Deoxyribonucleic acid: the inheritable, chemical substance found in all cells containing genetic codes required to make PROTEINS. DNA is a helical polymer, composed of two strands of NUCLEOTIDES, each joined by a deoxyribose sugar backbone. There are four different nucleotides, or bases, in DNA: adenine, thymine, guanine and cytosine, and the arrangement of these into a specific sequence provides the basis of GENES. Within each strand of DNA, bases are joined in a 5′-3′ manner by a phosphodiester bond, formed between the phosphate group at the 5′ carbon with the hydroxyl-containing 3′ carbon of the next nucleotide. Adjacent strands of DNA are held together by hydrogen bonds formed between bases. Base-pairing is specific, so that adenine will only bond with thymine, and guanine will bond with cytosine. Using one strand of DNA as a template, therefore, a complementary strand may be formed. Strands within a helix are antiparallel: that is the 5′ end of one strand corresponds to the 3′ end of the adjacent strand. Replication of DNA before cell division allows genes to be preserved between generations of cells. DNA replication is initiated by the enzyme DNA helicase, which untwists a region of DNA, causing the strands to part in a replication fork. Strands are held apart by single-strand DNA binding proteins, while a small piece of RNA (ribonucleic acid) known as a primer binds to each DNA strand.
Pairing of new complementary nucleotides to the 3' end of the RNA primer is initiated by another enzyme, DNA polymerase III. Because replication proceeds in a 5′–3′ manner, and the DNA strands are antiparallel, only one complementary strand, the 'leading' strand, will be formed by the continuous addition of nucleotides, as the replication fork progresses along the length of DNA. The other, 'lagging' strand, is formed by synthesis of many short 5′–3′ fragments called Okazaki fragments, each initiated by the addition of an RNA primer, and elongated by DNA polymerase III. RNA primers are excised and replaced with deoxyribose nucleotides by DNA polymerase I, and gaps in the strands are sealed by DNA ligase. In this manner, DNA is replicated to produce two identical molecules, each containing one original strand of DNA. While complementary base-pairing is highly specific, mismatches may occur and must be corrected. Some nucleotides are methylated following replication. If a mismatch is found during replication, the methylated strand of DNA will be preserved and mismatched bases will be replaced in the non-methylated strand. Similarly, irradiation or heat exposure may result in damaged regions of DNA. Enzymes called DNA nucleases remove damaged nucleotides and new bases are added by DNA polymerase I. However, these mechanisms sometimes lead to small alterations in sequences in bases, and consequently an altered gene sequence, or MUTATION. Be-sides gene sequences, DNA contains large amounts of non-coding sequences, including those which act as recognition sites for enzymes that initiate, enhance or terminate messenger RNA transcription. The precise functions of many regions of DNA are yet to be elucidated.
FIONA M.INGLIS
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