Everything from the 4 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A nucleotide is a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA) and a nitrogen-containing base. Adenine and guanine are purines with a double ring; cytosine, thymine and uracil are pyrimidines with a single ring. ATP is a phosphorylated nucleotide: adenine, ribose and three phosphates. Nucleotides are joined into a polynucleotide by phosphodiester bonds (condensation reactions) between the phosphate of one and the sugar of the next. DNA is a double helix of two antiparallel strands, one running 5′ to 3′ and the other 3′ to 5′, held together by hydrogen bonds between complementary bases: A with T (two hydrogen bonds) and C with G (three). mRNA is a single, short, unpaired polynucleotide with ribose and uracil in place of deoxyribose and thymine.
Do not say thymine is a purine because it pairs with adenine. Pairing tells you nothing about the class: every pair is one of each. Learn the ring count instead (memory aid: pure As Gold): adenine and guanine are the double-ringed purines; C, T and U are single-ringed pyrimidines.
Antiparallel does not just mean the strands are side by side. It means they run in opposite directions: where one strand has its 5′ end, the other has its 3′ end.
DNA is copied during the S phase of the cell cycle. The hydrogen bonds between bases break and the double helix unwinds, so each strand is exposed as a template. Free DNA nucleotides pair with the exposed bases by complementary base pairing (A with T, C with G). DNA polymerase joins the new nucleotides by forming phosphodiester bonds, but it can add nucleotides only to the 3′ end of a growing strand, so new DNA is always built in the 5′ to 3′ direction. On one template the new strand grows towards the replication fork continuously (the leading strand); on the other it must grow away from the fork, in short fragments (the lagging strand), which DNA ligase joins with phosphodiester bonds. Each daughter molecule has one original and one new strand: replication is semi-conservative.
Do not say DNA polymerase builds the lagging strand 3′ to 5′. It never does: both new strands are made 5′ to 3′. The lagging strand is in fragments precisely because its 5′ to 3′ direction points away from the fork.
The intermediate band never disappears in later generations: the two original heavy strands are kept, so there are always exactly two hybrid molecules. Only their proportion halves each generation.
A polypeptide is coded for by a gene, a sequence of nucleotides forming part of a DNA molecule. The genetic code is a triplet code: each group of three bases codes for one amino acid, or acts as a start or stop signal, and it is universal (the same in almost all organisms). In transcription, in the nucleus, RNA polymerase separates the strands of the gene and builds mRNA complementary to the template (transcribed) strand; the other is the non-transcribed strand. In eukaryotes the primary transcript is modified: introns (non-coding) are removed and exons (coding) are joined. In translation, at a ribosome in the cytoplasm, each tRNA carries a specific amino acid and has an anticodon complementary to an mRNA codon. Starting at AUG, amino acids are joined by peptide bonds until a stop codon is reached.
mRNA is complementary to the template strand, not to the coding strand. So the mRNA reads the same as the coding (non-transcribed) strand, with U instead of T. If you are given the template strand, pair every base and swap T for U.
Remember which is which: exons are expressed (they stay in the mRNA); introns are in the way (they are removed). It is the RNA that is edited, not the DNA: the introns stay in the gene.
A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide. In a substitution one base pair is replaced by another, so only one codon changes: because the code is degenerate it may still code for the same amino acid (no effect), it may code for a different amino acid (as in sickle cell anaemia, where glutamic acid becomes valine in β-globin), or it may become a stop codon (a shortened polypeptide). A deletion or insertion of one nucleotide causes a frameshift: every codon after the mutation is read differently, so many amino acids change and a stop codon often appears early. The polypeptide usually folds wrongly and cannot function.
Do not write that a mutation is a change in an amino acid or in a protein. A gene mutation is a change in the DNA base sequence; a changed polypeptide is a possible consequence.