Genetics & DNA lessons
What is DNA?
The real HBB gene unzips, is copied into mRNA and read codon by codon into a protein; then change one base.
DNA is the molecule that holds the instructions for building and running a living thing. Nearly every cell in your body carries a full copy.
A gene is a stretch of DNA that holds the recipe for one protein. The cell copies the gene into a messenger molecule, then reads that message to build the protein.
The gene above is HBB, which makes part of haemoglobin, the protein that carries oxygen in your blood. Change one base and the protein can change too.
The double helix
DNA is two long strands wound around each other in a double helix, like a twisted ladder. Each strand has a backbone made of sugar and phosphate.
The rungs of the ladder are pairs of bases. There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C).
A 3D tour of a DNA double helix: the backbones, an A-T pair, a G-C pair and one full turn.
The order of the bases along a strand is its sequence. The sequence spells out the instructions, the way letters spell out words.
Base pairing
The bases always pair the same way: A with T, and G with C. This is called complementary base pairing.
Each pair is held by weak hydrogen bonds: two between A and T, and three between G and C. Weak bonds let the strands unzip when the cell needs to read or copy them.
A random strand gains its partner base by base, with two or three hydrogen bonds per pair and a running count of each base.
So one strand decides the other. If you know one strand, you can write its partner.
Practice: Build the partner strand
Drag bases into place to build the partner of a random DNA strand.
Where your DNA lives
Your DNA sits in the nucleus of each cell. It is split into 46 long pieces called chromosomes, which come in 23 pairs.
You get one chromosome of each pair from each parent. One full set of DNA is a genome: about 3 billion base pairs, with about 20,000 genes that code for proteins.
A 3D zoom from a whole animal cell into its nucleus and down to the DNA double helix.
Stretched out, the DNA in one cell would be about 2 metres long. It is wound tightly around proteins to fit in a nucleus a few micrometres across.
Practice: Genome numbers
Match each number to what it counts in the human genome.
Copying DNA
Before a cell divides, it copies all of its DNA. This is called DNA replication.
An enzyme called helicase unzips the double helix. Then DNA polymerase builds a new partner strand along each old strand, following the pairing rules.
Helicase unzips a short stretch of DNA and polymerase builds a new strand on each old one, giving two half-old, half-new copies.
Each new molecule keeps one old strand and gains one new strand. This is called semi-conservative replication.
Practice: Order the copying steps
Put the steps of DNA replication in order.
Transcription
To use a gene, the cell first copies it into mRNA, short for messenger RNA. This step is called transcription and happens in the nucleus.
RNA polymerase unzips the gene and reads one strand, the template strand. It builds mRNA that pairs with it, using uracil (U) where DNA would use T.
RNA polymerase steps along a template strand, adding the matching mRNA base from the pairing rules at each step.
So the mRNA matches the other DNA strand, the coding strand, with U in place of T. The mRNA then leaves the nucleus.
Practice: Transcribe the template
Tap in the mRNA built on a random template strand.
The genetic code
In the cytoplasm, a ribosome reads the mRNA three bases at a time. Each group of three bases is a codon.
There are 64 codons. 61 of them code for the 20 amino acids, and 3 are stop codons that end the protein.
Look up codons in the genetic code: first base, then second, then third.
Molecules called tRNA carry each amino acid to the ribosome, which joins them into a chain. This is translation, and the finished chain folds into a protein.
Practice: Translate the mRNA
Drag the right amino acid under each codon of a random mRNA, using the genetic code.
Mutations
A mutation is a change in the DNA sequence. It can happen when DNA is copied, or be caused by things like UV light.
A substitution swaps one base for another. An insertion adds a base, and a deletion removes one.
The start of the HBB gene with a substitution, an insertion and a deletion, and what each does to the codons and amino acids.
Adding or removing a base shifts how every later base is grouped into codons. This frameshift usually ruins the protein.
Practice: Name the mutation
Compare a sequence before and after a random mutation, and name the change and its effect.
Alleles and genotypes
Most genes come in different versions, called alleles. You carry two alleles of each gene, one from each parent.
Your pair of alleles is your genotype, and the trait it gives you is your phenotype. Two matching alleles make you homozygous, and two different ones make you heterozygous.
Pick two alleles of the pea flower colour gene and see the genotype, its name and the flower it gives.
A dominant allele shows even with one copy, and is written as a capital letter. A recessive allele only shows when there are two copies.
Practice: Sort the genotypes
Sort genotypes into homozygous dominant, heterozygous and homozygous recessive.
Punnett squares
A Punnett square predicts the offspring of two parents. One parent's two alleles go across the top, and the other parent's go down the side.
Each box joins one allele from the top with one from the side. The four boxes are equally likely, so each one is a 1 in 4 chance.
Pick two parents and watch the Punnett square fill in, with the genotype and phenotype ratios.
Two Pp parents give purple and white flowers in a 3 to 1 ratio. Gregor Mendel found this ratio in pea plants over 150 years ago.
Practice: Fill the Punnett square
Fill in the four boxes of a random Punnett square.
Practice: Predict the odds
Type the chance that a child of a random cross has a given trait.
Mitosis and meiosis
Mitosis makes two cells with the same 46 chromosomes as the parent cell. It is how your body grows and repairs itself.
Meiosis makes four sex cells, or gametes, each with 23 chromosomes, one from each pair. When a sperm fertilises an egg, the full 46 come back.
Two chromosome pairs go through mitosis or meiosis, with crossing over and the chromosome count in each cell.
Meiosis also mixes genes up. Pairs swap pieces in crossing over, and each pair splits at random, so one person can make over 8 million different gametes.
Ways to sort 23 pairs: 2^{23} = 8\ 388\ 608
Practice: Mitosis or meiosis?
Decide whether each statement describes mitosis, meiosis or both.
Genetic engineering
Genetic engineering changes an organism's DNA on purpose. One way is to move a useful gene from one living thing into another.
Since 1982, bacteria given the human insulin gene have made insulin for people with diabetes. Crops have also been given genes that help them resist pests.
How bacteria are engineered to make human insulin, step by step.
CRISPR gene editing
CRISPR-Cas9 edits DNA at one exact spot. A short guide RNA matches the target sequence and leads the Cas9 enzyme there to cut both strands.
The cell then repairs the cut. The repair can switch the gene off, or copy in a new sequence that scientists supply.
Cas9 and its guide RNA scan along DNA, stop where the guide matches and cut both strands.
In 2023, Casgevy became the first approved CRISPR medicine, for sickle cell disease. It edits a patient's own blood stem cells so they make fetal haemoglobin again, which does not sickle.
Practice: Gene editing
Match each part of gene editing to its job.