🧬 Strand C: Molecular Genetics

Course: SBI4U — Biology, Grade 12
Focus: DNA replication, gene expression, mutations, biotechnology

1. DNA Structure

DNA (deoxyribonucleic acid) is a double helix of two antiparallel strands connected by complementary base pairs: A-T (2 hydrogen bonds) and G-C (3 hydrogen bonds). Each strand has a sugar-phosphate backbone with nitrogenous bases extending inward.
Central Dogma of Molecular Biology DNA mRNA Protein Transcription Translation Replication DNA → DNA Semi-conservative DNA polymerase DNA → mRNA In nucleus RNA polymerase mRNA → Protein At ribosomes tRNA + amino acids

2. DNA Replication

DNA replication is semi-conservative — each new double helix contains one original strand and one new strand.

Key enzymes: Helicase (unwinds), Primase (RNA primer), DNA Polymerase III (adds nucleotides 5'→3'), Ligase (seals fragments)

3. Transcription & Translation

FeatureTranscriptionTranslation
LocationNucleusRibosomes (cytoplasm or rough ER)
TemplateDNA (template strand)mRNA
ProductmRNAPolypeptide (protein)
Key enzymeRNA polymeraseRibosome (rRNA)
Direction5' → 3' (mRNA)5' → 3' (reading mRNA)

4. Mutations

Point mutations: Substitution (silent, missense, nonsense), Insertion, Deletion (frameshift)

📝 Worksheet — Strand C: Molecular Genetics

1. [K/U — 3 marks] Given the DNA template strand: 3'-TACGGATCCAATTAGC-5'

a) Write the complementary mRNA strand:

b) Using a codon chart, determine the amino acid sequence:

2. [Thinking — 4 marks] A mutation changes the DNA template from TAC to TAT at the start of a gene.

a) What is the original mRNA codon and amino acid?

b) What is the mutated mRNA codon and amino acid?

c) What type of mutation is this (silent, missense, nonsense, frameshift)?

d) Predict the effect on the resulting protein.

3. [Communication — 3 marks] Explain why DNA replication is described as "semi-conservative." Include a labeled diagram showing the parent molecule and two daughter molecules.

4. [Application — 4 marks] CRISPR-Cas9 is a gene editing technology that can cut DNA at specific locations.

a) Describe one potential medical application of CRISPR.

b) Identify one ethical concern about using CRISPR on human embryos.

SBI4U — Biology, Grade 12 | Strand C: Molecular Genetics | Ontario Curriculum 2008