The Structure of DNA

The structure
of DNA

T Thymine

+
Adenine

A

Chromosome

G

T

A

Guanine

C

Cytosine

+

A chromosome contain double-stranded DNA,
which is made from nucleotides with four
different bases. Adenine always pairs with
thymine, and guanine with cytosine. Together they
form “base pairs.” The cell’s 46 chromosomes
comprise approximately 6 billion base pairs.

DNA-helix

C

G

G

A

C

G

T

A

G

G

T

A

C

C

T

G

T

C

T

G

C

A

T

C

C

A

T

G

G

A

C

A

T

G

A

C

T

C

A

T

C

A

A

T

G
C

T
A

A
T

T
A

G

T

When the cell divides, all the chromosomes
are copied. The DNA replication machinery
unwinds the DNA helix and forms two new
DNA strands, using the old strands as
templates. Again, adenin always pairs with
thymine, and guanine with cytosine.

T

G

A

C

A

A
T
G
T
C
A

New DNA
strand

T

G

A

New DNA strand

Illustration: © Johan Jarnestad/
The Royal Swedish Academy of Sciences