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  Section: Principles of Horticulture » Pollination and fertilization
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The genetic code

Pollination and fertilization
  Introductory principles
  The genetic code
  Cell Division
  Inheritance of characteristics
  Other breeding programmes
  The Plant Varieties and Seeds Act, 1964
  Gene Banking

All living plant cells contain a nucleus which controls every activity in the cell. Within the nucleus is the chemical deoxyribonucleic acid (DNA), a very large molecule made up of thousands of atoms (see also carbon chemistry). DNA contains hundreds of sub-units (nucleotides), each of which contains a chemically active zone called a ‘ base ’ . There are four different bases: guanine, cytosine, thymine and adenine. The sequence of these bases is the method by which genetic information is stored in the nucleus, and also the means by which information is transmitted from the nucleus to other cell organelles (this sequence is called the genetic code). A change in the base sequencing of a plant’s code will lead to it developing new characteristics. These very long molecules of DNA are called chromosomes. Each species of plant has a specific number of chromosomes. The cells of tomato (Lycopersicum esculentum) contain 24 chromosomes, the cells of Pinus and Abies species 24 and onions 16 (human beings have 46). Each chromosome contains a succession of units, called genes , containing many base units. Each gene usually is the code for a single characteristic such as flower colour or disease resistance. Scientists have
been able to correlate many gene locations with plant characteristics that they control. Microscopic observation of cells during cell division reveals two similar sets of chromosomes, e.g. in tomatoes a total of 12 similar or homologous pairs. The situation in a nucleus where there are two sets of chromosomes is termed the diploid condition. A gene for a particular characteristic, such as flower colour, has a precise location on one chromosome, and on the same location of the homologous chromosome. For each characteristic, therefore, there are at least two alleles (alternative forms of the gene), one on each chromosome in the homologous pair, which provide genetic information for that characteristic. The fact that every living plant cell which has a nucleus has a complete set of all genetic information (totipotency) means that cells have the information to become any specialized cell in the plant. Therefore, when organs are removed from their usual place, as in vegetative propagation, they are able to develop new parts, such as adventitious roots, using this information. Vegetative propagation is described in detail in Plant propagation.

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