Cambridge International AS and A Level Biology 6: Nucleic acids and protein synthesis

Nucleic Acids and Protein Synthesis is Cambridge International Biology 9700 Topic 6. It connects nucleotide structure to DNA replication, RNA, the genetic code, transcription, RNA processing, translation and gene mutation. This note focuses on nucleotide components and the roles of DNA polymerase and DNA ligase; the exact official examination scope still needs verification.

1. Nucleotides and ATP

A nucleotide contains a pentose sugar, phosphate group and nitrogenous base. DNA nucleotides contain deoxyribose and use adenine, thymine, cytosine or guanine. RNA nucleotides contain ribose and use uracil instead of thymine.

Adenine and guanine are double-ring purines. Cytosine, thymine and uracil are single-ring pyrimidines. Check the current official syllabus before assuming which structural details an examination requires.

ATP is a phosphorylated nucleotide with adenine, ribose and three phosphate groups. ATP hydrolysis can be coupled to energy-requiring cellular processes; the overall reaction, rather than bond breaking alone, supplies the free-energy change. ATP is not a long-term energy store and is regenerated continually.

2. DNA structure

DNA contains two antiparallel polynucleotide strands. One runs 5-prime to 3-prime while the other runs 3-prime to 5-prime. Within each strand, phosphodiester bonds link the sugar of one nucleotide to the phosphate of the next, producing a sugar-phosphate backbone.

The strands form a double helix and are held together by complementary base pairing. Adenine pairs with thymine through two hydrogen bonds. Cytosine pairs with guanine through three. Complementarity permits accurate copying and information retrieval.

Hydrogen bonds are individually weak enough for strand separation, but their large number stabilises the molecule. Phosphodiester bonds are covalent and hold each backbone together.

3. Semi-conservative replication

In the normal eukaryotic nuclear cell cycle, DNA replication occurs in S phase. The two strands separate and each acts as a template. Complementary free nucleotides align. DNA polymerase catalyses phosphodiester-bond formation and adds nucleotides only in the 5-prime to 3-prime direction.

Because the templates are antiparallel, one new strand can be made continuously toward the replication region, while the other is made discontinuously in fragments. DNA ligase joins the fragments of the lagging strand. Each daughter DNA molecule contains one original strand and one newly synthesised strand, making replication semi-conservative.

Keep the roles of polymerase and ligase distinct. This focus is not an independently verified statement that all other replication factors are excluded from examination requirements.