Processing of pre-mRNA occurs in the nucleus. After transcription, three crucial modification take place. A 7-methylguanosine molecule is added to the 5' end to form a cap. Polyadenylation is added to the 3' end to create a poly-A tail. Introns are spliced out by spliceosomes, removing the non-coding regions of the RNA. The final product after modifications is considered a mature mRNA; prior to this, the transcript is known as heteronuclear RNA (htRNA).

Nucleotide excision repair is a method of proofreading after DNA replication to reduce the frequency of mutation.

Exons contain the actual genetic information coding for protein. In contrast, introns are intervening non-coding segments of DNA. During the splicing process of pre-mRNA modification, introns are removed from the sequence. Alternative splicing patterns allow multiple exon sequences to be created from a single gene, resulting in multiple proteins.

An operon is a set of genes that are adjacent to one another in the genome and are coordinately controlled. The silencer region is a site of a gene where repressor proteins bind to regulate gene expression.

snRNPs are RNA protein complexes that combine with pre-mRNA and other proteins to form a spliceosome. Spliceosomes remove introns from pre-mRNA. After final modifications, the spliced pre-mRNA is considered mature mRNA and can be exported to the cytoplasm for translation.

mRNA is an RNA molecule that conveys genetic material from DNA to the ribosome. tRNA is an RNA molecule that serves as the link between the nucleotide sequence of nucleic acids and the amino acid sequence of proteins. rRNA is the RNA component of the ribosome that is essential for protein synthesis in all living organisms.

mRNA, tRNA, and rRNA are the most commonly recognized types of RNA, though there are several more divisions. Messenger RNA (mRNA) is the product of gene transcription and is used to carry genetic information to ribosomes for translation. Transfer RNA (tRNA) is used to transport amino acid residues to active ribosomes during translation and contains anticodon sequences to bind to mRNA. Ribosomal RNA (rRNA) forms part of the ribosomes structure.

Though cRNA is not a class of RNA molecule, complementary DNA (cDNA) is used to store and analyze genomes. cDNA is the non-coding complement to the template strand used for transcription, and can be used to analyze genetic sequences.

Skipped exon, dual splice site, and alternative 3' splicing are all types of alternative RNA splicing. Exon reversal is a nonsense term, and does not represent an actual biochemical process.

Major RNAs are not a category of RNAs. Long non-coding RNAs are non-protein coding transcripts typically longer than 200 base pairs (bp) and play a role in regulating gene expression and epigenetic regulation. MicroRNAs are small RNAs (~20 bp) and play a role in RNA silencing and post-transcriptional regulation of gene expression. Short interfering RNAs are double-stranded (20-25 bp) and play a role in post-transcriptional gene silencing. Piwi-interacting RNAs are small non-coding RNAs that interact with piwi proteins in epigenetic and post-transcriptional silencing of genetic elements such as retroposons. 

Only 2-5% of the total RNA content in a cell is mRNA. Approximately 10% is transfer RNA (tRNA), and approximately 85% is ribosomal RNA (rRNA).

The poly-A tail adds to the stability of the mRNA transcript. The tail becomes shorter and shorter over time due to exonucleases, eventually signaling an enzyme to break down the mRNA to stop further translation. The 5' methylated guanosine cap facilitates binding of ribosomes, and specific splicing sequences signal removal of introns, not the poly-A tail. The tail cannot signal the end of transcription by the RNA polymerase because the poly-A tail is a post-transcriptional change, meaning it is not present during transcription. Lastly, the poly-A tail lies at the end of the 3' UTR (untranslated region), and is not included in the protein product of the mRNA.

Introns are regions included in genes that are not actually part of the final protein generated. Scientists first observed that some areas of genes are removed before mRNA translation by visualizing that not all of a gene hybridizes with its cognate mRNA, and hence there are pieces that are spliced out and not used. Note that splicing of introns, like all other post-transational modifications, only occurs in eukaryotes. The function of intron regions is thought to be mostly regulatory.

The correct answer is repressors bind splicing silencers and increase nearby splice junction activity. Activators bind splicing enhancers and decrease likelihood of proximal sites as splice junction. These enhancer sites can be in the intron or exon of the nascent RNA molecule and are most commonly bound by serine and arginine (SR) proteins. Moreover, the relative abundance of these bound RNA regulatory elements in proximity to a splice junction confers differential splicing activity.