SPLICING OVERVIEW
Rna is very reactive for the presence of ribose instead of deoxyribose. The OH of ribose reacts with
the phosphate and this is a key for understanding the process of splicing.
The passage from nucleus to cytoplasm is very controlled. The splicing or maturation is the process
through which the introns are removed, and the exons are kept. This occurs in the nucleus and it is
extremely quick and starts during transcription. In bacteria and phages the coding sequence is
continuous, and the genome is not organized in introns and exons, in this sense all the sequence of
amino acids are collinear to the codons of mrna. In eukaryotes the situations is different cause we
have exons and introns. The reason why is supposed to be the fact that the bacteria are unicellular,
and the presence of introns slow down all cellular processes also the absence of introns favours the
quick adaptability of prokaryotes. Presence of introns is responsible for the biodiversity of
multicellular organism and are responsible for the genetic exchange recombination and
accumulation of non-dangerous errors (buffer).
Introns are constituted by non-coding rna and they need to be removed, this removal is important
because it brings together all the coding sequences. The genome of eukaryotic organism is an
alternation of exons and introns.
The introns vary enormously in number between genes and species. Usually, the introns is
correlated with the complexity of the organism, single cell organisms tend to have less introns,
humans can afford an enormous number of introns (ex: dihydrofolate reductase enzyme).
Alternative splicing is a method through which the same mrna can be spliced in more ways giving
different proteins.
Trans splicing exons of different massagers are joined and the intron will not have a lariat shape
CHEMISTRY OF SPLICING:
Enzymes and factor involved, splicing occurs thanks to specific nucleotide sequences present
between introns and exons called either 3’ or 5’ splice sites, another important factor is an adenine
within the intron branch and next to the adenine we have a polypyrimidine tract (py tract).
The intron is removed in a form called Lariat as the exons are joined. The process needs two
successive transesterification reaction to reform the phosphodiester bond
Step1: the adenine in the branch point gets activated and it present an OH in position 2’ of the
ribose (the one that reacts with the phosphate) reacts with the phosphoryl group of the G in the 5’
splice site, as a result the 5’ exon is released, and the 5’-end of the introns forms a 3
Step2: The G-OH at the 5’ splice sites attack the P at 3’ splice sites and the introns is released as a
lariat.
2 phosphodiester bonds are broken and 3 are formed so the delta G is 0 but I don’t want the reverse
reaction to occur, so the intron is completely destroyed. The energy is given by the structure of rna
itself
The spliceosome is the machinery in charge for the elimination, is composed by 150 proteins and
several SNURPs (from U1 to U5 100-300 nt) the small rnas carry several processes, when fused with
proteins they form snurps. The spliceosome is the larger snurps we know and different components
of the spliceosome are added or removed (very dynamic structure)
