Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 76

Exon
1

Exon

Exon Exon

2 3 4

Gene

Tr anscription

1 2 3 4

mRNA

Spliceosome

Alternative
splicing

123 12 4

RNA Isoforms

Figure 2.9 Demonstration of AS: In DNA, the genetic information that includes the code
for making a protein is located in fragments (exons, shown by green boxes), which are
interrupted by noncoding fragments (introns, shown by blue boxes). By the process of AS,
the introns are removed and the exons spliced together in different combinations, generating
different mRNAs that are decoded (translated) into distinct proteins.

Source : From Kashyap and Tripathi (2008).

2012 ). However, many AS variants in plants may be nonfunctional ( McGuire et al.,
2008; Tress et al., 2007 ). It is thought that “the role of AS as a mechanism for expan-
sion of function proteome diversity in plants is very limited” ( Severing et al., 2009 ).
AS is a function of an epigenetic mechanism for the combinatorial manipulation
of linear DNA information to increase diversity of the proteome. AS combines mul-
timeric blocks of pre-mRNA ( exons ) to construct new proteins that otherwise would
be impossible to build genetically. A crude estimation of the number of proteins gen-
erated by AS in millions of animal and plant species suggests that it may exceed by
far the number of proteins that evolved via genetic mutations during almost 4 billion
years of the existence of life on Earth.
Both common sense and intuition tell us that AS and AS-derived proteins evolved
via an intrinsic information-generating mechanism, which in all likelihood may be
identical to the mechanism that performs AS in extant organisms. Hence, the study
of the AS mechanism is important not only for our understanding of cell functions
but it is also of paramount evolutionary theoretical importance. AS implies selective
excision of introns and ligation of exons of pre-mRNA transcripts in specific ways
( Figure 2.9 ).

Splicing Machinery
AS is the function of spliceosomes, which are nuclear organelles, that remove non-
coding introns from pre-mRNA and recombine exons to produce protein isoforms.
Spliceosomes are dynamic macromolecular organelles composed of five types of
small nuclear ribonucleoproteins (snRNPs) as well as a number of other proteins