lecture 31 regulating gene expression Flashcards
why is gene expression regulation important
organisms respond to changes in their internal and external environment by monitoring their gene expression.
however gene expression is very costly in terms of metabolism
hence, animals need to monitor their gene expression carefully
to grow
to compete for food and resources
the survive adverse conditions
what does gene regulation determine
- where a gene is expressed eg rubisco gene in plants only expressed in the leaf
- for how long a gene is expressed
- to what extent is a gene expressed - ie high or low levels of expression - rubisco most abundant protein in leaves 50 percent
- response to external stimuli - eg. regulation by light rubisco gene only expressed in response to sunlight
what are expressed genes
genes that are actively being transcribed by RNA polymerase
the resulting mrna being translated to produce the encoded protein.
what is used for regulation of gene expression in prokaryotes
operons
what is an operon
a set of genes which encode another set of enzymes all of which are required in the same metabolic pathway
the genes are all clustered together on the chromsome and form a single transcription unit
transcription of the operon produces a single mRNA which codes for all of the enzymes needed in the metabolic pathway
function of the lac operon
the presence of lactose induces the production of the lactose gene
codes for all the enzymes needed to metabolise lactose
lac operon how does it work
if lactose is present the lac operon turns on , if not present the lac operon turns off
if lactose is absent, the repressor protein binds to the operator, and hence rna polymerase is unable to bind to the promotor., and is prevented from transcribing the genes downstream. The transcription unit is not transcribed and hence no lac operon is formed.
if lactose is present, it binds to the repressor , making it change its shape, hence it cannot bind to the operator and now the rna polymerase is free to bind to the promotor and transcribe downstream
regulatory genes
before the promotor on a dna sequence
either a repressor or an enhancer
the three proteins required for lactose metabolism and their functions
lactose permease - lacy gene - a membrane transporter, transports lactose into the cell.
beta-galactosidase - lac z gene - cleaves lactose to give glucose and galactose
galactoside acetyltransferase - lac a gene
inducible gene expression
the presence of lactose induces the production of the lactose gene
regulatory gene
lacl - which codes for a repressor protein
the operon contains an a regulatory gene called lacl which codes for an allosteric repressor protein
what does the control region consist of
a promoter and an operator
the repressor binds to the operator
as a result RNA polymerase cannot bind to the promoter and transcription is halted
what general principles of gene regulation are revealed by the lac operon
the existence of regulatory genes, which code for regulatory proteins which control gene expression
the existence of regulatory proteins, that can act as repressors or activators
the exitsence of regulatory sequences, promoters and operators- to which regulatory proteins bind and nifluence gene expression
how is eukaryotic DNA packaged
with histone proteins to form chromatin
why is gene expression in eukaryotes more complex than in prokaryotes
DNA is packaged within a nucleus
it is not naked, but packaged with histone proteins
the coding sections of DNA are interrupted by none coding sections called introns
