biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

, two

biopolymer Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, ...

s are antiparallel if they run

parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster o ...

to each other but with opposite directionality (alignments). An example is the two complementary strands of a DNA

double helix A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * ...

, which run in opposite directions alongside each other.

Nucleic acids

Nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main ...

molecules have a phosphoryl (5') end and a

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...

(3') end. This notation follows from organic chemistry nomenclature, and can be used to define the movement of enzymes such as

DNA polymerase A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to crea ...

s relative to the DNA strand in a non-arbitrary manner.

G-quadruplexes

G-quadruplex In molecular biology, G-quadruplex secondary structures (G4) are formed in nucleic acids by sequences that are rich in guanine. They are helical in shape and contain guanine tetrads that can form from one, two or four strands. The unimolecular ...

es, also known as G4 DNA are secondary structures found in nucleic acids that are rich in

guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is ...

. These structures are normally located at the

telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...

s (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 Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

is located at the top of the

leading strand In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritanc ...

, and the 3' carbon is located at the lower section of the lagging strand. The

nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are us ...

s are complementary and parallel, but they go in opposite directions, hence the antiparallel designation. The antiparallel structure of DNA is important in

DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritan ...

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 pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both D ...

s. 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 Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its deriv ...

cytosine Cytosine () ( symbol C or Cyt) is one of the four nucleobases found in DNA and RNA, along with adenine, guanine, and thymine ( uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached ( ...

, and

thymine Thymine () ( symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidin ...

, 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

Polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

s have an

N-terminus The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...

and a

C-terminus The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein i ...

, which refer to the ends of the polymer in a way that reflects the direction in which the polymer was synthesized. The chronological sequence of each

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...

sub-unit is the basis for directionality notation in polypeptides; a given

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

can be represented as its set of unique amino acid abbreviations within an N-terminus and a C-terminus.

Beta sheet

Many proteins may adopt a

beta sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a ge ...

as part of their

secondary structure Protein secondary structure is the three dimensional form of ''local segments'' of proteins. The two most common secondary structural elements are alpha helices and beta sheets, though beta turns and omega loops occur as well. Secondary struct ...

. In beta sheets, sections of a single polypeptide may run side-by-side and antiparallel to each other, to allow for

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

ing between their backbone 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 aligned H-bonds, which are at a 90° angle.

References

{{DEFAULTSORT:Antiparallel (Biochemistry) Biochemistry