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In molecular biology, the term double helix [1] refers to the structure formed by double-stranded molecules of nucleic acids such as DNA. The double helical structure of a nucleic acid complex arises as a consequence of its secondary structure , and is a fundamental component in determining its tertiary structure .
The nitrogenous bases of the two separate polynucleotide strands are bound together, according to base pairing rules (A with T and C with G), with hydrogen bonds to make double-stranded DNA. The complementary nitrogenous bases are divided into two groups, the single-ringed pyrimidines and the double-ringed purines. In DNA, the pyrimidines are ...
In most cases, naturally occurring DNA molecules are double-stranded and RNA molecules are single-stranded. [19] There are numerous exceptions, however—some viruses have genomes made of double-stranded RNA and other viruses have single-stranded DNA genomes, [20] and, in some circumstances, nucleic acid structures with three or four strands ...
These enzymes, along with accessory proteins, form a macromolecular machine which ensures accurate duplication of DNA sequences. Complementary base pairing takes place, forming a new double-stranded DNA molecule. This is known as semi-conservative replication since one strand of the new DNA molecule is from the 'parent' strand.
The tertiary arrangement of DNA's double helix in space includes B-DNA, A-DNA, and Z-DNA. Triple-stranded DNA structures have been demonstrated in repetitive polypurine:polypyrimidine Microsatellite sequences and Satellite DNA. B-DNA is the most common form of DNA in vivo and is a more narrow, elongated helix than A-DNA. Its wide major groove ...
DNA strand 1: antisense strand (transcribed to) → RNA strand (sense) DNA strand 2: sense strand. Some regions within a double-stranded DNA molecule code for genes, which are usually instructions specifying the order in which amino acids are assembled to make proteins, as well as regulatory sequences, splicing sites, non-coding introns, and ...
From the DNA double helix model, it was clear that there must be some correspondence between the linear sequences of nucleotides in DNA molecules to the linear sequences of amino acids in proteins. The details of how sequences of DNA instruct cells to make specific proteins was worked out by molecular biologists during the period from 1953 to 1965.
The chemical structure of DNA base-pairs . 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 DNA and RNA.