Agriculture Reference
In-Depth Information
contributed to a higher ei ciency in the
screening and selection of particular hybrids
after a breeding process. Restriction frag-
ment length polymorphism (RFLP) was the
i rst molecular marker, developed in early
1980. Another breakthrough was the
emergence of polymerase chain reaction
(PCR) in 1990. With this technology, a
new generation of DNA markers was intro-
duced into modern plant-breeding systems.
Examples are randomly amplii ed poly-
morphic DNA (RAPDs), sequence char-
acterized amplii ed regions (SCARs),
sequence tagged sites (STS), single poly-
morphic amplii cation test (SPLAT), variable
number of tandem repeats (VNTRs),
amplii ed fragment length polymorphism
(AFLP), DNA amplii cation i ngerprinting
(DAF), single-strand conformational poly-
morphism (SSCP), single-nucleotide poly-
morphism (SNP), microsatellites or short
tandem repeats (STRs), cDNA, DNA micro-
arrays and rDNA-internal transcribed
spacers (ITS).
Molecular markers are DNA-based
markers and they make it possible to select
not for the trait but for the presence of the
genetic information coding for the trait of
interest. h is is done by searching for DNA
markers that have been proven to be
genetically linked to the genes coding for the
trait. It also allows the whole breeding
process to be speeded up. After crossing a
particular variety containing an elite genetic
background with a plant containing the
desired trait in an unfavourable genetic
background, the use of molecular markers
allows identii cation within the population
of of spring plants, with maximal con-
servation of the elite genotype but
additionally with the new trait of interest.
Besides gaining time, this approach also
makes it possible to carry out selection on
small plants, even without the phenotypic
expression of the genotype and the trait of
interest. DNA markers are also very
important for the fast introduction of a
particular transgene-encoded trait in a
variety of existing cultivars through con-
ventional breeding, allowing valorization of
GMO elite events.
Its Importance to Plant Breeding
h is section describes the dif erent steps of
developing a transgenic crop. h e description
will be general and rather theoretical.
Depending on the crop and the gene of
interest to be introduced, the procedure is
slightly dif erent. However, in general terms,
it can be stated that the whole process of
developing a commercial GMO variety
consists of the following steps: isolation and
functional analysis of genes encoding a
particular trait; the assembly of a functional
gene construct; transfer and integration of
the gene construct into the genome; identii -
cation of elite events and breeding with the
elite events; and preparation of the
authorization dossier, needed for com-
mercialization.
functional analysis of genes
Before discussing the process of gene
isolation and characterization, it is relevant
to describe what is meant by the term 'gene'.
A gene is a molecular unit of heredity of a
living organism. It is a name given to a
stretch of DNA that codes for a polypeptide
or for an RNA chain that has a function in
the organism. Living beings depend on genes,
as they specify all proteins and functional
RNA chains. Genes hold the information to
build and maintain an organism's cells and
pass genetic traits to its of spring. All
organisms have many genes corresponding
to various biological traits, some of which are
immediately visible and some of which are
not. However, over the last decades, this
simple dei nition of a stretch of linked
nucleic acids that code for a protein has been
broadened and i ne-tuned. New terms have
been introduced, such as gene families,
iso-proteins, as a result of alternative
splicing, and ribozymes, which are RNA
molecules that catalyse particular reactions.
In isolating a gene, the researcher makes
use of the following characteristics of a gene
to start his research: a gene has a dei ned
