L3: Operons, Bacteria Transcription And Regulation Flashcards Preview

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Flashcards in L3: Operons, Bacteria Transcription And Regulation Deck (19):

What is meant by the “one gene-one enzyme” hypothesis (Beadle and Tatum)?

Each gene controls the production, specified and function of a particular enzyme


What is a genome?

The whole of an organisms dna including non coding dna
Set of genes


How many genes do bacteria and animals have?



Eukaryotic genes are transcribed individually but what are operons?

Clusters of bacterial genes with related functions and whose products are needed under the same conditions
Genes in operons are transcribed from one promoter to give a polycistronic mRNA


What problem is there for bacterial translation initiation and how is it overcome?

Methionine found in middle of proteins as well as beginning
In operon, translation must start at more than one place on mRNA molecule
So cell must be able to distinguish start codons from other methionine codons
In eukaryotes the ribosome binds to mRNA and moves to first AUG in mRNA
In bacteria the ribosome recognises the shine dalgarno sequence in ribosome binding site next to start codon in mRNA so 3’ end of rRNA binds to it


What is a promoter

Specific DNA sequence that provides binding site for RNA polymerase and specifies the start point and direction of transcription


What is the transcription terminator?

Specific DNA sequence that species where transcription ends


What are consensus sequences?

Conserved regions identifying most promoters in bacteria
-35 and -10 (35 and 10 bases before the start of transcription)


What is core enzyme made up of,

5 polypeptides:



How is bacterial RNA polymerase formed?

Core enzyme + σ = holoenzyme
Catalysed transcription as it binds specifically to promoters


How does bacterial transcription termination take place?

End of mRNA folds into stem loop structure followed by succession of uracils.. this destabilises DNA-RNA interactions and RNA synthase dissociates from DNA causing transcription to terminate


What is the lac operon?

An example of bacterial gene expression regulation
It’s function is to allow E.coli to grow on lactose so it can metabolise it
Uses lactose permease enzyme to transport lactose into cell and beta-galactosidase to hydrolyse lactose to glucose and galactose


Which 3 genes are transcribed together in the lac operon and what do they code for?

LacZ (z) codes for beta-galactosidase
LacY (y) codes for lactose permease
LacA (a) codes for beta-galactoside transacetylase which’s biological function is unknown


When is lac operon transcribed?

When glucose is absent and lactose present as it is only transcribed when it’s gene products are needed


What regulates transcription in response to lactose?

Regulatory protein decreases transcription when lactose is absent (repressor)


How does the repressor (regulatory protein) work?

By binding to operator (specific DNA sequence) in operon


What causes repressor to stop?

Signal molecule signals presence of lactose by binding to repressor causing it to dissociate from operator


What is the difference between wild type (+) and mutant (-) alleles?

Plus is active, negative mutants are inactive e.g Z+ codes for active beta-galactosidase whereas Z- doesn’t


How can Z+ be detected?

By screening for beta-galactosidase activity (on agar converts substrate from clear to blue)