4.3 Fungi

The Fungi are a large group of organisms composed of about 80 000 named species. For many years they were classified with the plants but are now recognized as a separate kingdom. This separation is based on the presence of the polysaccharide chitin found in their cell walls, rather than the cellulose present in plant cell walls. The Fungi lack chlorophyll and are therefore unable to photosynthesize. They feed heterotrophically, generally as saprobionts or parasites.

Economic importance of fungi

Many fungi are beneficial to humans. Examples include:

1. Decomposition of sewage and organic material in the soil.

2. Production of antibiotics, notably from Penicillium and Aspergillus.

3. Production of alcohol for drinking and industry.

4. Production of other foods. Citric acid for lemonade is produced by the fermentation of glucose by Aspergillus. Yeasts are used in bread.

5. Experimental use, especially for genetic investigations.

Translation

Taxonomic ranks

It is convenient to distinguish large groups of organisms from smaller subgroups and a series of rank names has been devised to identify the different levels within this hierarchy. The rank names used today are largely derived from those used by Linnaeus over 200 years ago. The largest groups are known as phyla and the organisms in each phylum have a body plan radically different from organisms in any other phylum. Diversity within each phylum allows it to be divided into classes. Each class is divided into orders of organisms which have additional features in common. Each order is divided into families and at this level differences are less obvious. Each family is divided into genera and each genus into species. Every organism is given a scientific name according to an internationally agreed system of nomenclature, first devised by Linnaeus. The name is always in Latin and is in two parts. The first name indicates the genus and is written with an initial capital letter; the second name indicates the species and is written with a small initial letter. These names are always distinguished in text by italics or underlining. This system of naming organisms is known as binomial nomenclature.

Living organisms are divided into 5 kingdoms: Prokaryotae, Fungi, Protoctista, Plantae and Animalia. It is difficult to fit viruses into this scheme of classification because they are on the border of living and non-living. For this reason they are dealt with separately.

Retroviruses

Probably the best known retrovirus is the Human Immunodeficiency Virus (HIV) which causes AIDS (Acquired Immune Deficiency Syndrome).

The genetic information in a retrovirus is RNA. While many viruses possess RNA, retroviruses are different in that they can use it to synthesize DNA. This is a reversal of the usual genetic process in which RNA is made from DNA and the reason retroviruses are so called (retro – behind or backwards).

In 1970 the enzyme capable of synthesizing DNA from RNA was discovered and given the name reverse transcriptase. This discovery has considerable importance for genetic engineering.

The DNA form of the retrovirus genes is called the provirus and is significant in that it can be incorporated into the host’s DNA. Here it may remain latent for long periods before the DNA of the provirus is again expressed and new viral RNA produced. During this time any division of the host cell results in the proviral DNA being duplicated as well. This explains why individuals infected with HIV virus often display no symptoms for many years before suddenly developing full-blown AIDS.

When incorporated into the host DNA the provirus is capable of activating the host genes. Where these genes are concerned with cell division or growth, and are ‘switched off’, their activation by the provirus can result in a malignant growth known as cancer. The RNA produced by these newly activated genes may become packaged inside new retrovirus particles being assembled inside the host cell. This RNA may then be delivered, along with the retroviral RNA, to the next cell the virus infects. This new cell will then become potentially cancerous.

Host genes which have been acquired by retroviruses in this way are called oncogenes (oncos = tumour). Very few human cancers are caused by retroviruses in this way but research into them has led to the discovery of similar genes found in human chromosomes. These genes can be activated by chemicals or forms of radiation rather than viruses, and their investigation has already helped to prevent some cancers and may, in time, provide a cure.

Speaking

Заполните и прокомментируйте таблицу:

Organisms	Harmful properties	Useful properties
Viruses
Bacteria
Fungi

Unit 5. Inheritance

Did you know?

Every cell contains all the DNA

needed to make the entire organism

from which it comes. And yet the DNA

from a single cell of a human weight 0.024g.

Essential vocabulary

Accuracy to adapt alternating

adenine to assert compatible

base to breed delicate

boundary to compete insecticidal

constancy to elucidate resistant

cytosine to ensure suitable

erythropoietin to fail variable

evidence to lead

female to predict

guanine to prevail

habitat to succeed

hazard to supply

helix to withstand

make-up

male

medium

messenger

offspring

ribose

uracil

Образуйте однокоренные слова, используя префиксы и суффиксы:

Prefix	Base word	Suffix
In- Mis- Non- Re- Un-	accuracy constancy evidence habitat hazard to adapt to compete to lead to predict resistance to suit to vary	-able - al -ant -ate -ent -er - ety - ion - ity - ive - tion -or - ous

Аналитическое чтение.