IP Biology Notes: Molecular Genetics (Upper Sec 12)

Study guide26 Nov 2025, 00:00 ZUpdated 17 Jul 2026

Free IP Biology notes on DNA, genes, protein synthesis basics, and genetic engineering for Sec 3 to Sec 4.

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For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.

How this chapter applies

Eclat core: DNA, genes, chromosomes, nucleotides, complementary base pairing, polypeptide coding, transgenic organisms, insulin production, and ethical evaluation form the main route.
School-sensitive extension: transcription, translation, mRNA processing, codon tables, restriction maps, plasmid-selection detail, and wider biotechnology should be used only where the current school teaches them.
2027 national comparison: K325 Topic B12 requires the DNA-gene-chromosome relationship and a qualitative gene-to-polypeptide link, but explicitly does not require transcription and translation details.
Check your school: the expected treatment of mRNA, enzymes, vectors, marker genes, and bioethics can differ. Use the school task to set the molecular depth.
Exam-track route: use the separate O-Level and SEC G3 Biology notes for K325 topic ownership.

If molecular genetics is where DBQ evidence, sequence changes, or biotechnology explanations start to break down, use the IP Upper Secondary Biology tuition page for the focused Year 3-4 route.

The core idea is simple: DNA stores instructions by the order of its bases.

Use it as a working check: A gene is a DNA section that codes for a polypeptide. Base order affects amino acid order, which affects protein shape and function.

Then go one layer deeper: Example: changing one base can change one amino acid. Sometimes nothing changes, but sometimes the protein folds differently and works poorly.

What you must know

DNA is a double helix of two antiparallel strands; nucleotide = deoxyribose sugar + phosphate + base (A, T, C, G); complementary pairing A-T, C-G via hydrogen bonds.
Genes are sections of DNA on chromosomes; each gene codes for a polypeptide; base sequence determines amino acid sequence.
Basic flow: DNA sequence → mRNA copy (concept), mRNA read to assemble amino acids into polypeptide (no detailed steps needed at this level).
Genetic engineering: isolate target gene (e.g., human insulin), insert into vector (plasmid) using restriction enzymes and ligase, transfer to host (bacteria) to express protein. Consider benefits (medicine, crops) vs risks/ethics (allergies, escape, equity).

Reviewed by

Ezekiel Tan·Academic Advisor (Biology)

Question clue	First move	What to write next
Complementary strand	Pair A with T and C with G.	Keep the base order aligned with the original strand.
Gene to protein	Split the base sequence into triplets.	Each triplet stands for one amino acid, so triplet order controls amino acid order.
Base substitution	Check whether the changed triplet codes for a different amino acid.	If the amino acid changes, the protein shape or active site may change. If not, the mutation can be silent.
Genetic engineering	Name the target gene, vector, and host.	Explain how the inserted gene lets the host make the desired protein.

Question asks for	Pairing move	What to check	Common trap
Complementary DNA strand	A pairs with T; C pairs with G.	The answer is still DNA, so it can contain T.	Writing U in a DNA strand.
mRNA copied from a DNA template	DNA A pairs with RNA U; DNA T pairs with RNA A; C pairs with G.	The answer is mRNA, so it uses U instead of T.	Copying the DNA sequence without changing T to U where needed.
Amino acid order	Split the mRNA into triplets before interpreting.	The triplet order controls amino acid order.	Jumping from one changed base straight to "disease" without checking the triplet.

Layer to check	Original sequence	After one base change	What to decide
DNA triplets	TAC CTT GGA	TAC CAT GGA	Which triplet changed?
mRNA triplets	AUG GAA CCU	AUG GUA CCU	Which copied triplet changed?
Amino acid order	Start - amino acid A - amino acid B	Start - amino acid C - amino acid B	Did the amino acid sequence change?
Protein effect	Normal folding possible	Folding or active-site shape may change	Would this affect the protein's function?

Tool or part	Job in the process	Wording to use	Common trap
Target gene	Carries the instructions for the desired protein	Identify and isolate the gene for the protein wanted.	Naming the product but not the gene that codes for it.
Restriction enzyme	Cuts DNA at specific points	Cut the target gene and plasmid so compatible ends can join.	Saying it "sticks" DNA together.
Plasmid vector	Carries the inserted gene into the host cell	Insert the gene into a plasmid vector.	Treating the plasmid as the final protein.
Ligase	Joins DNA fragments	Seal the target gene into the plasmid.	Confusing ligase with the enzyme that cuts DNA.
Host bacterium	Reads the inserted gene and makes the protein	Transfer the recombinant plasmid into bacteria so they produce the protein.	Forgetting that the host cell must express the gene.

If the question is about...	Do not skip...	A strong answer should connect...
DNA sequence	Base pairing and triplet order	The changed bases to the copied sequence.
Mutation	Amino acid sequence	The changed triplet to possible protein shape or function.
Genetic engineering	Vector and host cell roles	The inserted gene to expression of the desired protein.
DBQ evidence	The data pattern	The quoted evidence to the biological conclusion.

IP Biology Notes: Molecular Genetics (Upper Sec 12)

How this chapter applies

What you must know

Sources

Base sequence reasoning checkpoint

DNA strand vs mRNA copy checkpoint

Triplet mutation worked checkpoint

Genetic engineering tool-purpose checkpoint

Molecular genetics tuition checkpoint

Worked walkthroughs

Pitfalls and fixes

Practice drills

Quick applications

Exam cues

Next steps

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