DNA, Genes and Chromosomes

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.14 — The genome as an organism's entire DNA; a gene as a section coding for a protein
• 3.15 — The nucleus contains chromosomes on which genes are located
• 3.16 — DNA as a double helix with paired bases A–T and C–G (bold B statement — Paper 2 only)

Specification section 3(a) Reproduction — also taught in Sexual and Asexual Reproduction, Flowers, Pollination and Fertilisation, Seeds and Germination, The Human Reproductive Systems.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Almost every difficulty on this topic comes from meeting five words — genome, chromosome, gene, DNA, base — as though they were five different things. They are not. They are one thing seen at different magnifications, and once you can zoom in and out along that chain, every question about inheritance becomes easier to hold.

Zooming in, one step at a time

1. Start with a CELL. Inside it is a NUCLEUS.
2. Zoom into the nucleus and you find CHROMOSOMES — long structures made of coiled-up DNA. A human body cell has 46 of them, in 23 pairs.
3. Zoom into one chromosome and you find it is a single enormously long molecule of DNA, wound up tightly so it fits.
4. Along that DNA molecule are GENES — sections of DNA, each one coding for a particular protein. A chromosome carries hundreds or thousands of them, each at its own fixed position.
5. Zoom into a gene and you find a sequence of BASES — the four chemical letters A, T, C and G — and it is the ORDER of those bases that carries the information.
6. The GENOME is the whole lot: the entire DNA of an organism, every chromosome and every gene it contains.

Think of it like a library, a book and a sentence

The genome is the whole library. Each chromosome is one very long book on a shelf. A gene is a single set of instructions inside that book — a recipe, say, occupying a few pages at a known position. The bases are the letters those pages are printed in, and just as with letters, it is the ORDER that carries the meaning: the same four letters in a different sequence spell something else entirely. And notice what the analogy gets right that a diagram cannot — every cell in your body holds a complete copy of the entire library, and uses only the few recipes it needs.

Now the molecule itself. DNA is a DOUBLE HELIX: two strands twisted round each other, held together by pairs of bases reaching across between them like the rungs of a twisted ladder. There are four bases — adenine, thymine, cytosine and guanine — and they do not pair at random. Adenine always pairs with thymine, and cytosine always pairs with guanine. A with T, C with G, every time, without exception.

That rule is worth more than it first appears, and it is why the structure was such an important discovery. Because the pairing is fixed, EACH STRAND CARRIES ALL THE INFORMATION: if you know one strand reads A-T-G-C, you know the other must read T-A-C-G, and you could reconstruct either from the other with no further knowledge. Separate the two strands and each is a template for rebuilding its partner exactly. The molecule's shape is therefore also a mechanism — a way of copying information faithfully, without anything having to understand what the information means.

Two precise points to hold, because both are examined and both are easy to blur. First, a GENE is a section of DNA that codes for a PROTEIN — that is the whole definition, and 'codes for a characteristic' is not quite it, because the gene's product is a protein and the characteristic follows from what that protein does. Second, chromosomes come in PAIRS in a body cell, one of each pair inherited from each parent, which is why you have two copies of every gene and why the next lesson on alleles works at all.

It is worth pausing on the scale, because it is genuinely strange. The DNA in a single human cell is about two metres long if stretched out, packed into a nucleus a few millionths of a metre across — which is the coiling those chromosome pictures are showing. The genome contains roughly three billion base pairs and around twenty thousand genes, which means the coding sections are a small fraction of the whole. A genome is not a tidy list of genes; it is a vast sequence with genes distributed along it.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Gene

A section of DNA that codes for a particular protein.

Learn this definition · Genome

The entire DNA of an organism — all of its chromosomes and all of the genes they carry.

Learn this definition · Chromosome

A structure found in the nucleus, made of a long coiled molecule of DNA, along which genes are located at fixed positions.

The five levels, largest to smallest — the order to recall them in

Genome

The entire DNA of an organism.

Chromosome

One long coiled DNA molecule in the nucleus. Humans have 46 in a body cell, in 23 pairs.

Gene

A section of a chromosome's DNA coding for one protein, at a fixed position.

DNA

The molecule itself: a double helix of two strands held together by paired bases.

Base

One of the four chemical letters A, T, C and G. Their ORDER carries the information.

Statement 3.16 — the structure of DNA, in the phrases that score

• DNA is a DOUBLE HELIX — two strands twisted around each other
• The two strands are held together by PAIRS OF BASES between them
• There are FOUR bases: adenine (A), thymine (T), cytosine (C) and guanine (G)
• A always pairs with T; C always pairs with G — the COMPLEMENTARY base pairs
• The ORDER of the bases along a gene determines the protein that the gene codes for
• Because pairing is fixed, each strand carries all the information and can act as a template to rebuild the other

Working out the complementary strand — the routine for the data question

1. Write out the given strand of bases in order.
2. Underneath each base, write its partner: A becomes T, T becomes A, C becomes G, G becomes C.
3. Check by counting: the number of A's on one strand must equal the number of T's on the other, and likewise for C and G.
4. Worked example: a strand reading A-T-G-C-C-A gives a complementary strand reading T-A-C-G-G-T.

Gene against chromosome — the two most frequently swapped words on the topic

A. A CHROMOSOME is the whole structure: one very long DNA molecule, coiled, carrying thousands of genes. Humans have 46 in a body cell.

B. A GENE is one SECTION of that DNA, coding for a single protein, at a fixed position along the chromosome.

Not this: A gene codes for a characteristic, such as eye colour.

This: A gene codes for a PROTEIN. The characteristic follows from what that protein does — and most characteristics depend on several genes, not one.

Model answer [4 marks]

Describe the structure of a DNA molecule. [4]

DNA is a double helix, made of two strands twisted around each other. The two strands are held together by pairs of bases reaching between them. There are four bases: adenine, thymine, cytosine and guanine. Adenine always pairs with thymine, and cytosine always pairs with guanine.

Model answer [3 marks]

Explain the relationship between a genome, a chromosome and a gene. [3]

The genome is the entire DNA of an organism. It is divided among the chromosomes, which are long coiled molecules of DNA found in the nucleus — a human body cell contains 46. Each chromosome carries many genes, and a gene is a section of that DNA which codes for a particular protein.

Model answer [3 marks]

Explain why the fixed pairing of bases allows DNA to be copied accurately. [3]

Because adenine always pairs with thymine and cytosine always pairs with guanine, the sequence of bases on one strand determines the sequence on the other exactly. If the two strands are separated, each one can act as a template on which a new complementary strand is built. The two resulting molecules are therefore identical to the original.

Mark-losing trap. A gene codes for a PROTEIN. Saying it codes for a characteristic loses the mark.

Mark-losing trap. A chromosome is not a gene. A chromosome carries thousands of genes along its length.

Mark-losing trap. A pairs with T, C pairs with G. Learn the pairs, not just the four letters.

Mark-losing trap. It is the ORDER of the bases that carries information, not which bases are present — all four are in every gene.

Mark-losing trap. Say DOUBLE HELIX, not 'twisted ladder' or 'spiral'. The specification's word is the one that scores.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Define [1 mark] — What is a gene?
2. Grade 7 · Apply [1 mark] — One strand of a short length of DNA has the base sequence A-T-G-C-C-A. What is the sequence of the complementary strand?
3. Grade 8 · Describe [5 marks] — Describe the relationship between the nucleus, chromosomes, DNA, genes and the genome. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Calculate [4 marks] — A sample of DNA is found to contain 1200 adenine bases, and adenine makes up 20 % of all the bases present. Calculate the number of cytosine bases in the sample.
5. 9+ · Analyse [5 marks] — Before the structure of DNA was known, some scientists argued that a molecule made of only four different units was too simple to carry the huge amount of information an organism needs, and that proteins — made of twenty different amino acids — must carry it instead. Select every statement that belongs in a full-mark explanation of why that argument was wrong.

The people behind this science

Two ways into the same idea — the one whose data showed the shape, and the one who deduced genes without ever seeing one. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Rosalind Franklin — the one whose data showed the shape

The double helix on this page is a shape somebody had to establish, and Franklin's X-ray diffraction images are the evidence it rests on. Her Photograph 51 shows the cross pattern that a helix produces, and her measurements gave the spacing and the diameter — enough to show that the strands run in opposite directions and that the phosphate backbone is on the outside. That photograph was shown to Watson without her knowledge, and the 1962 Nobel Prize went to three men four years after she died at 37.

• “What does a photograph of a crystal actually tell you about a molecule?”
• “What did your images show about the shape of DNA?”
• “How did you know the backbone was on the outside?”
• “Why were you cautious about announcing a helix when others were not?”
• “What was it like to work in a laboratory that did not want you there?”

Gregor Mendel — the one who deduced genes without ever seeing one

Mendel had no microscope capable of showing a chromosome and no idea that DNA existed, and he still worked out that inheritance is carried by discrete units passed on whole rather than by a blending of the parents. He did it by counting: tens of thousands of pea plants, generation after generation, until the ratios were unmistakable. Everything this page describes physically — genes at fixed positions on paired chromosomes — is the material explanation of a pattern he had established from numbers alone, thirty years before anyone could see it.

• “How can you discover something you have no way of seeing?”
• “What made you certain inheritance came in separate units rather than blending?”
• “Why did counting thousands of plants matter so much?”
• “What would you have wanted to look at, with a modern microscope?”

Carry on through the course

• Previous lesson: The Placenta and the Amniotic Fluid
• Next lesson: Protein Synthesis
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification