○ All RNA is manufactured from a DNA template in a process called RNA transcription.
○ Transcription requires a promoter; Replication requires a primer.
○ The beginning of transcription is called initiation.
○ In initiation, a group of proteins called initiation factors finds a promoter on the DNA strand, and assembles a transcription initiation complex, which includes RNA polymerase
○ Prokaryotes have 1 type of RNA polymerase; Eukaryotes have 3 types of RNA polymerase (one for each type of RNA).
○ A promoter is a sequence of DNA nucleotides that designates a beginning point for transcription, and promoter recognition is the rate limiting step in transcription.
○ The promoter in prokaryotes is located at the beginning of the gene (said to be upstream).
○ The transcription start point is part of the promoter.
○ The first base-pair located at the transcription start point is designated +1; base-pairs located before the start point, such as those in the promoter, are designated by negative numbers.
○ The most commonly found nucleotide sequence of a promoter recognized by the RNA polymerase of a given species is called the consensus sequence.
○ Variation from the consensus sequence causes RNA polymerase to bond less tightly and less often to a given promoter, which leads to those genes being transcribed less frequently.
○ After binding to the promoter, RNA polymerase unzips the DNA double helix creating a transcription bubble.
○ Next the complex switches to elongation mode.
○ In elongation, RNA polymerase transcribes only one strand of the DNA nucleotide sequence into a complementary RNA nucleotide sequence.
○ Only one strand in the molecule of double stranded DNA is transcribed.
○ This strand is called the template strand or (-) antisense strand.
○ The other strand, called the coding strand or (+) sense strand protects its partner from degradation.
○ The coding strand/sense strand resembles the universal code sequence of RNA.
Diagram Representing Regulation of Transcription in Eukaryotes
○ Just like DNA polymerase, RNA polymerase reads in the 3’ → 5’ direction and builds in the 5’ → 3’ direction, but it DOESN’T have proofreading ability.
○ The end of transcription is called termination, and requires a special termination sequence (high G-C content) and special proteins to dissociate RNA polymerase from DNA.
○ Genes are activated or deactivated at the level of transcription.
○ For all cells, most regulation of gene expression occurs at the level of transcription via proteins called activators and repressors.
○ Activators and repressors bind to DNA close to the promoter, and either activate or repress the activity of RNA polymerase.
○ Activators and repressors are often allosterically regulated by small molecules such as cAMP.
○ The primary function of gene regulation in prokaryotes is to respond to the environmental changes.
○ In contrast, lack of change or homeostasis of the intracellular and extracellular compartments is the hallmark of multicellular organisms.
○ The primary function of gene regulation in multicellular organisms is to control the intra- and extracellular environments of the body.
○ Prokaryotic mRNA typically contains several genes in a single transcript (polycistronic), whereas eukaryotic mRNA includes only one gene per transcript (monocistronic).
○ The genetic unit usually consisting of the operator, promoter, and genes that contribute to a single prokaryotic mRNA is called the operon.
○ Genes of an operon are transcribed on one mRNA.
○ Genes outside the operon may code for activators and repressors
○ An operator is a segment of DNA that a regulatory protein binds to.
○ It is classically defined in the lac operon as a segment between the promoter and the genes of the operon.
○ A repressor or activator can bind to an operato.
○ A good example of an operon is the lac operon.
○ The lac operon codes for enzymes that allow E. Coli to import and metabolize lactose when glucose is not present in sufficient quantities.
○ Low glucose levels lead to high cAMP levels.
○ cAMP binds to and activates catabolite activator protein (CAP).
○ The activated CAP protein binds to a CAP site located adjacent and upstream from the promoter to the lac operon.
○ The promoter is now activated allowing the formation of an initiation complex and subsequent transcription and translation of the 3 proteins.
Diagram Representing Regulation of Transcription in Prokaryotes
○ A second regulatory site on the lac operon, called the operator, is located adjacent and downstream to the promoter.
○ The operator provides a binding site for a lac repressor protein.
○ The lac repressor protein is inactivated by the presence of lactose in the cell.
○ The lac repressor protein will bind to the operator unless lactose binds to the lac repressor protein and inactivates it.
○ The binding of the lac repressor to the operator in the absence of lactose prevents the transcription of the lac genes.
○ Lactose, then, can induce the transcription of the lac operon only when glucose is not present.
○ Gene regulation in eukaryotes is more complicated involving the interaction of many genes.
○ Thus more room is required than is available near the promoter.
○ Enhancers are regulatory proteins commonly used by eukaryotes.
○ Their function is similar to activators and repressors, but they act at a much greater distance from the promoter.
RNA Transcription Citations