Chapter 2 of 8

All 8 chapters
  1. 1DNA, genes and chromosomes, readunderstood
  2. 2The structure of DNA, readunderstood
  3. 3Meiosis and sex cells, readunderstood
  4. 4Boy or girl: X and Y, readunderstood
  5. 5Dominant and recessive, readunderstood
  6. 6Family trees and genetic disorders, readunderstood
  7. 7Variation, readunderstood
  8. 8Mutation and genetic engineering, readunderstood

S4.2.2 Chapter 2

The structure of DNA

DNA is a code with only four letters. How can four letters hold the instructions for a whole body?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Describe DNA as a double helix with paired bases
  • Pair A with T and C with G to complete a strand

You have seen that a gene is a code. What is the code made of? To find out, we zoom in on the DNA molecule itself.

DNA has two long strands twisted round each other. This shape is called a double helix. A helix is a spiral, like a spring or a spiral staircase.

A sketch of DNA: two strands twisted round each other, with rungs between them. The backbone and a base pair are named, and the words two strands twisted sit under the picture.backbonebase pairtwo strands twisted
The double helix. The rungs are not drawn with letters here. The next pictures show them.

Each strand has a backbone. It is a chain of sugar molecules and phosphate molecules that take turns along the strand. The backbone holds the strand together.

On every sugar of the backbone sits one base. There are four kinds. They are called adenine, thymine, cytosine and guanine. We write them as the letters A, T, C and G.

A base on one strand joins a base on the other strand. The joined pair is a base pair, and base pairs are the rungs of the ladder. They do not pair at random. A always pairs with T, and C always pairs with G.

Draw it first

Build a ladder from one strand

  1. A straight ladder with four rungs. The top strand reads A, T, G, C. In the first picture the lower boxes are empty. In the second picture each lower box holds the partner base: T, A, C, G. The backbone and a base pair are named.ATGCbackbonebase pair

    1 One strand is given. It reads A, T, G, C. The boxes of the other strand are empty.

  2. A straight ladder with four rungs. The top strand reads A, T, G, C. In the first picture the lower boxes are empty. In the second picture each lower box holds the partner base: T, A, C, G. The backbone and a base pair are named.ATTAGCCGbackbonebase pair

    2 Fill each box with the partner of the base above it. The partner of A is T, of T is A, of G is C, and of C is G.

A table with the columns Base and Pairs with. A pairs with T. T pairs with A. C pairs with G. G pairs with C.BasePairs withATTACGGC
The pairing rule. There is no other pair in DNA.

See it three ways

As a picture

A short ladder with two rungs. The top strand reads G, C and the lower strand reads C, G. The backbone and a base pair are named.GCCGbackbonebase pair

In words

Every base has one partner. G always has C opposite it, and C always has G.

In symbols

G with C, and C with G. In general, A with T and C with G.

The rule has a big result. If you know one strand, you know the other. The two strands are complementary: each one is the partner of the other.

Worked example

One strand of DNA reads ATGC. What does the other strand read?

  1. Pair each base in turn: A with T, T with A, G with C, and C with G.
  2. Write the partners in the same order: T, A, C, G.
  3. The other strand reads TACG.

Answer: TACG

One strand can hold the bases in any order. But in a double helix, the number of A equals the number of T. The number of C equals the number of G. This is because every A has a T opposite it, and every C has a G.

Worked example

In a sample of DNA, 28% of the bases are A. What percentage of the bases are C?

  1. A pairs with T, so there are as many T as A. 28% are T.
  2. A and T together make 28 + 28 = 56%.
  3. The rest are C and G: 100 − 56 = 44%.
  4. C pairs with G, so there are as many C as G: 44 ÷ 2 = 22%.

Answer: 22%

A bar chart of the bases in a sample of DNA. The axis is labelled percentage of bases, with a scale from 0 to 40 in steps of 10. The bar for A reaches 28. The bar for T reaches 28. The bar for C reaches 22. The bar for G reaches 22.010203040ATCGpercentage of bases
The sample of the worked example. A and T are equal. C and G are equal.

How does the order of bases make a protein? A cell reads the bases of a gene three at a time. Each group of three bases is the code for one amino acid. Amino acids are joined in a chain, and the chain folds to make a protein.

Four bases can make 4 × 4 × 4 = 64 different groups of three. Cells use 20 kinds of amino acid, so several groups code for the same amino acid, and a few groups mean "stop". A different order of bases makes a different protein.

The pairing rule also lets a cell copy its DNA. Before a cell divides, the two strands come apart. Each strand then builds a new partner by the rule. The result is two double helixes, each the same as the first. Each has one old strand and one new strand.

Four boxes joined by arrows, from top to bottom: one double helix, the two strands come apart, each strand builds a new partner by the pairing rule, two identical double helixes.one double helixthe two strands come aparteach strand builds a newpartner by the pairing ruletwo identical double helixes
How DNA is copied before a cell divides.

Try it

Twist a paper ladder

You need: an old sheet of paper, a pencil.

  1. Tear a strip about 5 cm wide and 30 cm long from the paper. Tear it, so that you do not need scissors.
  2. Draw a long line down each long edge. These are the two backbones.
  3. Draw a short rung across the strip every 3 cm. At the two ends of each rung write a pair: A and T, or C and G.
  4. Hold the two ends. Turn one end round and round so that the strip twists.

What you will see: The flat ladder twists into a spiral. The rungs stay straight and the backbones wind round them. A twisted ladder is the shape of a double helix.

If you cannot try it, this is what happens.
A ladder with four rungs. The top strand reads A, C, G, T and the lower strand reads T, G, C, A, so each rung is a pair: A with T, C with G, G with C, T with A. The backbone and a base pair are named.ATCGGCTAbackbonebase pair

A wrong idea many people have

The two strands of DNA are exact copies of each other.

Why it feels right: The word "double" suggests two of the same thing.

The test that breaks it: Write one strand: ATGC. Pair each base by the rule and you get TACG. The two strands are not the same, and every base differs from the one opposite it.

What is true: The strands are complementary: each is the partner of the other. Together they hold the same information twice, in a matching form.

Key words

double helix
the shape of DNA: two strands twisted round each other.
backbone
the chain of sugar and phosphate molecules that holds a DNA strand together.
base
one of the four letters of DNA: A, T, C and G.
base pair
two bases joined across the two strands, A with T or C with G.
complementary
matching by the pairing rule. One strand is complementary to the other.
amino acid
a small molecule that is a link in the chain of a protein. Three bases code for one.

Back to your guess

The answer is A: The four letters are used in a very long order, like the letters of a long book.

The four bases are used in a very long order. The order is the code. A human cell has billions of bases in a row, so four letters are more than enough.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which pairs of bases join in DNA?

    Tap your answer.

    1. Not this one. That swaps the partners. In DNA, A pairs with T, and C pairs with G. Try another.

    2. Not this one. A base pairs with a different base across the two strands, never with its own kind. Try another.

    3. Yes, that is right. The pairing rule is the same in every living thing: A with T, and C with G.

    4. Not this one. Those pairs do not form. A needs T, and C needs G. Try another.

    See the answer

    A with T, and C with G

    The pairing rule is the same in every living thing: A with T, and C with G.

    • If you chose A: That swaps the partners. In DNA, A pairs with T, and C pairs with G.
    • If you chose B: A base pairs with a different base across the two strands, never with its own kind.
    • If you chose D: Those pairs do not form. A needs T, and C needs G.

  2. One strand of DNA reads TTAGC. What does the other strand read?

    Tap your answer.

    1. Yes, that is right. T pairs with A, T with A, A with T, G with C, and C with G. So the partner strand is AATCG.

    2. Not this one. That is a copy of the strand you were given. The partner strand has the matching base at each place, not the same one. Try another.

    3. Not this one. You paired A with C and T with G. The rule is A with T, and C with G. Try another.

    See the answer

    AATCG

    T pairs with A, T with A, A with T, G with C, and C with G. So the partner strand is AATCG.

    • If you chose B: That is a copy of the strand you were given. The partner strand has the matching base at each place, not the same one.
    • If you chose C: You paired A with C and T with G. The rule is A with T, and C with G.

  3. What is the backbone of a strand of DNA made of?

    Tap your answer.

    1. Not this one. The bases hang off the backbone and meet the bases of the other strand. They are not the backbone. Try another.

    2. Yes, that is right. Each strand has a backbone of sugar and phosphate. A base is attached to each sugar.

    3. Not this one. Amino acids make proteins. DNA is a different molecule, with sugar and phosphate in the backbone. Try another.

    See the answer

    Sugar and phosphate molecules, taking turns

    Each strand has a backbone of sugar and phosphate. A base is attached to each sugar.

    • If you chose A: The bases hang off the backbone and meet the bases of the other strand. They are not the backbone.
    • If you chose C: Amino acids make proteins. DNA is a different molecule, with sugar and phosphate in the backbone.

  4. What are the rungs of the DNA ladder?

    Tap your answer.

    1. Not this one. Sugars are part of the backbone, the sides of the ladder, not the rungs. Try another.

    2. Not this one. A gene is a long section of DNA with many rungs. A single rung is only one base pair. Try another.

    3. Not this one. A chromosome is a whole DNA molecule, ladder and all. A rung is one small part of it. Go to S4.2.1. Try another.

    4. Yes, that is right. Each rung is one base pair: a base on one strand joined to its partner on the other.

    See the answer

    Pairs of bases, one from each strand

    Each rung is one base pair: a base on one strand joined to its partner on the other.

    • If you chose A: Sugars are part of the backbone, the sides of the ladder, not the rungs.
    • If you chose B: A gene is a long section of DNA with many rungs. A single rung is only one base pair.
    • If you chose C: A chromosome is a whole DNA molecule, ladder and all. A rung is one small part of it. Go to S4.2.1.

  5. Why does the pairing rule help a cell copy its DNA?

    Tap your answer.

    1. Not this one. The strands are partners, not copies. The rule gives each base its matching base. Try another.

    2. Not this one. The rule does not cut anything. It says which base goes opposite which. Try another.

    3. Yes, that is right. When the strands come apart, each one tells the cell which bases to add. That gives two identical double helixes.

    See the answer

    Each old strand acts as the pattern for its new partner.

    When the strands come apart, each one tells the cell which bases to add. That gives two identical double helixes.

    • If you chose A: The strands are partners, not copies. The rule gives each base its matching base.
    • If you chose B: The rule does not cut anything. It says which base goes opposite which.

  6. From an earlier chapter

    How are a gene and a chromosome related?

    Tap your answer.

    1. Not this one. It is the other way round. A gene is a section of the DNA of a chromosome. Go to S4.2.1. Try another.

    2. Yes, that is right. A chromosome is one long DNA molecule, and a gene is one section of it with the code for one protein.

    3. Not this one. A chromosome is a whole coiled DNA molecule with many genes. A gene is just one section. Go to S4.2.1. Try another.

    See the answer

    A gene is a section of the DNA that makes up a chromosome.

    A chromosome is one long DNA molecule, and a gene is one section of it with the code for one protein.

    • If you chose A: It is the other way round. A gene is a section of the DNA of a chromosome. Go to S4.2.1.
    • If you chose C: A chromosome is a whole coiled DNA molecule with many genes. A gene is just one section. Go to S4.2.1.

  7. Complete the lower strand of the ladder. The strand above reads GATC. What does the lower strand read?

    A straight ladder with four rungs. The top strand reads G, A, T, C. The boxes of the lower strand are empty. The backbone and a base pair are named.GATCbackbonebase pair
    See the answer

    CTAG

    G pairs with C, A with T, T with A, and C with G. So the lower strand reads CTAG.

  8. In a sample of DNA, 30% of the bases are T. What percentage are G?

    See the answer

    20%

    T pairs with A, so 30% are A as well. A and T make 60%. The other 40% are C and G in equal shares, so G is 20%.

  9. A part of a gene has 90 bases. Each group of three bases codes for one amino acid. How many amino acids does this part code for?

    See the answer

    30 amino acids

    Three bases make one group, so 90 ÷ 3 = 30 groups. Each group is one amino acid.

Keep it

Copy this into your exercise book.

DNA is a double helix. Sugar and phosphate make each backbone. The rungs are base pairs: A with T, C with G. Three bases code for one amino acid.

Draw: A ladder with four rungs and the letters A, T, C and G on it, with the pairs matched.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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