Nucleic acid molecules have a
phosphoryl (5') end and a
hydroxyl (3') end. This notation follows from
organic chemistry nomenclature, and can be used to define the movement of enzymes such as
DNA polymerases relative to the DNA strand in a non-arbitrary manner.
G-quadruplexes G-quadruplexes, also known as G4 DNA are secondary structures found in nucleic acids that are rich in
guanine. These structures are normally located at the
telomeres (the ends of the
chromosomes). The G-quadruplex can either be parallel or antiparallel depending on the loop configuration, which is a component of the structure. If all the DNA strands run in the same direction, it is termed to be a parallel quadruplex, and is known as a strand-reversal/propeller, connecting adjacent parallel strands. If one or more of the DNA strands run in opposite direction, it is termed as an anti-parallel quadruplex, and can either be in a form of a lateral/edgewise, connecting adjacent anti-parallel strands, or a diagonal, joining two diagonally opposite strands. The structure of these G-quadruplexes can be determined by a cation.
DNA replication In DNA, the 5'
carbon is located at the top of the
leading strand, and the 3' carbon is located at the lower section of the
lagging strand. The
nucleic acid sequences are complementary and parallel, but they go in opposite directions, hence the antiparallel designation. The antiparallel structure of DNA is important in
DNA replication because it replicates the leading strand one way and the lagging strand the other way. During DNA replication, the leading strand is replicated continuously whereas the lagging strand is replicated in segments known as
Okazaki fragments.
Anti-parallelism in biochemistry The importance of an antiparallel DNA double helix structure is because of its hydrogen bonding between the complementary
nitrogenous base pairs. If the DNA structure were to be parallel, the hydrogen bonding would not be possible, as the base pairs would not be paired in the known way. The four base pairs are:
adenine,
guanine,
cytosine, and
thymine, where adenine complements thymine, and guanine complements cytosine.
Transcription would be another problem if the DNA structure were to be parallel, making no sense of the information being read from the DNA. This would further lead to the production of incorrect proteins. ==Polypeptides==