The molecular machinery of inheritance is remarkably precise. This HL lesson covers D1.1 DNA replication, D1.2 Protein synthesis and D1.3 Mutation & gene editing in depth.
You will study leading and lagging strands, translation in detail, mutation types and CRISPR gene editing.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.
D1.1 DNA replication
Replication in detail (HL)
Replication is semi-conservative. Helicase unwinds the helix; because DNA polymerase only builds in the 5→3 direction:
the leading strand is made continuously toward the fork;
the lagging strand is made in short Okazaki fragments, later joined by DNA ligase.
Primase lays RNA primers; polymerase proofreads to keep the copy accurate.
Calculate
Your turn — calculate
1A double-stranded DNA molecule contains 20% thymine. Calculate the percentage that is cytosine.
%
Hint: %T = %A = 20, so G + C = 60%, shared equally between G and C.
D1.2 Protein synthesis
Transcription and translation (HL)
Transcription: RNA polymerase makes mRNA from a DNA template.
Translation at the ribosome has three phases — initiation, elongation and termination. tRNAs, activated with their amino acids, read codons via anticodons; the ribosome forms peptide bonds. Several ribosomes can read one mRNA as a polysome.
Calculate
Your turn — calculate
2A polypeptide is 150 amino acids long. Calculate the minimum number of nucleotides in the mRNA coding sequence (ignore the stop codon).
nucleotides
Hint: each amino acid is coded by one triplet, so 150 × 3.
D1.3 Mutation & gene editing
Mutation and gene editing
Mutations change DNA: substitutions (point mutations, e.g. sickle-cell), and insertions/deletions that cause a frameshift. Mutagens raise the rate.
CRISPR-Cas9 is a guided nuclease that cuts DNA at a chosen sequence, allowing precise gene editing — powerful but ethically debated.
Sort it
Sort each item into its group
Sort each item into the correct category.
Enzyme in replication
Molecule in translation
Type of mutation
Quick check
Question
?On the lagging strand, DNA is synthesised as short pieces called...
Quick check
Question
?During translation, the amino acid sequence of a protein is determined by...
Quick check
Question
?A single base substitution that changes one amino acid in a protein is a...
Match it
Match each item to its partner
Match each replication term to its description.
Item
Partner
Quick check
Question
?DNA polymerase can only add new nucleotides to the...
Recap
The big ideas to take away
D1.1 (HL): semi-conservative; leading (continuous) vs lagging (Okazaki) strands; ligase
D1.2 (HL): transcription then translation (initiation, elongation, termination); polysomes