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> > >Unit 4: > > >Post Transcriptional Modifications, Processing of Eukaryotic RNA and
Gene Regulation > > >
Exon shuffling | RNA editing | And there different methods |
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> > >Unit 4: > > >Post Transcriptional Modifications, Processing of Eukaryotic RNA and
Gene Regulation > > >
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>Exon shuffling >
>Exon shuffling is a mechanism of gene evolution that rearranges exons
within a gene or fuses exons from different genes to create new genes with
new functions. >
>Exons are the coding regions of a gene that are assembled to form mRNA and
then translated into protein. Exon shuffling occurs when different exons are
recombined in different ways to create new proteins with new structures and
functions. >
>Exon shuffling can occur through multiple mechanisms, including
intron-mediated recombination, retrotransposition and trans-splicing.
Intron-mediated recombination involves exon recombination within introns,
the noncoding regions of DNA located between exons. In retrotransposons,
mobile genetic elements that can copy themselves and insert themselves into
new sites in the genome, retrotransposons can insert themselves into
introns, bringing exon sequences with them. In trans-splicing, exons of
different genes are spliced together to form a chimeric mRNA. >
>Exon shuffling is thought to be a key driver of protein evolution, enabling
the rapid generation of new protein functions and the adaptation of
organisms to changing environments. It is thought to have played an
important role in the evolution of complex biological structures and
processes such as the immune system and brain. >
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>Different types of exon shuffling >
>There are several types of exon shuffling that can occur during genetic
evolution. Here are a few examples: >
>Exon shuffling within a gene: In this type of exon shuffling, exons
within a single gene are rearranged to form new combinations. This can
occur through intron-mediated recombination, in which exons are spliced
together in a new order, or through exon skipping or inclusion, in which
certain exons are removed or retained in the final mRNA transcript. >
>Domain shuffling: In domain shuffling, exons encoding functional domains,
which are regions of a protein that have a distinct function, are
recombined to create new protein structures. This can occur through exon
shuffling within a gene or through the fusion of exons from different
genes. >
>Gene fusion: In gene fusion, exons from different genes are fused
together to create a new hybrid gene. This can occur through chromosomal
rearrangements or retrotransposition of mobile genetic elements. >
>Alternative splicing: Alternative splicing can also be considered a type
of exon shuffling, as it involves the recombination of exons in different
ways to create different protein isoforms. >
>These different types of exon shuffling can result in the rapid generation
of new protein functions and the evolution of complex biological structures
and processes. However, they can also lead to genetic disorders if they
disrupt the normal function of essential genes. >
RNA editing
>RNA editing is a process by which the nucleotide sequence of RNA is altered
after transcription, leading to changes in the protein sequence that it
encodes. This process can occur in both coding and non-coding regions of
RNA, and it can be either site-specific or non-specific. >
>There are several types of RNA editing, including: >
>Substitution editing: In substitution editing, a specific nucleotide is
changed to a different nucleotide. This can lead to a change in the amino
acid sequence of the protein that the RNA encodes. >
>Insertion/deletion editing: In insertion/deletion editing, nucleotides are
added to or removed from the RNA sequence, resulting in a frameshift
mutation that alters the protein sequence. >
>RNA splicing editing: In RNA splicing editing, the splicing of pre-mRNA is
altered, leading to the inclusion or exclusion of specific exons in the
final mRNA transcript. >
>RNA editing by base modification: In RNA editing by base modification, the
nucleotide bases in RNA are modified chemically. For example, adenosine can
be converted to inosine by the action of adenosine deaminase enzymes, which
changes the base-pairing properties of the RNA molecule. >
>RNA editing can occur naturally as a part of gene expression, or it can be
induced by external factors such as stress or disease. RNA editing is known
to play a role in various biological processes, including the regulation of
gene expression, the immune response, and the maintenance of neuronal
function in the brain. >
>Adenosine-to-inosine (A-to-I) editing: This is the most common form of RNA
editing in humans. A-to-I editing is catalyzed by the adenosine deaminase
acting on RNA (ADAR) enzyme, which converts adenosine to inosine. Inosine is
recognized as guanosine by the translation machinery, so this type of
editing can change the coding sequence of mRNA and alter the amino acid
sequence of the encoded protein. >
>Cytosine-to-uracil (C-to-U) editing: C-to-U editing is catalyzed by enzymes
known as apolipoprotein B mRNA editing enzymes (APOBECs). This type of
editing can occur in both coding and non-coding regions of RNA, and can
result in changes in protein function or gene regulation. >
>Adenosine-to-cytosine (A-to-C) editing: A-to-C editing occurs in
trypanosomes, a type of protozoan parasite, and involves the conversion of
adenosine to inosine, followed by deamination to form a cytosine base. >
>Uracil-to-cytosine (U-to-C) editing: U-to-C editing occurs in kinetoplastid
protozoa, such as Trypanosoma brucei and Leishmania major. In these
organisms, most of the mRNAs undergo U-to-C editing, which involves the
conversion of uracil to cytosine by a multiprotein editing complex. >
>Non-coding RNA editing: RNA editing can also occur in non-coding RNAs, such
as transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs), where it can affect RNA
stability, folding, and function. >
>These different types of RNA editing can have profound effects on gene
expression and protein function, and they play important roles in various
biological processes, including development, differentiation, and
disease. >
