrogen-containing base. DNA uses deoxyribose and the bases adenine, thymine, cytosine and guanine. RNA uses ribose and uracil instead of thymine. A nucleotide is not the same as a base; it includes all three components.
Condensation joins neighbouring nucleotides through phosphodiester bonds between sugar and phosphate groups, forming a polynucleotide backbone. The base sequence can vary while the backbone is chemically repetitive. This allows information to be carried in sequence rather than in a different type of sugar for every instruction.
DNA usually consists of two antiparallel polynucleotide strands wound into a double helix. Complementary bases pair by hydrogen bonding: adenine with thymine and cytosine with guanine. There are two hydrogen bonds in an A–T pair and three in a C–G pair, but both strands still have strong covalent backbones.
Complementarity means either strand can act as a template for the other. In a complete double-stranded DNA molecule, total A equals T and total C equals G. These equalities need not hold within one isolated strand. RNA in the syllabus is typically a shorter single polynucleotide chain; some RNA regions can also fold through internal base pairing.
In living cells, DNA stores hereditary information and RNA participates in transferring and using that information. Messenger RNA carries a transcribed sequence to ribosomes; transfer RNA participates in translation. Ribosomes contain ribosomal RNA and proteins, so RNA is not simply a disposable copy with no structural role.
A gene is a particular DNA sequence contributing to a functional product; a DNA molecule can contain many genes and non-coding regions. Historically, DNA’s relatively simple chemistry made some scientists doubt its informational capacity. Evidence concerning sequence, inheritance and replication was needed, rather than the visual appeal of a helix alone.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.