Transcription and Translation

• Genetic information in chromosomes is converted into proteins in the cytosol.
• DNA information is first transcribed into RNA by RNA polymerase.
• RNA undergoes processing to become messenger RNA (mRNA).
• mRNA molecules are exported from the nucleus to the cytosol and translated into proteins by ribosomes.
• Some RNA molecules do not code for proteins and are processed differently.
• Transcription and translation are highly regulated and require energy to maintain high fidelity.
• Information flows from DNA to RNA to protein in prokaryotes and eukaryotes.
• Information content goes from DNA > mRNA > protein.

RNA Transcription

• Transcription generates a single-stranded RNA molecule complementary to a DNA template strand.
• RNA is synthesized 5'->3', while DNA is read 3'->5' by RNA polymerase, a complex multi-subunit enzyme.
• Eukaryotes have 3 different RNA polymerases, while prokaryotes only have one RNA pol.
• In prokaryotes, RNA polymerase strongly binds to the promoter sequence.
• Eukaryotes needs general transcription factors which position RNA polymerase and start transcription.
• Extension of the RNA chain requires elongation factors and ATP hydrolysis.
• RNA transcription stops after RNA polymerase encounters a special DNA sequence called a terminator.
• In eukaryotes, transcription ends after reaching a polyadenylation signal.

MRNA Processing

• Processing is required for RNA's that encode proteins before they are considered mRNA.
• The first modification occurs after the 5' end of RNA exits polymerase, which adds a 7-methylguanosine "cap" to the 5' end.
• Protein-coding genes contain introns (intervening sequences) that interrupt coding sequences (exons).
• Introns must be removed via RNA splicing carried out by the spliceosome.
• Spliceosome carries out RNA splicing, using small nuclear ribonucleoproteins (snRNPs), small nuclear RNAs (snRNAs) and multiple proteins.
• The spliceosome assembles while pre-mRNA is being transcribed.
• The splicing process is flexible and a given transcript may have different splicing patterns.
• Once transcription is complete, the 3' end of the RNA receives a poly-A tail.
• The 3' end of the original RNA is cleaved off and ~200 A's are added by poly-A polymerase.
• Poly-A tail involves poly-A binding proteins and is important for export from the nucleus and later protein synthesis.
• RNA synthesis and processing occur in the nucleus, while protein synthesis occurs in the cytosol.
• Mature mRNA binds to nuclear export receptor, which guides it through the nuclear pore complex into the cytosol.

Non-Coding RNAs

• mRNA represents only ~5% of cellular RNA.
• Up to 80% of cellular RNA is ribosomal RNA(rRNA) which is the structural and catalytic core of ribosomes.
• rRNA is synthesized by RNA pol I (18S, 5.8S, and 26S) and RNA pol III (5S).
• Non-coding RNAs have functions in pre-mRNA splicing (snRNAs), ribosome assembly (snoRNAs), protein synthesis (tRNAs), regulation of gene expression (siRNAs and miRNAs), telomere synthesis, and more.

RNA Types and Functions

• Messenger RNAs (mRNAs) code for proteins.
• Ribosomal RNAs (rRNAs) form the basic structure of the ribosome and catalyze protein synthesis.
• Transfer RNAs (tRNAs) transfer RNAs, central to protein synthesis as adaptors between mRNA and amino acids
• Small nuclear RNAs (snRNAs) function in nuclear processes, including pre-mRNA splicing.
• Small nucleolar RNAs (snoRNAs) are used to process and chemically modify rRNAs.
• Small cajal RNAs (scaRNAs) modify snoRNAs and snRNAs.
• MicroRNAs (miRNAs) regulate gene expression by blocking translation of selective mRNAs.
• Small interfering RNAs (siRNAs) turn off gene expression by directing degradation of selective mRNAs.
• Other noncoding RNAs function in telomere synthesis, X-chromosome inactivation, and transport of proteins into the ER.

RNA Polymerases and Genes Transcribed

• RNA polymerase I transcribes 5.8S, 18S, and 28S rRNA genes.
• RNA polymerase II transcribes all protein-coding genes, plus snoRNA, miRNA, siRNA, and most snRNA genes.
• RNA polymerase III transcribes tRNA, 5S rRNA, some snRNA genes, and genes for other small RNAs.
• The rRNAs are named according to their “S” values referring to sedimentation rate in an ultracentrifuge.

Genetic information in chromosomes is converted into proteins. DNA information is transcribed into RNA by RNA polymerase. mRNA molecules are exported to the cytosol and translated into proteins by ribosomes. Information flows from DNA to RNA to protein.