FUNDAMENTAL DISCIPLINES
Life is divided into many levels of organization; atoms, molecules, cells, tissues, organs, organ systems, organisms, and populations. The basic disciplines of biology may study life at one or more of these levels.

1- Taxonomy attempts to arrange organisms in natural groups based on common features. It is concerned with the identification, naming, and classification of organisms (see classification, biological). The seven major taxonomic categories, or taxa, used in classification are kingdom, phylum, class, order, family, genus, and species. Most modern systems use five: Monera (bacteria and blue-green algae), Protista (Protozoa and the other algae), fungi, plant, and animal.

2- Ecology is concerned with the interrelationships of organisms, both among themselves and between them and their environment. Studies of the energy flow through communities of organisms and the environment (the ecosystem approach) are especially valuable in assessing the effects of human activities. An ecologist must be knowledgeable in other disciplines of biology.

3- Behaviorists are concerned with how organisms respond to stimuli from other organisms and from the environment. Most of them study animals as individuals, groups, or entire species; in describing animal behavior patterns. These patterns include animal migration, courtship and mating, social organization, territoriality, instinct, and learning. When humans are included, biology overlaps with psychology and sociology. Growth and orientation responses of plants can also be studied in the discipline of behavior, although they are traditionally considered as belonging under development and physiology, respectively.

4- Embryology (both descriptive and comparative) are the classic areas of development studies, although postembryological development, particularly the aging process, is also examined. The biochemical and biophysical mechanisms that control normal development are of particular interest when they are related to birth defects, cancer, and other abnormalities.

5- Genetics studies the inheritance of physical and biochemical characteristics, and the variations that appear from generation to generation. The emphasis may be on improving domestic plants and animals through controlled breeding, or it may be on the more fundamental questions of molecular and cellular mechanisms of heredity.  (More information below)

6- Molecular Biology is a branch of biology growing in importance since the 1940s, and essentially developed out of genetics and biochemistry. It seeks to explain biological events by studying the molecules within cells, with a special emphasis on the molecular basis of genetics; nucleic acids in particular and its relationship to energy cycling and replication. Evolution, including the appearance of new species, the modification of existing species, and the characteristics of extinct ones, is based on genetic principles. Information about the structure and distribution of fossils that is provided by paleontologists is essential to understanding these changes (see paleontology).

7- Morphology (from the Greek, meaning "form study") traditionally has examined the anatomy of all organisms. The middle levels of biological organization, cells, tissues, and organsÑare the usual topics, with comparisons drawn among organisms to help establish taxonomic and evolutionary relationships.

8 Physiology studies the functions of an organism. Physiology is concerned with the life processes of entire organisms as well as those of cells, tissues, and organs. Metabolism and hormonal controls are some of the special interests of this discipline.

Genetics is an important aspect of both pure and applied biology. Viral, bacterial, plant, animal, and human genetics focus on the genes of specific organisms. Molecular genetics studies the chemical structure and function of genes. Developmental genetics focuses on the ways in which genes program the development of complex organisms, starting with a single cell. Population genetics is concerned with the genetic structure of populations, especially as a way of studying the evolutionary process.

Genetics has had a profound impact at the applied level. Breeding for specific genetic traits has extensively redesigned the plants and animals that are used in commerce. Food, clothing, and pharmaceutical products come from organisms that have been specifically bred by geneticists. The traditional practices of breeding have been extended by molecular genetic technologies that allow gene transfer between organisms, increasing the scope of genetic manipulation. Genetics is especially relevant to humans because a large proportion of human disease and ill health has a genetic basis. Hereditary disease is passed on from generation to generation because of defective genes, and genetic defects in individual cells are the root cause of cancers.

Multicellular organisms such as plants and animals are composed of large numbers of cells, each with a central nucleus that contains the genes. Genes are united into structures called chromosomes (see genetic code).  A chromosome is one very long molecule of a chemical called deoxyribonucleic (de-ox-i ri-bo new-clay-ic) acid, or DNA. In humans the DNA of one chromosome is on average 5 cm long. Since chromosomes are visible only under a powerful microscope, the DNA is densely packed. A gene is a functional region along the DNA molecule. In general the DNA of plants and animals can be divided up into two types: unique regions present only once per genome (these are the many different kinds of genes) and repetitive DNAs present in many copies per genome.  Individual variation in repetitive DNA is the basis for DNA fingerprinting, a technique now used in forensic genetics to establish individual identity.

THE NATURE OF THE GENE
DNA is the genetic material of most organisms. It is a double-stranded intertwined helix of long chains of nucleotides (abbreviated A, T, G, and C) as proposed by James D. Watson and Francis H. C. Crick in 1953. The A nucleotide in one strand always pairs with a T nucleotide in the other, and the same is true of G and C. These A-T and G-C pairs are like the steps of a spiral staircase.

Most genes of the DNA in organisms and viruses code for the structure of proteins, which are the main determinants of phenotype. Organisms consist of either body protein such as muscle and skin or substances made by catalytic proteins called enzymes, which regulate chemical reactions in cells.

Plant, animal, and fungal genes contain structures called introns, noncoding regions of unknown function that are found in the coding regions of most genes. Many human genes contain large numbers of introns. The introns are cut out at the pre-mRNA stage and do not get translated into protein.

MUTATION
Mutation is the process by which genes change from one form to another. Mutations may be caused by high energy radiation (such as X rays or ultraviolet rays) or certain chemicals (such as nitrous acid or ethyl methane sulfonate). Mutations may also occur spontaneously as a result of accidental errors in the chemistry of the cell. Because a mutation is a type of random change, most mutations result in damaged genes that are nonfunctional. Common types of damage are nucleotide substitution or deletion that alter the amino acid sequence in the protein coded by that gene. The alteration of amino acids can have a drastic effect on function, as in the case of hemoglobin in sickle-cell disease. Mutant alleles are often recessive because in a heterozygote the accompanying normal allele can take over normal protein function. However, homozygotes arise in later generations from inbreeding or from mating with other heterozygotes. Mutants usually do not live long in nature; geneticists and breeders may keep mutants alive for study or for use in producing new plant and animal forms that have characteristics desirable for research or commerce. In research each mutation identifies a gene that is important in the process under study.

Mutations can occur either in germ cells (in the testes or ovaries in humans) or in somatic (body) cells. Recessive germinal mutations may be passed on to progeny and to later descendants. Somatic mutations are not passed on to the next generation; their effect depends on the gene that is mutated. In some cases the effect is to kill the individual cell, which probably would have little consequence. More important, if the mutation occurs in a proto-oncogene, the mutation may convert the cell into a cancer cell, which can then go through uncontrolled cell division and form a tumor.

MOLECULAR GENETICS
Traditional genetics involves manipulation of genomes through controlled matings of specific genotypes. In molecular genetics the genome itself is manipulated. The DNA is physically isolated, studied, modified if necessary, and reinserted back into another organism of the same species, or even a different species. This process, called genetic engineering, uses techniques beyond the reach of traditional genetics. Organisms that have been modified by a foreign gene from another species, or a designer gene from the same species, are called transgenic organisms. In the future, transgenic plants, animals, and microbes will become increasingly important in agriculture and commerce, providing improved types of food, clothing, and pharmaceutical products.

GENES IN POPULATIONS
The goal of population genetics is to understand the genetic structure of populations. One fundamental measure of the genetic structure of a population is allele frequency. In a simple example the frequency of an allele A is represented by p and that of its allele a by q. The values of p and q must add up to 1 (100%).

In a very large freely interbreeding population these values determine the frequencies of the three possible genotypes. All other things being equal, these proportions will be constant from generation to generation; the formula is called the Hardy-Weinberg equilibrium formula.

For changes to occur, allele frequency must change. This is possible by changes in the mutation rate, selection for or against specific genotypes, migration, founder effects, or chance variation from generation to generation (genetic drift). It is the interaction of these forces that determine allele frequency and hence genotype frequency.