Inbiochemistry, twobiopolymers areantiparallel if they runparallel to each other but with oppositedirectionality (alignments). An example is the twocomplementary strands of aDNAdouble helix, whichrun in opposite directions alongside each other.
Nucleic acid molecules have aphosphoryl (5') end and ahydroxyl (3') end. This notation follows fromorganic chemistry nomenclature, and can be used to define the movement of enzymes such asDNA polymerases relative to the DNA strand in a non-arbitrary manner.
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G-quadruplexes, also known as G4 DNA are secondary structures found in nucleic acids that are rich inguanine.[1] These structures are normally located at thetelomeres (the ends of thechromosomes). 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.[2] The structure of these G-quadruplexes can be determined by a cation.
In DNA, the 5'carbon is located at the top of theleading strand, and the 3' carbon is located at the lower section of thelagging strand. Thenucleic acid sequences are complementary and parallel, but they go in opposite directions, hence the antiparallel designation.[3] The antiparallel structure of DNA is important inDNA 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 asOkazaki fragments.
The importance of an antiparallel DNA double helix structure is because of its hydrogen bonding between the complementarynitrogenousbase 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.[4] The four base pairs are:adenine,guanine,cytosine, andthymine, 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.[5]
Polypeptides have anN-terminus and aC-terminus, which refer to the ends of the polymer in a way that reflects the direction in which the polymer wassynthesized. Thechronological sequence of eachamino acidsub-unit is the basis fordirectionality notation in polypeptides; a givenprotein can be represented as its set of uniqueamino acid abbreviations within an N-terminus and a C-terminus.
Many proteins may adopt abeta sheet as part of theirsecondary structure. In beta sheets, sections of a single polypeptide may run side-by-side and antiparallel to each other, to allow forhydrogen bonding between theirbackbone chains. Beta sheets can also be either a parallel or anti-parallel secondary structure. However, an anti-parallel beta sheet is significantly more stable than a parallel structure due to their well alignedH-bonds, which are at a 90° angle.[6]