DNA: Introns and Exons
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.
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