From Wikipedia,
the free encyclopedia
In
molecular biology, two
nucleotides on opposite
complementary
DNA or
RNA strands that are connected
via
hydrogen bonds are called a
base pair (often abbreviated
bp). In DNA,
adenine (A) forms a base pair
with
thymine (T), as does
guanine (G) with
cytosine (C). In RNA,
thymine is replaced by
uracil (U). As DNA is usually
double-stranded, the number of
base pairs given for a particular
DNA strand is the number of
nucleotides in one of the strands.
Thus, the following examples of
base-paired
nucleotide sequence are said
to be six base-pairs long:
- A base-paired DNA sequence:
ATCGAT
TAGCTA
- The corresponding
base-paired RNA sequence:
AUCGAU
UAGCUA
The following abbreviations are
commonly used to describe the
length of a DNA/RNA molecule:
- kbp = kilo base pairs =
1,000 bp
- Mbp = mega base pairs =
1,000,000 bp
- Gbp = giga base pairs =
1,000,000,000 bp
In case of single stranded
DNA/RNA we talk about
nucleotides, abbreviated nt
(or knt, Mnt, Gnt), rather than
base pairs, as they are not
paired.
The larger nucleic acids, adenine
and guanine, are members of a
class of doubly-ringed chemical
structures called
purines; the smaller nucleic
acids, cytosine and thymine (and
uracil), are members of a class of
singly-ringed chemical structures
called
pyrimidines. Purines are only
complementary with pyrimidines:
pyrimidine-pyrimidine pairings are
energetically unfavourable because
the molecules are too far apart
for hydrogen bonding to be
established; purine-purine
pairings are energetically
unfavourable because the molecules
are too close, leading to
electrostatic repulsion. The only
other possible pairings are GT and
AC; these pairings are mismatches
because the pattern of hydrogen
donors and acceptors do not
correspond.
Thymine and adenine bond
together through two hydrogen
bonds, while cytosine and guanine
bond together through three
hydrogen bonds.
As
hydrogen bonds are not very
strong, the two nucleotide strands
will separate on temperatures
higher than 94
°C.
Chemical analogs of nucleotides
can take the place of proper
nucleotides and establish
non-canonical base-pairing,
leading to errors in
DNA replication and
DNA transcription. Some
analogs are
carcinogens; others are
chemotherapy drugs.
Guanine and cytosine form 3
hydrogen bonds while adenine and
thymine form only 2 hydrogen
bonds. Consequently A-T pairs are
less stable.
