DNA

Deoxyribonucleic acid or DNA is the nucleic acid, containing the sugar deoxyribose, which forms the materials of which the chromosomes and genes of organisms are composed. It carries the blueprint for construction of all the cells of an organism and all instructions that control their activities, and which enables these instructions to be passed on from one generation to the next. DNA is found in all living organisms except certain viruses which contain RNA (ribonucleic acid), a closely related nucleic acid.

In eukaryotic ells- which make up the majority of organisms- DNA is found in the membrane bound sphere called the nucleus. Here, it forms chromosomes, normally long threads (but in cells that are about to divide chromosomes form separate X shapes as they duplicate themselves). Within the chromosomes DNA is tightly packed in coils and super coils. Once unraveled, DNA can be seen to consist of two long strands wound around each other in spiral structure or double helix, like a twisted ladder. The rungs of this ladder are made up of different bases. A rung is formed when adenine from one DNA strand binds with thymine on the other. The arrangement of bases along the double helix constitutes the genetic code, which controls production of the protein molecules that facilitate all cell activities, including metabolism and construction.

Classification of Plants and Animals

Every different kind of organism called a species, and closely related species are grouped together into genera (the singular of with is genus). Each species has a two part scientific name, composed of its generic name and its species name. For example, the house fly, which belongs to the genus Musca, is scientifically known as Musca domestica. Where a number of members of the genus Musca are mentioned, later examples may be abbreviated to, for instance, M. domestica. By convention the genus name begins with a capital letter.

Related genera are grouped into families, related families are grouped into Orders, Orders are grouped into classes, and classes are grouped into phyla. The latter are the largest divisions of each kingdom. Family names in the animal kingdom always end in  …idea, while  plant families always end in  ..aceae.

Viruses

Viruses are on the borderline of living and non living things and do not fit into the five kingdoms of the living world. They are much smaller than bacteria and each consists simply of a lump of DNA inside a protein envelope.

There are many different kinds of viruses and they all produce disease in plants and animals by invading living cells and interfering with their normal functions. The viruses can be converted to a crystalline form, in which they can survive for a very long time, but they cannot grow or reproduce on their own. They need the help of the host cells for this. The virus DNA takes control of the cell nucleus and instructs it to make more virus mater. The new virus particles then escape from the host cell and rapidly infect other cells. Human diseases caused by viruses include the common cold, measles, polio and aids

Animal and Plant Cells

All living things are made of cells and part from those of the bacteria the cells all have much the same internal structure, although their external appearance varies enormously according to the jobs they have to do. A variety of animal cells are shown on the right below. There are several hundred different kinds of cells in the human body, and the total number of cells may exceed ten trillion. Most of them are obviously extremely small, and it is only recently that powerful microscopes have revealed their detailed structure. The nucleus the most conspicuous of the internal structures or organelles is the cell’s control center. It is bounded by the nuclear membrane and it contains the chromosomes that carry all the hereditary material. The nucleus produces the ribosomes, which themselves are responsible for the formation of the cell’s proteins. This takes place on the surface of the endoplasmic reticulum, which is a network of flattened and connected to the nuclear membrane and spreading throughout the cell.