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DNADeoxyribonucleic acid Molecular biology A double-stranded linear macromolecule which encodes an organism's genetic information
DNAAbbrev. for deoxyribonucleic acid. The very long molecule that winds up to form a CHROMOSOME and that contains the complete code for the automatic construction of the body. The molecule has a double helix skeleton of alternating sugars (deoxyribose) and phosphates. Between the two helices, lying like rungs in a ladder, are a succession of linked pairs of the four bases adenine, thymine, guanine and cytosine. The molecules of adenine and guanine are larger than thymine and cytosine and so, to keep the rungs of equal length, adenine links only with thymine and guanine only with cytosine. This arrangement allows automatic replication of the molecule. The sequence of bases along the molecule, taken in groups of three (codons), is the genetic code. Each CODON specifies a particular amino acid to be selected, and the sequence of these, in the polypeptides formed, determines the nature of the protein (usually an ENZYME) synthesized. Polypeptide formation occurs indirectly by way of MESSENGER RNA and TRANSFER RNA. Periodicity of DNA is defined as the number of base pairs per turn of the double helix.
DNA (deoxyribonucleic acid)a complex NUCLEIC ACID molecule found in the chromosomes of almost all organisms, which acts as the primary genetical material, controlling the structure of proteins and hence influencing all enzyme-driven reactions.
- structure. DNA is a polymer of deoxyribonucleotides. The model proposed by WATSON and CRICK in 1953 has now become universally accepted for double-stranded DNA. The DNA is considered to consist of two POLYNUCLEOTIDE CHAINS joined together by hydrogen bonds between NUCLEOTIDE BASES, with COMPLEMENTARY BASE PAIRING between specific bases ensuring a parallel-sided, stable structure: ADENINE pairing with THYMINE (2H bonds) and CYTOSINE with GUANINE (3H bonds).
The two polynucleotide chains each have an opposite polarity due to the way the phosphates are attached to the sugar groups by 3'- 5' PHOSPHODIESTER BONDS. DNA can exist in a number of configurations, of which B-DNA is the predominant form. In this form the molecule is twisted into a right-handed double helix, with a complete turn every tenth base.
- replication (see SEMICONSERVATIVE REPLICATION MODEL). Replication is initiated at the ORIGIN OF REPLICATION. DNA HELICASE enzymes unwind the double-stranded DNA and each parental strand acts as a TEMPLATE for new DNA synthesis. The anti-parallel nature of the double-stranded DNA molecule affects the replication process. The DNA POLYMERASES involved in replication can only add NUCLEOTIDES to the 3'- OH group of a polynucleotide chain, that is, a DNA strand can only be synthesized in the 5'- 3' direction requiring a template running 3'-" 5'. Thus the two newly synthesized strands must grow in different directions. One daughter strand (the leading strand) is synthesized continuously in the 5'- 3' direction. The other daughter strand (the lagging strand) is synthesized discontinuously in fragments (called Okazaki fragments) that have been synthesized in the normal 5'- 3' direction, but the strand grows overall in the 3'-" 5' direction. These fragments are afterwards joined to make a continuous strand. The region in which replication occurs is called the replication fork. In E. coli, replication occurs as follows: the DNA unwinds, SSB protein is laid down on the single strands to stabilize them and to prevent rewinding. An RNA PRIMER initiates DNA synthesis on the leading strand. RNA primers initiate synthesis of each fragment on the lagging strand. These primers are later removed and the gaps left are filled in by the activity of a DNA polymerase. DNA LIGASE then joins the fragments together to form a complete strand.
- location. DNA is found in all chromosomes except those of certain viruses (such as tobacco mosaic virus, TMV), where the heritable material is RNA. In PROKARYOTES, DNA is generally in the form of a single coiled molecule in a continuous loop (see NUCLEOID and may also occur as extrachromosomal material in the cytoplasm. In EUKARYOTES the DNA is also highly coiled but is complexed with basic and acidic proteins. There is probably only one very long DNA molecule per chromosome. DNA is also found in CHLOROPLASTS and MITOCHONDRIA of eukaryote cytoplasm (see CYTOPLASMIC INHERITANCE).
- DNA as a genetic material. There are several pieces of evidence to suggest the role of DNA in inheritance: (i) TRANSFORMATION experiments with Streptococcus (Diplococcus) pneumoniae in 1928, by F. GRIFFITH; (ii) the identification of the ‘transforming principle’ as DNA by AVERY MacLeod and McCarty in 1944; (iii) the fact that the wavelength of ultraviolet light which causes most mutations in various prokaryotes and eukaryotes matches the ABSORPTION SPECTRUM of nucleic acids (260 nm); (iv) HERSHEY and Chase's experiment with labelled BACTERIOPHAGE. DNA has several features which make it an ideal genetic material: great stability (see structure above); accurate replication so that all cells contain an identical copy of information; four nucleotide bases to provide storage of coded information (see GENETIC CODE); it is capable of mutation by altering the base sequence; it may be broken and rejoined to form new genetic combinations (see RECOMBINATION); stored information can be accurately ‘read’ by other cell molecules (see TRANSCRIPTION).