Introns and Exons


March 31 1997
AP. Biology essay

The finding of the Introns and the exons was one of the most significant
discoveries in genetics in the past fifteen years. split genes were discovered
when lack of relation between DNA sequences were seen during. DNA- mRNA
hybridation. For all new mRNA, they must be transcribed by RNA polymerase
enzymes. The transcription begins at the promoter sequence on the DNA and works
down, thus the nucleotide sequence of the mRNA is complimentary to the one of
DNA. In eukaryotes the mRNA is processed in the nucleus before transport to the
cytoplasm for translation. In order for the mRNA to become true functioning
RNA it must under go several stages of modification.
At first, when the mRNA is produced, a cap is added enzymaticully to the
5 end of the RNA by linking a 7-methylguanosine residue by a triphosphate bond
this is called the G-cap. The G-cap is necessary for translation. The subunit
of the ribosome recognizes the G-cap and then finds the initiation codon to
start translation. As the mRNA comes finishes transcription, the Poly A tail is
added to the 3 end. As the two ends are placed the mRNA becomes pre-mRNA.
The pre-mRNA consists of splicing and non-coding regions. pre-mRNA
molecules are much longer than the mRNA molecule needed to code for its protein.
The regions that do not code for amino acids; aa, are scattered all along the
coding region. The genes are split with coding regions, called exons, short for
expressed regions; in between the exons the non-coding region called introns
exist. Before the translation of mRNA the introns must be spliced off.
Splicing is an complicated process for the cell. It must locate every intron in
the primary transcript. An average mRNA consists of eight to ten introns, some
even contain sixteen introns. exons, like introns are also spread apart. Some
of their codons may be split by introns, so information for a single amino acid
could be some distance apart. Splicing takes place in the nucleus but also
could take place in the cytoplasm and the mitochondria. After the splicing of
the introns, the G-caps and the Poly A tails remain on the mRNA.
A single gene can code for multiple proteins by alternative splicing. A
single strand was found to be coding for twenty different proteins, depending on
how the exons are assembled. Different splicing combinations are regulated in t
issue specific manner.
Most of the transcribed DNA are introns. ninety nine percent of the
information contained in the gene transcript is destroyed when the introns are
eliminated since exons are only translated. Most genes have introns. Only a
hand full of organisms are found without introns. Larger eukaryotes tend to
have bigger and more numerous amounts of introns compared to smaller eukaryotes.
There are sequence of nucleic acids at the exon-intron junction of mRNA
allowing intron splicing., From what is known there is an GU at the 5 splice
site and AG at the 3 splice site for most genes. This is called the GU-AG rule
Splicing enzymes recognize these sites with the help of ribonucleprotein called
snRNP or snurps. snurps are formed by small nuclear RNA fragments of less than
three hundred nucleotides called snRNAs. As an RNA molecule is being
transcribed, four snurps attach to it combining into a large spliceosome. The
abundant snRNAs catalyze the cutting and the splicing of the gene.
In a self splitting intron, the hair pin structure brings the ends o the
introns near to the branch point. Then the introns it self catalyze the making
of the loop joining the two exons. The difference between self splicing intron
and one which require the spliceosome is that the non-self splicing introns can
split any introns, almost any size. This helps the organism to survive
mutations. When a mutation forms, some times the self splicing introns lose it
s hairpin structure not allowing it self to be spliced off.

Category: Science