Genetics

DNA: the code of life

DNA is the molecule that stores genetic instructions. Its shape is a double helix of nucleotides.

CAPS

Grade 12 · Life Sciences · Genetics

Life Sciences is two papers. Use the strand to know which one.

Where it is examined
Paper 1 is reproduction, responding to the environment, and homeostasis. Paper 2 is DNA, meiosis, genetics and evolution. Each paper is 150 marks.
How to write it
Name the structure, then say what it does. A labelled diagram only scores if the labels are the biological names. “It helps the body” is not a function.
How a marker reads it
A common mix-up is to pair adenine with guanine, or to say RNA contains thymine.

A 2-mark “explain” wants two facts, or one fact and its reason. Extra sentences that contradict the first one can cancel a mark.

Diagram

Worked theory

Worked theory. A gene for a hair protein. The DNA base order is transcribed into mRNA in the nucleus. The mRNA leaves and a ribosome reads it three bases at a time. Each trio brings a particular amino acid. The chain of amino acids folds into that protein. If one DNA base is changed and the amino acid changes, the protein may work badly. If the copy of the whole DNA, before mitosis, mismatches a base and the cell does not repair it, the daughter cell inherits the change. DNA is not a protein. It is the instruction for making proteins.

More worked examples

One base in the haemoglobin gene
A stretch of the haemoglobin gene reads ...GAG... which codes for glutamic acid. One base change makes ...GTG... and the amino acid becomes valine. That single swap is sickle cell. The DNA is still a double helix. A with T and G with C still hold. Transcription copies the changed triplet into mRNA, with U where the DNA had T. The ribosome builds a protein that behaves differently under low oxygen. The whole chromosome was not rewritten. One base was.
TAC becomes a start
A DNA triplet on the template strand reads TAC. Transcription builds the mRNA codon AUG, because A pairs with U in RNA and T in DNA pairs with A. At the ribosome AUG means “start, and put methionine here”. Change the DNA to TAA and the mRNA becomes AUU, which is not the start codon. One base on the DNA moved the meaning. The ribosome did not invent the change. It read what transcription handed it.

In ordinary words

A nucleotide is a sugar, a phosphate and a base. The bases pair: adenine with thymine, guanine with cytosine. The order of bases is the code. Replication copies DNA before a cell divides. The helix unwinds, each strand is a template, and each new molecule has one old strand and one new strand. That is semi-conservative replication. Transcription makes messenger RNA from a gene. Translation reads that RNA at a ribosome and builds a protein. RNA uses uracil instead of thymine, and it is usually a single strand.

In South Africa

Every trait you will meet in genetics, from blood type to a pedigree, starts from this code.

Part 1 / 4

What it is

DNA is the molecule that stores genetic instructions. Its shape is a double helix of nucleotides.

DNA: the code of life / dna-structure — Diagram

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

A DNA sample contains 30% adenine. Calculate the percentages of thymine, guanine and cytosine.

4. Step-by-step working

01
Identify the given
T=A=30%.
02
Calculate
G+C=100−60=40%, so G=C=20%.

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: A DNA sample contains 30% adenine. Calculate the percentages of thymine, guanine and cytosine.

In other words — as if you have never seen this before

If you have never seen this before: A pairs with T, G pairs with C. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

A=30%.

3. Which method — and why this one?

A pairs with T, G pairs with C.

4. Step-by-step working

01 / 05
Identify the given
T=A=30%.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Calculate
G+C=100−60=40%, so G=C=20%.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
04 / 05
Step F — Box the answer with its unit
T 30%, G 20%, C 20%.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

T 30%, G 20%, C 20%.

Copy onto your answer sheet — in this order

Answer: T 30%, G 20%, C 20%.

6. Check it like a marker

30+30+20+20=100 ✓

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

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Meiosis

Life Sciences

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