9. Unit III. Molecular biology of the gene

10. Text 3.1. Dna Structure

■ Essential targets:

By the end of this text you should be able to:

●distinguish between a nucleoside, a nucleotide, and a polynucleotide;

●explain how a phosphodiester bond forms;

●discuss the significance of complementary base pairing in DNA.

Pre-reading

■ Working in pairs, try and answer the following questions before you read the text. Don`t be afraid of guessing the answers! When you have finished, check your answers by reading the text.

1. Who discovered the structure of DNA?

2. What do you know about a nucleoside and a nucleotide?

3. What shape does a molecule of DNA have?

4. What kind of information does a molecule of DNA contain?

Exercise A. Match the words with their definitions:

1	to join	A	only one or considered to its own
2	base	B	serious study of a subject that is intended to discover new facts or test new ideas
3	ring	C	to connect or fasten things together
4	support	D	the most important part of something from which new ideas develop
5	bond	E	a circular line or mark
6	single	F	sympathetic encouragement and help that you give to someone
7	research	G	the chemical force that holds atoms together
8	to discover	H	a single thin piece of thread, wire, hair etc.
9	double	I	something that is twice the size, quantity, value, or strength of something else
10	strand	J	to find something that was hidden or that people did not know about before

■ Read the given text and make your essential assignments:

The description of the double helical structure of DNA (deoxyribonucleic acid) by Watson and Crick in 1953 (see Fact of life) was a landmark in science history. Their discovery sparked off a new era in scientific research which has had, and will continue to have, far-reaching consequences.

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 reaction to 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 phosphodiester bonds.

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.