Khan Academy DNA video

DNA stands for deoxyribonucleic acid. separate it into deoxy ribo nucleic acid.


In DNA, the bases are:

  • adenosine (A)
  • cytosine (C)
  • guanine (G)
  • thymine (T)

Pyrimidines include cytosine (C) and thymine (T).

Purines include adenosine (A) and guanine (G).

Each of these bases is a nucleotide (nt)

DNA contains coded information. It is a linear sequence of base pairs. The sequence is known as the nucleotide sequence of base sequence.

Ends of chain a called 3' and 5'. When reading the base letters, start from the 5' end.

There are 3 billion base pairs in a human DNA.

Each base has a complement and forms a "base pair"


DNA is made of 2 strands that complement each other.

Structure is a 'double helix'. i.e. twisted ladder.

On each rung of the ladder, bases complement each other. Adenine pairs with thymine, and guanine pairs with cytosine.

There are 2 hydrogen bonds between A and T, and 3 hydrogen bonds between C and G.

This means if you know the sequence of one strand, you can deduce the sequence of the other strand. The 5' and 3' ends of the strands run in opposite directions, i.e. anti-parallel, so when describing the deduced strand, it is in reverse.

DNA replication

DNA polymerase can synthesize (make) a new complementary DNA strand using an existing DNA strand as a template. One double helix can help create 2 new DNA strands.

DNA is negatively charged due to the phosphate groups. Mg2+ helps stabilise the DNA.

The error rate is approximately 1 in 10^9


The exome is a subset of the genome that codes for proteins. It includes exons. Introns are excluded. DNA fragments that code for proteins can be described as exonic.

In humans, the exome is roughly 1.5% of the genome.