Chromosomes, genes and DNA

Almost every cell in your body has a nucleus — a control centre that holds your instructions. Those instructions are stored as three things that fit inside each other:

Watch out: the DNA and chromosomes are inside the cell's nucleus — not floating loose in the cell. Genes are the small sections; DNA is the long molecule they sit on.

What does a gene actually do?

A gene is a section of DNA that carries instructions that help control a feature, such as eye colour. Different genes carry different instructions.

• Humans have about 20 000 genes.
• A human body cell has 46 chromosomes — arranged as 23 pairs.
• You inherit one of each pair from each parent, which is why you share features with both.

Key idea: your full set of genes is a bit like a recipe book. The book (DNA) is huge; each recipe (gene) tells the cell how to make one thing.

Who worked out the shape of DNA?

In the early 1950s, scientists raced to find DNA's shape.

• In London, Rosalind Franklin and Maurice Wilkins shone X-rays through DNA. Photo 51, taken in Franklin's lab in 1952, showed an X shape: a sign that DNA is a helix (a coil, like a spring).
• In Cambridge, James Watson and Francis Crick used these results, including Photo 51 and Franklin's measurements, to build a model: two strands twisted into a double helix. They published it in 1953.
• In 1962, Watson, Crick and Wilkins shared a Nobel Prize. Franklin had died in 1958.

Common mistake: Watson and Crick did not take the X-ray pictures. They used results from Franklin's lab.

Who did what?

Where is your DNA?

Biggest to smallest

Counting chromosomes

Passing it on: heredity

Heredity is the passing of genetic information from parents to offspring. It is why offspring resemble their parents — but, in sexual reproduction, are never identical to them.

Reproduction matters: in sexual reproduction the mixing of two parents' genes is what makes each offspring unique. Which chromosome of each pair a parent passes on is down to chance, so brothers and sisters get different mixes (unless they are identical twins).

Variation: no two alike

Variation means the differences between living things. There are two levels:

• Between species — a cat and a dog are obviously different because they have very different genes.
• Within a species — even people (all one species) differ in height, eye colour, hair, and more.

Genes, environment, or both?

Variation has two causes — and lots of features are shaped by both:

• Genetic variation — caused by the genes you inherit (e.g. eye colour, natural hair colour, blood group).
• Environmental variation — caused by your surroundings and lifestyle (e.g. a scar, a suntan, a language you speak).
• Both — e.g. your height depends on your genes and on your diet as you grow.

Common mistake: not all variation is genetic. A scar or a tan is caused by the environment, not by your DNA — and features like height are caused by genes AND environment together.

What causes this feature?

Tap whether each feature is mostly genetic, mostly environmental, or a bit of both.

Genetic or not?

Continuous or discontinuous?

Variation within a species comes in two kinds:

• Discontinuous: separate groups, nothing in between. Example: blood group (A, B, AB or O). Count how many are in each group, then draw a bar chart with gaps between the bars.
• Continuous: any value in a range. Examples: height, hand span. Measure each one, group the values, then draw a histogram with the bars touching. Often, most values are near the middle, so the shape is like a bell.

Common mistake: don't leave gaps between the bars for continuous data. Height doesn't jump from group to group.

Which graph?

Same parents. Why aren't brothers and sisters the same?

Did giraffes get long necks by stretching?

Variation gives an edge

Living things must compete for food, water, mates and space. Because of variation, some individuals happen to have features that make them better suited — or adapted — to their environment.

• A faster gazelle is more likely to escape a lion.
• A moth whose colour matches the tree bark is harder for birds to spot.

Key link: the better-suited individuals are more likely to survive and reproduce — passing their helpful genes on. This is the engine of natural selection.

Natural selection, step by step

Charles Darwin explained how species change over time. The idea works in a simple sequence:

Nothing is "trying" to change

Natural selection is not a plan. Animals do not decide to change, and they do not grow a feature because they need it.

What really happens: the useful variation is already there by chance (from random differences in genes). The environment simply lets those individuals survive and breed more often.

Common mistake: giraffes did not stretch their necks longer because they wanted to reach high leaves. By chance, some already had longer necks; they survived better and bred more, so the genes were passed on.

Explaining long necks

Evolution: change over time

Evolution is the slow change in a species over many, many generations, driven by natural selection. Given enough time, it can produce entirely new species.

• Evolution usually happens over thousands to millions of years, not in one lifetime.
• We see the evidence in fossils, which show how living things have changed.
• All life today is related through this long, branching history.

Timescale tip: an individual animal does not evolve. It is the population (the whole group) that gradually changes across generations.

Environmental change & extinction

If the environment changes — a new predator, a warmer climate, a lost food source — a species may suddenly be less well suited to survive.

Extinction means every member of a species has died and none are left. Most dinosaurs died out about 66 million years ago, likely after a huge, sudden change to their environment. Birds are their living relatives.

Why species die out

Biodiversity and gene banks

Biodiversity is the variety of living things: how many different species there are, and the variation within each species.

• It matters. We depend on many species, such as the insects that pollinate crops. Wild plants may carry genes that could help future crops or medicines.
• Once a species is extinct, it is gone for ever.

A gene bank stores hereditary material to protect it. Seed banks keep seeds dried and frozen (about −20 °C), so plants could be grown again if they die out in the wild. Some gene banks freeze animals' sperm and eggs.

Common mistake: a gene bank does not replace protecting habitats. Living things still need somewhere to live.

Did giraffes get long necks by stretching?

Same species — or different?

Tap each example, then tap the box: is it variation within one species, or a difference between two species?

How fast is evolution?

Who and what?

Chromosome pairs

The big ideas to know

You've covered the key ideas in the KS3 Genetics and evolution topic. Press Finish to see your score.