RNA Transcription, Diagram, Definition, Process, Steps
RNA Transcription
â—‹ All RNA is manufactured from a DNA template in a process called RNA transcription.
â—‹ Transcription requires a promoter; Replication requires a primer.
â—‹ The beginning of transcription is called initiation.
â—‹ 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
â—‹ Prokaryotes have 1 type of RNA polymerase; Eukaryotes have 3 types of RNA polymerase (one for each type of RNA).
â—‹ 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.
â—‹ The promoter in prokaryotes is located at the beginning of the gene (said to be upstream).
â—‹ The transcription start point is part of the promoter.
â—‹ 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.
â—‹ The most commonly found nucleotide sequence of a promoter recognized by the RNA polymerase of a given species is called the consensus sequence.
â—‹ 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.
â—‹ After binding to the promoter, RNA polymerase unzips the DNA double helix creating a transcription bubble.
â—‹ Next the complex switches to elongation mode.
â—‹ In elongation, RNA polymerase transcribes only one strand of the DNA nucleotide sequence into a complementary RNA nucleotide sequence.
â—‹ Only one strand in the molecule of double stranded DNA is transcribed.
â—‹ This strand is called the template strand or (-) antisense strand.
â—‹ The other strand, called the coding strand or (+) sense strand protects its partner from degradation.
â—‹ 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.
â—‹ 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.
â—‹ Genes are activated or deactivated at the level of transcription.
â—‹ For all cells, most regulation of gene expression occurs at the level of transcription via proteins called activators and repressors.
â—‹ Activators and repressors bind to DNA close to the promoter, and either activate or repress the activity of RNA polymerase.
â—‹ Activators and repressors are often allosterically regulated by small molecules such as cAMP.
â—‹ The primary function of gene regulation in prokaryotes is to respond to the environmental changes.
â—‹ In contrast, lack of change or homeostasis of the intracellular and extracellular compartments is the hallmark of multicellular organisms.
â—‹ The primary function of gene regulation in multicellular organisms is to control the intra- and extracellular environments of the body.
â—‹ Prokaryotic mRNA typically contains several genes in a single transcript (polycistronic), whereas eukaryotic mRNA includes only one gene per transcript (monocistronic).
â—‹ The genetic unit usually consisting of the operator, promoter, and genes that contribute to a single prokaryotic mRNA is called the operon.
â—‹ Genes of an operon are transcribed on one mRNA.
â—‹ Genes outside the operon may code for activators and repressors
â—‹ An operator is a segment of DNA that a regulatory protein binds to.
â—‹ It is classically defined in the lac operon as a segment between the promoter and the genes of the operon.
â—‹ A repressor or activator can bind to an operato.
â—‹ A good example of an operon is the lac operon.
â—‹ The lac operon codes for enzymes that allow E. Coli to import and metabolize lactose when glucose is not present in sufficient quantities.
â—‹ Low glucose levels lead to high cAMP levels.
â—‹ cAMP binds to and activates catabolite activator protein (CAP).
â—‹ The activated CAP protein binds to a CAP site located adjacent and upstream from the promoter to the lac operon.
â—‹ 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
â—‹ A second regulatory site on the lac operon, called the operator, is located adjacent and downstream to the promoter.
â—‹ The operator provides a binding site for a lac repressor protein.
â—‹ The lac repressor protein is inactivated by the presence of lactose in the cell.
â—‹ The lac repressor protein will bind to the operator unless lactose binds to the lac repressor protein and inactivates it.
â—‹ The binding of the lac repressor to the operator in the absence of lactose prevents the transcription of the lac genes.
â—‹ Lactose, then, can induce the transcription of the lac operon only when glucose is not present.
â—‹ Gene regulation in eukaryotes is more complicated involving the interaction of many genes.
â—‹ Thus more room is required than is available near the promoter.
â—‹ Enhancers are regulatory proteins commonly used by eukaryotes.
â—‹ Their function is similar to activators and repressors, but they act at a much greater distance from the promoter.
RNA Transcription Citations
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