There are four different nucleotides

Definition

Nucleotide

The repeating unit of DNA, made of a sugar, a phosphate group and one of four bases.

All four nucleotides in DNA share the same sugar and the same phosphate group.
The one part that differs between them is the base attached to the sugar.
The four bases are adenine ( $A\text{A}$ ), thymine ( $T\text{T}$ ), cytosine ( $C\text{C}$ ) and guanine ( $G\text{G}$ ).
Because there are four possible bases, there are exactly four different nucleotides, and DNA is built from those four alone.
A strand of DNA is a chain of these four nucleotides in a particular order, and that order is what carries the coded information.
A single gene can be thousands of nucleotides long, so the number of possible orders is enormous, which is how a four-letter alphabet codes for every protein an organism makes.

Common Mistake

The sugar and the phosphate group never change from one nucleotide to the next, so a question asking what makes the four nucleotides different is asking about the base.

Bases join in fixed complementary pairs

Definition

Complementary base pairing

The fixed pairing of bases between the two DNA strands, in which adenine pairs with thymine and cytosine pairs with guanine.

Definition

Hydrogen bond

A weak bond that forms between a pair of bases and holds the two DNA strands together.

The bases on the two strands are not paired at random, they follow a fixed rule called complementary base pairing.
Adenine always pairs with thymine, written as the pair $A\text{A}$ and $T\text{T}$ .
Cytosine always pairs with guanine, written as the pair $C\text{C}$ and $G\text{G}$ .
Each base pair is held together by weak hydrogen bonds that form across the middle of the molecule.
One hydrogen bond on its own is easily broken, but a DNA molecule contains millions of them, so together they hold the two strands firmly.
Every pair contains one larger base and one smaller base, so each rung of the ladder is the same width and the helix keeps a constant shape along its length.
A pairing such as $A\text{A}$ with $G\text{G}$ never happens, because the shapes of those two bases do not fit together across the gap.

Complementary base pairing between two DNA strands, with adenine held to thymine and cytosine held to guanine by hydrogen bonds.

Hint

$A\text{A}$ and $T\text{T}$ are both straight-sided letters and $C\text{C}$ and $G\text{G}$ are both curved letters, which is a quick way of keeping the two pairs apart under pressure.

Common Mistake

Never write that $A\text{A}$ pairs with $G\text{G}$ or that $C\text{C}$ pairs with $T\text{T}$ , because only the pairs $A\text{A}$ with $T\text{T}$ and $C\text{C}$ with $G\text{G}$ occur in DNA.

One strand sets the sequence of the other

Because the pairing rule is fixed, knowing the base order of one strand tells you the base order of the other.
To work out the partner strand, replace every $A\text{A}$ with $T\text{T}$ , every $T\text{T}$ with $A\text{A}$ , every $C\text{C}$ with $G\text{G}$ and every $G\text{G}$ with $C\text{C}$ .
The two strands are called complementary because each is the matching partner of the other, not an identical copy of it.
This also means the amount of $A\text{A}$ in a DNA molecule always equals the amount of $T\text{T}$ , and the amount of $C\text{C}$ always equals the amount of $G\text{G}$ .
When the hydrogen bonds break and the strands separate, each exposed strand can act as a template, because the pairing rule leaves only one possible partner for each base.
Fixed pairing is therefore what lets the base sequence be rebuilt without error whenever the molecule is copied or read.

How the fixed pairing rules let one strand rebuild its partner exactly, so the base sequence is copied without error.

Example

Building the complementary strand

One strand of a DNA molecule reads $T\text{T}$ $A\text{A}$ $C\text{C}$ $G\text{G}$ $G\text{G}$ $A\text{A}$ .
Take the bases one at a time and swap each for its partner: $T\text{T}$ gives $A\text{A}$ , $A\text{A}$ gives $T\text{T}$ , $C\text{C}$ gives $G\text{G}$ , $G\text{G}$ gives $C\text{C}$ , $G\text{G}$ gives $C\text{C}$ and $A\text{A}$ gives $T\text{T}$ .
The complementary strand therefore reads $A\text{A}$ $T\text{T}$ $G\text{G}$ $C\text{C}$ $C\text{C}$ $T\text{T}$ .
Both strands contain six bases, which is the check that no base has been missed out.

Example

Working out the proportion of each base

A sample of DNA is found to contain $30%30\%$ adenine.
Adenine pairs only with thymine, so thymine must also make up $30%30\%$ of the bases.
That accounts for $60\%$ , leaving $40\%$ for cytosine and guanine together.
Cytosine and guanine pair with each other in equal numbers, so each makes up $40 \div 2 = 20\%$ .

Exam technique

Completing a base sequence

Work along the sequence base by base rather than trying to read the whole strand at once, because a single slip loses the mark for the entire line.
Count that your answer has exactly as many bases as the strand you were given.
Write the letters as clear capitals, since a $G\text{G}$ that looks like a $C\text{C}$ cannot be credited.
If the question asks you to explain the pairing, name the bases in full rather than using only the letters.

Self review

Why are there exactly four different nucleotides in DNA?
Name the base that pairs with adenine and the base that pairs with cytosine.
What type of bond holds a base pair together, and why does a molecule need so many of them?
Write the complementary strand for the sequence $G\text{G}$ $C\text{C}$ $A\text{A}$ $T\text{T}$ $T\text{T}$ $G\text{G}$ .
A DNA sample contains $22%22\%$ guanine. What percentage is adenine?

There are four different nucleotides

Definition

Nucleotide

The repeating unit of DNA, made of a sugar, a phosphate group and one of four bases.

All four nucleotides in DNA share the same sugar and the same phosphate group.
The one part that differs between them is the base attached to the sugar.
The four bases are adenine ( $A\text{A}$ ), thymine ( $T\text{T}$ ), cytosine ( $C\text{C}$ ) and guanine ( $G\text{G}$ ).
Because there are four possible bases, there are exactly four different nucleotides, and DNA is built from those four alone.
A strand of DNA is a chain of these four nucleotides in a particular order, and that order is what carries the coded information.
A single gene can be thousands of nucleotides long, so the number of possible orders is enormous, which is how a four-letter alphabet codes for every protein an organism makes.

Common Mistake

The sugar and the phosphate group never change from one nucleotide to the next, so a question asking what makes the four nucleotides different is asking about the base.

Bases join in fixed complementary pairs

Definition

Complementary base pairing

The fixed pairing of bases between the two DNA strands, in which adenine pairs with thymine and cytosine pairs with guanine.

Definition

Hydrogen bond

A weak bond that forms between a pair of bases and holds the two DNA strands together.

The bases on the two strands are not paired at random, they follow a fixed rule called complementary base pairing.
Adenine always pairs with thymine, written as the pair $A\text{A}$ and $T\text{T}$ .
Cytosine always pairs with guanine, written as the pair $C\text{C}$ and $G\text{G}$ .
Each base pair is held together by weak hydrogen bonds that form across the middle of the molecule.
One hydrogen bond on its own is easily broken, but a DNA molecule contains millions of them, so together they hold the two strands firmly.
Every pair contains one larger base and one smaller base, so each rung of the ladder is the same width and the helix keeps a constant shape along its length.
A pairing such as $A\text{A}$ with $G\text{G}$ never happens, because the shapes of those two bases do not fit together across the gap.

Complementary base pairing between two DNA strands, with adenine held to thymine and cytosine held to guanine by hydrogen bonds.

Hint

$A\text{A}$ and $T\text{T}$ are both straight-sided letters and $C\text{C}$ and $G\text{G}$ are both curved letters, which is a quick way of keeping the two pairs apart under pressure.

Common Mistake

Never write that $A\text{A}$ pairs with $G\text{G}$ or that $C\text{C}$ pairs with $T\text{T}$ , because only the pairs $A\text{A}$ with $T\text{T}$ and $C\text{C}$ with $G\text{G}$ occur in DNA.

One strand sets the sequence of the other

Because the pairing rule is fixed, knowing the base order of one strand tells you the base order of the other.
To work out the partner strand, replace every $A\text{A}$ with $T\text{T}$ , every $T\text{T}$ with $A\text{A}$ , every $C\text{C}$ with $G\text{G}$ and every $G\text{G}$ with $C\text{C}$ .
The two strands are called complementary because each is the matching partner of the other, not an identical copy of it.
This also means the amount of $A\text{A}$ in a DNA molecule always equals the amount of $T\text{T}$ , and the amount of $C\text{C}$ always equals the amount of $G\text{G}$ .
When the hydrogen bonds break and the strands separate, each exposed strand can act as a template, because the pairing rule leaves only one possible partner for each base.
Fixed pairing is therefore what lets the base sequence be rebuilt without error whenever the molecule is copied or read.

How the fixed pairing rules let one strand rebuild its partner exactly, so the base sequence is copied without error.

Example

Building the complementary strand

One strand of a DNA molecule reads $T\text{T}$ $A\text{A}$ $C\text{C}$ $G\text{G}$ $G\text{G}$ $A\text{A}$ .
Take the bases one at a time and swap each for its partner: $T\text{T}$ gives $A\text{A}$ , $A\text{A}$ gives $T\text{T}$ , $C\text{C}$ gives $G\text{G}$ , $G\text{G}$ gives $C\text{C}$ , $G\text{G}$ gives $C\text{C}$ and $A\text{A}$ gives $T\text{T}$ .
The complementary strand therefore reads $A\text{A}$ $T\text{T}$ $G\text{G}$ $C\text{C}$ $C\text{C}$ $T\text{T}$ .
Both strands contain six bases, which is the check that no base has been missed out.

Example

Working out the proportion of each base

A sample of DNA is found to contain $30%30\%$ adenine.
Adenine pairs only with thymine, so thymine must also make up $30%30\%$ of the bases.
That accounts for $60\%$ , leaving $40\%$ for cytosine and guanine together.
Cytosine and guanine pair with each other in equal numbers, so each makes up $40 \div 2 = 20\%$ .

Exam technique

Completing a base sequence

Work along the sequence base by base rather than trying to read the whole strand at once, because a single slip loses the mark for the entire line.
Count that your answer has exactly as many bases as the strand you were given.
Write the letters as clear capitals, since a $G\text{G}$ that looks like a $C\text{C}$ cannot be credited.
If the question asks you to explain the pairing, name the bases in full rather than using only the letters.

Self review

Why are there exactly four different nucleotides in DNA?
Name the base that pairs with adenine and the base that pairs with cytosine.
What type of bond holds a base pair together, and why does a molecule need so many of them?
Write the complementary strand for the sequence $G\text{G}$ $C\text{C}$ $A\text{A}$ $T\text{T}$ $T\text{T}$ $G\text{G}$ .
A DNA sample contains $22%22\%$ guanine. What percentage is adenine?

1.2.2 Nucleotides and complementary bases

There are four different nucleotides

Bases join in fixed complementary pairs

One strand sets the sequence of the other

Building the complementary strand

Working out the proportion of each base

Completing a base sequence

Review 1.2.2 Nucleotides and complementary bases by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.2.2 Nucleotides and complementary bases

There are four different nucleotides

Bases join in fixed complementary pairs

One strand sets the sequence of the other

Building the complementary strand

Working out the proportion of each base

Completing a base sequence

Review 1.2.2 Nucleotides and complementary bases by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.2.2 Nucleotides and complementary bases

There are four different nucleotides

Bases join in fixed complementary pairs

One strand sets the sequence of the other

Building the complementary strand

Working out the proportion of each base

Completing a base sequence

Review 1.2.2 Nucleotides and complementary bases by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.2.2 Nucleotides and complementary bases Revision Guide

1.2.2 Nucleotides and complementary bases

There are four different nucleotides

Bases join in fixed complementary pairs

One strand sets the sequence of the other

Building the complementary strand

Working out the proportion of each base

Completing a base sequence

Review 1.2.2 Nucleotides and complementary bases by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

1.2.2 Nucleotides and complementary bases Revision Guide