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Chapter 11: Regulation of Gene Expression
Gene expression is tightly regulated
Gene expression may be modified to counteract environmental changes, or gene expression may change to alter function in a cell
Constitutive proteins are actively expressed all the time
Inducible genes are expressed only when their proteins are needed by the cell
Genes can be regulated at the level of transcription
Gene expression begins at the promoter where transcription is initiated
In selective gene transcription a "decision" is made about which genes to activate
Two types of regulatory proteins-- also called transcription factors-- control whether a gene is active
These proteins bind to specific DNA sequences near the promoter
Negative Regulation--a repressor protein prevents transcription
Positive regulation-- an activator protein & stimulant
Acellular viruses use gene regulation to take over host cells
A phage injects a host cell with nucleic acid that takes over synthesis
New viral particles (virions) appear rapidly and are soon released from the lysed cell.
This lytic cycle is a typical viral reproductive cycle-- in a lysogenic phase, the viral genome is incorporated into the host genome and is replicated too.
A bacteriophage may contain DNA or RNA and may not have a lysogenic phase
The lytic cycle has two stages:
Early stage-- promoter in the viral genome binds host RNA polymerase and adjacent viral genes are transcribed
Early genes shut down transcription of host genes, and stimulate viral replication and transcription of viral late genes
Host genes are shut down by a post-transcriptional mechanism
Viral nucleases digest the host's chromosome for synthesis n new viral particles
Late Stage--viral late genes are transcribed
They encode the viral capsid proteins and enzymes to lyse the host cell and release new virions.
The whole process from binding and infection to release of new particles takes about 30 minutes.
Human immunodeficiency virus (HIV) is a retrovirus with single-stranded RNA.
HIV is enclosed in a membrane from the previous host cell--it fuses with the new host cell's membrane
After infection, RNA- directed DNA synthesis is catalyzed by reverse transcriptase
Two strands of DNA are synthesized and reside in the host's chromosome as a provirus.