Genetics

Genetics is the science that studies heredity (or inheritance). It began in the early 20th century when the works that the Austrian monk Gregor Mendel (1822-1884) made in the preceding century were rediscovered. Geneticists have revealed that the fundamental characteristics of all living organisms are determined by biological entities called 'genes.' Heredity is accomplished by the passage of genes from one generation to the next. Genetics, therefore, consists in the study of everything related to genes.

Genes are contained in the cells in a type of structure called chromosome. Chromosomes are mainly located at the nucleus of the cell and organized in pairs, although there are also some ones in the cytoplasm. Chromosomes are made of proteins and DNA (deoxyribonucleic acid). DNA is a chemical substance with a very long molecule in the form of a double helix. Each gene is a segment of this long molecule. There are several chromosomes in the nucleus of the cell, but its number is constant for a given species. For example, humans have 46 chromosomes--23 pairs--and the fruit fly, Drosophila melanogaster, has four pairs. The set composed by all genes of all chromosomes of a cell is called 'genome.'

The DNA molecule of a human chromosome is about five centimeters long, thus allowing the existence of many genes. According to the Human Genome Project, the number of genes in a human cell exceeds fifty thousand. Yet, chromosomes can accommodate within the cell nucleus because the DNA molecule that composes each chromosome is highly convoluted. Not all of the DNA molecule is composed by genes.

The cells in an organism can be divided into body cells and sex cells. All body cells have the same genetic information, that is, the same genes. Sex cells, instead, have half the number of genes. This is due to the different processes by which the cells are produced starting from the primary cell, the zygote. Body (or somatic) cells are produced by the replication process call 'mitosis,' and sex cells by the one called 'meiosis.'

Mitosis is the process that provides for growth and cell replacement in multicellular organisms. It is also the reproductive process in the majority of plants. During mitosis, each chromosome duplicates and each copy travels to a different end of the cell. Two nuclei are formed and the cell is divided in two. In this way the gene content of the original cell is equal to that of the daughter cells.

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