2. Согласование времен в сложноподчиненных

предложениях.

1. Найдите в тексте Inheritance примеры инфинитива. Определите их функции в предложении.

2. Объясните правила согласования времен в следующих предложениях:

a) It was stated that the female's role was nothing more than a convenient incubator.

b) Around this period, Regnier de Graaf discovered in ovaries what later would be called the Graafian follicle.

c) It was originally believed that new species would arise spontaneously in some manner.

d) By the end of the century it was more or less accepted that they were formed by adaptation of existing forms.

Summary

5.1 Nucleic acids

Ribonucleic acid (RNA)

RNA is a single-stranded polymer of nucleotides where the pentose sugar is always ribose and the organic bases are adenine, guanine, cytosine and uracil. There are three types of RNA found in cells, all of which are involved in protein synthesis.

Ribosomal RNA (rRNA) is a large, complex molecule made up of both double and single helices. Although it is manufactured by the DNA of the nucleus, it is found in the cytoplasm where it makes up more than half the mass of the ribosomes. It comprises more than half the mass of the total RNA of a cell and its base sequence is similar in all organisms.

Transfer RNA (tRNA) is a small molecule (about 80 nucleotides) comprising a single strand. Again it is manufactured by nuclear DNA. It makes up 10-15% of the cell's RNA and all types are fundamentally similar. There are at least 20 types of tRNA, each carrying a different amino acid.

Messenger RNA (mRNA) is a long single-stranded molecule, of up to thousands of nucleotides, which is formed into a helix. It enters the cytoplasm where it associates with the ribosomes and acts as a template for protein synthesis. It makes up less than 5% of the total cellular RNA. It is easily and quickly broken down, sometimes existing for only a matter of minutes.

Did you know?

Each cell contains about 2 metres

of DNA. If all the DNA of a single

human were stretched out, it would

reach the moon and back 8000 times.

Deoxyribonucleic acid (DNA)

DNA is a double-stranded polymer of nucleotides where the pentose sugar is always deoxyribose and the organic bases are adenine, guanine, cytosine and thymine, but never uracil. Each of these polynucleotide chains is extremely long and may contain many million nucleotide units.

Using the accumulated evidence, James Watson and Francis Crick in 1953 suggested a molecular structure which proved to be one of the greatest milestones in biology. They postulated a double helix of two nucleotide strands, each strand being linked to the other by pairs of organic bases which are themselves joined by hydrogen bonds. The pairings are always cytosine with guanine and adenine with thymine. However, this is no ordinary ladder; instead it is twisted into a helix.

5.2 The genetic code

Once the structure of DNA had been elucidated and its mechanism discovered, one important question remained: how exactly are the genetic instructions stored on the DNA to construct new cells and organisms? Most chemicals within cells are similar regardless of the type of cell or species of organism. It is in their proteins and DNA that cells and organisms differ. It seems a reasonable starting point to assume that the DNA in some way provides a 'code' for an organism's proteins. Moreover, most chemicals in cells are manufactured with the aid of enzymes, and all enzymes are proteins. Therefore by determining which enzymes are produced, the DNA can determine an organism's characteristics. Every species possesses different DNA and hence produces different enzymes. The DNA of different species differs not in the chemicals which it comprises, but in the sequence of base pairs along its length. This sequence must be a code that determines which proteins are manufactured.

The part of the DNA molecule which specifies a polypeptide is termed a gene. Perhaps the most significant scientific advance in recent years has been the development of technology which allows genes to be manipulated, altered and transferred from organism to organism - even to transform DNA itself. This has enabled us to use rapidly reproducing organisms such as bacteria as chemical factories producing useful, often life-saving, substances. The list of these substances expands almost daily and includes hormones, antibiotics, interferon and vitamins.