X. Read and translate the short text without any dictionary:

Fact of life:

A new microscope, called a scanning tunneling microscope, was invented in 1980. It measures surface features by moving a sharp probe over the object’s surface. It can achieve magnifications of 100 million, allowing scientists to view atoms on the surface of a solid. This type of microscope is likely to have a major impact on biology. Recently, it has been used to view DNA directly.

XI. Food for thought:

Suggest which unit should be used when calculating the diameter of the DNA molecule. Why might there be a discrepancy between the actual diameter and that estimated from the scanning tunneling micrograph?

■ Have Some Fun! Biologist Jokes!

A young college student stayed up all night studying for his zoology test the next day. As he entered the classroom, he saw ten stands with ten birds on them with a sack over each bird and only legs showing. He sat right on the front row because he wanted to do the best job possible. The professor announced that the test would be to look at each set of bird legs and give the common name, habitat, genus, species, etc.

The student looked at each set of bird legs. They all looked the same to him. He began to get upset. He had stayed up all night studying, and now had to identify birds by their legs. The more he thought about it, the madder he got. Finally, he could stand it no longer. He went up to the professor`s desk and said: “What a stupid test! How could anyone tell the difference between birds by looking at their legs?” With that the student threw his test on the professor`s desk and walked out the door.

The professor was surprised. The class was so big that he didn`t know every student`s name, so as the student reached the door the professor called:

“Mister, what`s your name?” The enraged student pulled up his pant legs and said: “You guess, buddy! You guess!”

UNIT III. MOLECULAR BIOLOGY OF THE GENE

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:

■ 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 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).

Не нашли, что искали? Воспользуйтесь поиском по сайту: