The conversion of DNA to a function mobile form of biological information (messenger RNA) requires multiple steps of sequential molecular interactions and modification. DNA is used as a templet for synthesizing a single strand of RNA. There are similarities to DNA replication and numerous differences. We will focus on the most complicated form of this conversion process, that found in eukaryotic systems. Keep is mind this is a very ancient process that started billions of years ago.
The first phase of this conversion in eukaryotes, is in the synthesis of a pre-mRNA, sometimes called heterogenous nuclear RNA. This has no biological activity and needs to be modified extensively to produce a biologically active molecular capable to effectively transporting the information in a usable form to the cytoplasm, for use in translation.
The conversion of pre-mRNA to mature mRNA requires three modifications to the molecule.
1.) A 5 prime cap structure is added, this is a docking signal structure for binding of the small subunit of the ribosome. No 5' cap= No translation.
2.) A 3 prime poly adenylate tail is added, this structure also plays a role in ribosome binding but he major role this structure plays is in mRNA stability.
3.) Creation of a continuous reading frame of code to provide a functional nucleotide template for a polypeptide sequence (complete "bioinformatic" sentence). This requires splicing of the relevant exons together.
These are really complicated processes but we will focus on a few key molecular components and look at how they operate to carrying the specific function they are responsible for in support of the larger process.
How much do you know of the the entire process? What are all of the jobs that need to be done that all together results in a functional mRNA ready for translation?
I'm fairly familiar with the processes involved in going from DNA to a functional mRNA (genetics and mol gen helped out there), so from transcription in the nucleus, to capping with a modified guanine to prevent degradation in the cytosol and aid in transport, mRNA are transported through a nuclear pore to the cytosol. Before translation, the immature mRNA has its introns spliced out so that only the protein-coding exons remain (mature mRNA). I know the cap must also be removed before translation at the rER, but my memory is a little fuzzy on the details...
ReplyDeleteI also don't remember all the precise molecular steps of this process, but it's worthy to note that this where alternative splicing can occur, giving rise to multiple gene products from one gene. Alternative splicing involves the removal of a different "combinations" of introns, ultimately leading to different mRNAs.
DeleteThe cap structure remains part of the mRNA so that ribosomes can recognize the 5" end and bind to the RNA. The PolyA tail plays a role in cytosolic stability and shortens over time until it is gone, then the mRNA is very rapidly degraded. Thus, the length of the PolyA tail is a major determinant of RNA half-life.
DeleteSome mRNAs will actually be edited before translation. An example of this is apolipoprotein B mRNA which will be truncated due to the insertion of an earlier stop codon. This stop codon will result in the production of a shorter protein after translation. This isn't a defect, but rather a functional necessity for this particular type of protein depending on where it is located in the body.
ReplyDeletePolyadenylation is also critical in the RNA processing phase. The added adenine ribonucleotides lend enhanced stability to the RNA and also helps to regulate its transport.
ReplyDelete