Central Dogma: DNA to RNA to Protein

• The central dogma describes the flow of information from DNA to RNA to protein.
• Genetic information in chromosomes must be read and converted into proteins in the cytosol to be useful.
• DNA-encoded information is initially transcribed into an RNA intermediary.
• RNA polymerase is responsible for the transcription of DNA.
• After transcription, the RNA molecule undergoes processing steps to become messenger RNA (mRNA).
• mRNA molecules are exported from the nucleus to the cytosol, where they are translated into proteins by ribosomes.
• Some RNA molecules do not code for proteins and are processed differently from mRNA.
• Both transcription and translation are highly regulated and use energy-dependent steps to ensure high fidelity.
• Information flows from DNA to RNA to protein in all prokaryotes and eukaryotes.
• Mechanisms are similar between prokaryotes and eukaryotes, but there are differences to address specific difficulties in eukaryotes.
• Information content decreases from DNA to mRNA to protein.

RNA Transcription

• Transcription produces a single-stranded RNA molecule complementary to the DNA template strand.
• RNA is synthesized in the 5' to 3' direction, while DNA is read in the 3' to 5' direction by RNA polymerase.
• RNA polymerase is a complex multi-subunit enzyme.
• Eukaryotes possess three different RNA polymerases, each synthesizing different types of RNAs.
• Prokaryotes have only one RNA polymerase.
• Transcription initiates at special DNA sequences called promoters.
• In prokaryotes, RNA polymerase binds strongly to the promoter sequence.
• In eukaryotes, transcription requires the binding of general transcription factors to position RNA polymerase and start the process.
• Additional proteins are needed to modify chromatin structure and fully activate transcription in eukaryotes.
• Extension of RNA chain requires elongation factors, which use ATP hydrolysis to help RNA polymerase move along chromatin
• RNA transcription stops when RNA polymerase encounters a specific DNA sequence.
• In prokaryotes, the sequence is a terminator.
• In eukaryotes, transcription ends after reaching a polyadenylation signal.
• Most eukaryotic RNA needs post-transcriptional processing to be functional.

mRNA Processing

• RNA that encodes proteins requires processing before it is considered mRNA.
• The first modification occurs immediately after the 5' end of RNA exits polymerase where 7-methylguanosine "cap" is added to the 5' end.
• Addition of the cap marks RNA as an mRNA-to-be.
• Most protein-coding genes contain introns, which are intervening sequences that interrupt the actual coding sequences (exons).
• Introns must be removed by RNA splicing, which is carried out by the spliceosome.
• The spliceosome is composed of snRNPs (small nuclear ribonucleoproteins), which include small nuclear RNAs (snRNAs) and multiple proteins.
• Splicing is directed by RNA sequences found at intron-exon boundaries.
• The spliceosome assembles on pre-mRNA while it is still being transcribed; however, splicing can be delayed.
• Splicing is extremely flexible, allowing a given transcript to have numerous possible splicing patterns.
• Once transcription is complete, the RNA 3' end receives a poly-A tail.
• First, 3' end of the original RNA is cleaved off, and then approximately 200 A's are added by a poly-A polymerase.
• Poly-A binding proteins bind to the tail, which is important for export from the nucleus and subsequent protein synthesis.

mRNA Export

• RNA synthesis and processing occur in the nucleus, but protein synthesis takes place in the cytosol.
• Only fully processed, mature mRNA can be exported from the nucleus.
• Export depends on the removal of some proteins and the addition or retention of others, such as cap-binding proteins and poly-A binding proteins.
• Mature mRNA binds to a nuclear export receptor, which guides it through the nuclear pore complex into the cytosol.

Other RNAs

• mRNA represents only about 5% of cellular RNA.
• Ribosomal RNA (rRNA) constitutes up to 80% of cellular RNA and forms the structural and catalytic core of ribosomes.
• rRNA is synthesized by RNA polymerase I (18S, 5.8S, and 26S) and RNA polymerase III (5S).
• rRNA is heavily processed and assembled with ribosomal proteins in the nucleolus.
• Other non-coding RNAs have functions in pre-mRNA splicing (snRNAs), ribosome assembly (snoRNAs), protein synthesis (tRNAs), regulation of gene expression (siRNAs and miRNAs), and telomere synthesis.