Final answer:
Exons are coding sequences in DNA, while introns are non-coding sequences that are removed during RNA processing. Splicing, carried out by spliceosomes, accurately removes introns and joins exons, which is crucial for creating functional mRNA. RNA may also undergo further modifications including 5' capping, poly-A tail addition, and editing to produce different protein variants.
Step-by-step explanation:
Exons and Introns
Exons and introns are components of eukaryotic genes. Exons are sequences in the DNA that code for proteins and are expressed, while introns are non-coding sequences that intervene between exons and do not code for proteins. During the process of gene expression, a large precursor messenger RNA (pre-mRNA) is first synthesized from the DNA template which includes both exons and introns.
RNA Processing and Splicing
The editing of pre-RNA into messenger RNA (mRNA) involves several steps. This process includes the precise removal of introns by splicing. The splicing is carried out by protein complexes called spliceosomes, which are composed of both proteins and small nuclear RNAs (snRNAs). Spliceosomes recognize specific sequences at the 5' and 3' ends of the intron, ensuring that the introns are accurately and precisely removed, and that the exons are reconnected without altering the reading frame. Incorrect splicing can result in a dysfunctional protein.
After splicing, additional modifications may occur such as the addition of a 5' cap and a poly-A tail to the mRNA, which are essential for proper translation and stability of the mRNA. In some cases, there's a process called RNA editing which alters the nucleotide sequence of the mRNA, leading to the production of different protein variants. An example of this is the human APOB protein, which can exist in two forms due to RNA editing that introduces a premature stop signal, resulting in a smaller, distinct version of the protein.