Principles of Genetics and Breeding

1.2.3.6 Genome

• The genome is a store of biological information but on its own it is unable to release that information to the cell. Utilization of the biological information contained in the genome requires the coordinated activity of enzymes and other proteins, which participate in a complex series of biochemical reactions referred to as genome expression.
• The initial product of genome expression is the transcriptome, a collection of RNA molecules derived from those protein-coding genes whose biological information is required by the cell at a particular time. The transcriptome is maintained by the process called transcription, in which individuals genes are copied into RNA molecules.
• The second product of genome expression is the proteome, the cell’s repertoire of proteins, which specifies the nature of the biochemical reactions that the cell is able to carry out. The proteins that make up the proteome are synthesized by translation of the individual RNA molecules present in the transcriptome.

Genome

↓ Transcription

Transcriptome

(RNA copies of the active protein-coding genes)

↓ Translation

Proteome

(The cell’s repertoire of proteins)

Fig.1. The genome, transcriptome and proteome

• Functional sequences only represent a small fraction of the total genome, for example around 3% in humans. The rest is made up of what has been called ‘ junk DNA’ whether all of it is really ‘junk’ is not known, but it is possible that much of it will have some function in the organism.
• Some of this junk DNA consists of Psuedogenes, genes that for some reason or another have become non - functional. Other non - coding DNA consist of dispersed or clustered repeated sequences of varying length, from one base pair (bp) to thousands of bases (kilobases, kb) in length.
• The dispersed repeated sequences occur as copies spread across the genome and can be categorized as long or short interspersed nuclear elements (LINE or SINE), long terminal repeats (LTR) and DNA transposons.
• The clustered repeated sequences, where the repeated sequences occurs in tandem copies, are called as satellites, minisatellites or microsatellites depending on the length of the repeat unit, and these turned out to be useful genetic markers. These repeated elements can constitute up to 40% of the genome.