Life Sciences Grade 12 · Year Summary

Life Sciences · Grade 12 · Year Summary Date: Top Journal

Every distinction begins with one topic at a time.- start now

Paper 2 · ~27 marks

TERM 1 · DNA: CODE OF LIFE DNA: CODE OF LIFE

Learning outcomes · you must be able to

Describe the structure of a nucleotide, and explain the differences between DNA and RNA.
Explain the process of DNA replication, and state why accurate replication matters.
Describe how a DNA profile is used to identify an individual or confirm parentage.
Explain transcription and translation as the two stages of protein synthesis.
Convert between a DNA base triplet, an mRNA codon, a tRNA anticodon and an amino acid.
Explain how a gene mutation can (or cannot) change the protein that is produced.

Quick refresher · structures you already know

The nucleus houses the genetic material; the nucleoplasm is the fluid inside it holding free nucleotides and enzymes.
Nuclear pores in the nuclear membrane are the exit route mRNA uses to reach the cytoplasm.
The chromatin network is DNA's loose, thread-like form in a non-dividing nucleus; it condenses into visible chromosomes only when a cell is about to divide.
Ribosomes (free in the cytoplasm, or studded on the rough endoplasmic reticulum) are built from protein and RNA, and are where proteins are physically assembled.
The nucleolus is a small dense body inside the nucleus that manufactures ribosomes.

Core content · DNA and RNA structure

📖 KEY DEFINITION Nucleotide: the repeating monomer of a nucleic acid, made of 1 phosphate group + 1 pentose sugar + 1 nitrogenous base. Gene: a section of a chromosome that codes for one characteristic. Genome: the complete set of all the genes an organism has. Base-pairing rule: adenine always pairs with thymine (or uracil in RNA), and guanine always pairs with cytosine.

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	Adenine, Thymine, Guanine, Cytosine	Adenine, Uracil, Guanine, Cytosine
Strands	Double-stranded (double helix)	Single-stranded
Kinds	One kind	Three kinds: mRNA, tRNA, rRNA
Job	Stores the genetic code long-term	Carries, reads or builds the code (per type)

RNA type	Job	Carries
mRNA	Carries the copied message from the nucleus to a ribosome	Codons
tRNA	Delivers one specific amino acid to the ribosome	An anticodon
rRNA	Forms the physical structure of the ribosome, with protein	Neither codon nor anticodon

⚡ MUST MEMORISE Base pairing: A-T and G-C in DNA; A-U and G-C when DNA is transcribed into RNA. DNA is double-stranded, RNA is single-stranded. A codon lives on mRNA, an anticodon lives on tRNA, and one codon (3 bases) always corresponds to exactly one amino acid.

Core content · DNA replication

Happens during interphase, before mitosis or meiosis begins, inside the nucleus, driven by enzymes.
The double helix unwinds.
The hydrogen bonds between paired bases break, so the two strands separate ("unzip").
Each original strand now acts as a template.
Free nucleotides in the nucleoplasm attach to each exposed template strand, pairing by the base-pairing rule (A with T, G with C).
Result: two identical double-stranded DNA molecules, each built from one old strand plus one newly made strand - biologists call this outcome semi-conservative replication.

DNA double helix at a replication fork — Learners must label: the sugar-phosphate backbone, a base pair (A-T or G-C), the double helix twist, and the unwound replication fork region

⭐ EXAM FAVOURITE A labelled DNA replication or protein-synthesis diagram with sub-questions: name a numbered part, name the phase or process shown, and state the bond or base pairing at a given point. Column-matching questions also often pair a DNA/protein-synthesis term against a meiosis term, testing whether you can tell the two topics apart.

🔥 FREQUENTLY TESTED Stating the significance of DNA replication: it doubles the genetic material so it can be shared evenly between daughter cells, keeps daughter cells genetically identical to the parent cell, and keeps chromosome number constant across cell generations.

⚠️ COMMON MISTAKE Placing DNA replication in a division phase (prophase, etc.) instead of correctly in interphase, before division starts. Also watch for a replication error changing the new strand's base sequence: that is exactly how a gene mutation arises.

Core content · protein synthesis: transcription and translation

Transcription (in the nucleus): a section of the double helix unwinds and separates, using the same "unzip" mechanism as replication.
Only ONE of the two DNA strands is used as the template (unlike replication, which uses both).
Free RNA nucleotides pair against the template following the base-pairing rule, but uracil takes thymine's place (DNA adenine pairs with RNA uracil).
This builds a single mRNA strand, complementary to the template, carrying the code as three-base codons. The finished mRNA leaves the nucleus through a nuclear pore.
Translation (in the cytoplasm, at a ribosome): the mRNA attaches to a ribosome, and tRNA molecules, each carrying one specific amino acid, move in and out.
A tRNA's anticodon only fits an mRNA codon it is complementary to; when it matches, it delivers its amino acid.
Adjacent amino acids link by a peptide bond, building a growing chain (polypeptide) in the exact order the codons dictate. The finished chain folds into the final protein.

Ribosome translating mRNA with tRNA — Learners must label: the ribosome, the mRNA strand, a tRNA molecule with its anticodon, the growing amino acid chain, and a codon-anticodon match point

🔥 FREQUENTLY TESTED Converting straight through the chain: DNA base triplet → mRNA codon → tRNA anticodon → amino acid. This exact skill is what most Section A diagram sub-questions are testing.

⚠️ COMMON MISTAKE Mixing up replication and transcription: replication uses two templates and produces DNA, held together by real base pairs; transcription uses only one template and produces single-stranded mRNA, which does not stay bonded to the DNA. Also, reversing codon (on mRNA) and anticodon (on tRNA) is a common slip.

💡 EASY MARK "Give the biological term for..." questions on this topic are quick 1-mark gifts: peptide bond, double helix, mutation, DNA profiling. Learn the exact terms; examiners want precise wording, not a rough description.

Core content · mutation and protein structure

A gene mutation changes the identity or arrangement of one or a few nitrogenous bases in a DNA sequence.
That changed base sequence carries through to the mRNA when it is transcribed.
If the new codon still codes for the same amino acid (the genetic code has built-in redundancy: several codons can code for one amino acid), the protein is unchanged.
If the new codon codes for a different amino acid, the amino acid sequence changes, and the protein may be altered.

⚠️ COMMON MISTAKE Jumping straight to "a different protein will form" without first checking whether the new codon actually codes for a different amino acid. The safe answer pattern is that the protein may change once the amino acid itself has changed, never an automatic guarantee.

Core content · DNA profiling

A DNA profile is a pattern of bands of different lengths, thicknesses and positions, generated from a person's DNA.
Every profile is unique, except identical twins, who share one.
Comparison logic: line up an unknown/evidence sample against a known/reference sample; closely matching band positions and thickness point to the same source.
Real-world uses: paternity or maternity testing, forensic suspect identification, identifying relatives or unidentified remains, and transplant tissue-compatibility checks.

Paternity-testing step	What it establishes
1. Profile the mother, the child, and the possible father	Gives three band patterns to compare
2. Match the child's bands against the mother's	Those matching bands are accounted for as maternal
3. Identify the leftover, unmatched bands in the child	These must have come from the biological father
4. Compare the leftover bands to the possible father	Full match confirms him; any mismatch excludes him

⭐ EXAM FAVOURITE Describing the paternity-testing procedure, or interpreting a gel-band diagram to identify the biological father, is a recurring multi-mark Section B sub-question, often folded into a bigger genetics question rather than asked on its own.

💡 EASY MARK Naming a lab precaution is a quick 1-2 mark question: label samples clearly, wear gloves and a mask to avoid contaminating the sample with the handler's own DNA, and use sterile equipment throughout.

Worked example 1 · DNA replication

Q: A short stretch of one DNA strand belonging to a fictional insect, the amber lacewing, reads: GTA CGA TCC AAT GGA. Write down the base sequence of the new strand that would form on this template during replication.

Original template strand	GTA	CGA	TCC	AAT	GGA
New strand (A-T, G-C pairing)	CAT	GCT	AGG	TTA	CCT

∴ New strand: CAT GCT AGG TTA CCT

Worked example 2 · transcription and translation

Q: Using the same amber lacewing strand as a template for transcription (GTA CGA TCC AAT GGA), determine (a) the mRNA formed, (b) the amino acid sequence using the codon table below, and (c) the anticodons of the tRNA molecules delivering them.

mRNA codon	Amino acid
CAU	Histidine
GCU	Alanine
AGG	Arginine
AGU	Serine
UUA	Leucine
CCU / CCC	Proline

(a) mRNA (DNA adenine pairs with RNA uracil): CAU GCU AGG UUA CCU

(b) Amino acids, read off the table: Histidine - Alanine - Arginine - Leucine - Proline

Worked example 3 · effect of a gene mutation

Q: Two separate mutations occur on the amber lacewing DNA strand from Worked example 2. Mutation 1 changes the third triplet from TCC to TCA. Mutation 2 changes the fifth triplet from GGA to GGG. Using the codon table above, explain the effect of each mutation on the protein.

Mutation 1: TCA transcribes to mRNA codon AGU, which codes for serine, not arginine (the original codon AGG's amino acid). This IS a different amino acid, so the protein may be altered.

Mutation 2: GGG transcribes to mRNA codon CCC, which - like the original codon CCU - still codes for proline. This is the SAME amino acid, so the protein is unaffected.

∴ Mutation 1 may change the protein; Mutation 2 does not, because of redundancy in the genetic code.

Worked example 4 · DNA profiling to confirm parentage

Q: At a game reserve, a calf is born after two bulls (Bull A and Bull B) both had access to the mother during the breeding season. A profile with 6 band positions is run for the mother, the calf, and both bulls. The calf's profile shows bands at positions 1, 2, 4, 5 and 6. The mother's profile matches the calf exactly at positions 1, 4 and 6. Bull A's profile matches the calf at positions 2 and 5. Bull B's profile matches the calf only at position 2. Identify the biological father, with reasons.

Bands 1, 4 and 6 in the calf match the mother, so they are accounted for as maternal. The remaining calf bands, 2 and 5, must have come from the sire.

Bull A matches both remaining bands (2 and 5): confirmed as the biological father. Bull B matches only band 2, not band 5: excluded.

∴ Bull A is the biological father.

Every protein your body will ever build starts as three letters at a time, read off a strand of DNA.- on protein synthesis

Why it matters: every mark in this chapter runs off one rule, A with T and G with C. Replication uses both strands and makes DNA, while transcription uses one strand and makes mRNA, and confusing the two is the fastest way to lose a whole conversion question.

🧠 ACTIVE RECALL

Which sugar is found in a DNA nucleotide but NOT in an RNA nucleotide?(1)

During DNA replication, which base pairs with adenine on the template strand?(2)

In which phase of the cell cycle does DNA replication take place?(2)

The three-base units on an mRNA strand that specify an amino acid are called:(1)

The three-base unit carried by a tRNA molecule, which is complementary to an mRNA codon, is called:(1)

A DNA profile is unique to every individual, except:(2)

A section of a DNA template strand reads CGA TTC AGT. Write down the mRNA sequence that would be transcribed from it.(4)

Explain, using the idea of codon redundancy, why a base substitution in a gene does not always change the protein that is produced.(4)

State TWO precautions a laboratory technician should take when handling DNA samples for profiling, and explain why each is necessary.(3)

Explain to a Grade 8 learner why a cell has to "unzip" its DNA before it can either copy it or use it to build a protein.

ONE-MINUTE SUMMARY

Base pairing (A-T, G-C, and A-U when RNA is involved) is the one rule behind replication, transcription and translation. Replication uses BOTH strands as templates and produces DNA; transcription uses only ONE strand and produces single-stranded mRNA. Translation converts mRNA codons into amino acids via tRNA anticodons, linked by peptide bonds. A base substitution changes the protein only if it changes the amino acid coded for; redundancy in the genetic code means it sometimes does not. A DNA profile is unique to every individual except identical twins.

IN THE EXAM

Expect this content mostly inside Question 1 (Section A): multiple choice, one-word term recall, column-matching and diagram-labelling on either a replication fork or a translation-at-the-ribosome diagram.
DNA profiling and paternity content often reappears as a sub-question inside a bigger Section B genetics question, not only in Section A.
Diagram questions hinge on correctly tracing which strand is the template versus the newly built one, and on reading a numbered part by what it connects to, not just its position.
Always show the base-pairing substitution working, not just a final answer - marks are awarded for the method.

🔒 MEIOSIS

🔒 REPRODUCTION IN VERTEBRATES

🔒 HUMAN REPRODUCTION

🔒 NERVOUS SYSTEM

🔒 SENSES: EYE & EAR

🔒 ENDOCRINE SYSTEM

🔒

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