The genetic code
Content 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. |