A polymer of nucleotides
Each DNA strand is a polymer made up of nucleotide subunits. Тhe nucleotides join together to form long unbranched polynucleotide chains.
Each nucleotide consists of deoxyribose (a five-carbon or pentose sugar), an organic nitrogen-containing base (of which there are four different types), and phosphoric acid.
The sugar and the organic base join together by a condensation reactionto form a nucleoside. (A condensation reaction results in the removal of a water molecule.)
Another condensation reaction joins the nucleoside with phosphoric acid to form the nucleotide. This bond forms between carbon 5 of the sugar and the phosphate, and is called a phosphoester bond.
The organic bases present in DNA are either purines (guanine, G and adenine, A) or pyrimidines (cytosine, С and thymine, T). Purines have a double ring structure; pyrimidines have a single ring structure.
Two nucleotides can join together by a condensation reaction between the phosphate group of one nucleotide and the hydroxyl group on carbon 3 of the sugar of the other nucleotide. The bonds linking the nucleotides together are strong, covalent phosphodiesterbonds.
The process can be repeated so that a polynucleotide chain builds up. The chain has a sugar-phosphate backbone with the organic bases projecting outwards.
Each chain has two distinct ends: a 3' ('three prime') end and a 5'('five prime') end. At the 3' end, the carbon 3 of the deoxyribose is closest to the end; at the 5' end, the carbon 5 of the deoxyribose is closest to the end.
The double helix
DNA consists of two polynudeotide chains coiled around each other to form a double helix. The double helix is held together by hydrogen bonds between pairs of bases in the two chains. The pairings depend on the shapes of the bases (a purine can only bond with a pyrimidine) and on their ability to form hydrogen bonds:
Adenine (a purine) pairs with thymine (a pyrimidine), forming two hydrogen bonds (A=T).
Guanine (a purine) pairs with cytosine (a pyrimidine), forming three hydrogen bonds (G = C).
Complementary base pairing
These complementary base pairs are the only ways the bases can bond and join the two nucleotide chains. Thus, the sequence of bases along one polynudeotide chain determines the sequence along the other: an adenine on one chain means there must be a thymine on the other chain at that point, and so on. Complementary base pairing forms the basis of DNA replication and its ability to form messenger RNA during protein synthesis.
Complementary base pairing can happen only if the two polynudeotide chains are antiparallel. Antiparallel chains run in opposite directions; one chain runs from 3' to 5', and the other from 5' to 3'.
Watson and Crick's model of DNA showed that the base pairs are 0.34 nm apart, and that each complete turn of the helix has ten base pairs.
DNA is a double helix made of two polynucleotide chains.
Each chain has a sugar-phosphate backbone on the outside with organic bases on the inside.
The two chains are held together by complementary base pairing.
The chains are antiparallel (the 5' end of one chain lies next to the 3' end of the other chain).
■Glossary of essential terms for you to know
■ Your Essential Assignments
I. Quick check
I. Distinguish between a nucleoside and a nucleotide.
2. By what type of chemical reaction is a phosphodiester bond formed?
3. If one strand of DNA has the base sequence AATCCG, what will be the corresponding base sequence of bits complementary strand ?
II. Fill in the missing words:
III. Use a monolingual English dictionary and give the definitions of the words below:
landmark; bond; chain; sequence; to coil.
IV. Find English equivalents to the following word combinations:
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