CONTROL OF GENE EXPRESSION Flashcards
What are transcription factors
they control the transcription of target genes
in eukaryotic cells how do transcription factors work
Move from the cytoplasm to nucleus
In nucleus they bind to specific DNA sites, near the start of the target genes
They control expression by controlling the rate of transcription
What are activators
They stimulate the rate of transcription
Eg they help RNA polymerase bind to the start of the target gene and activate transcription
What are repressors
They inhibit the rate of transcription
Eg they bond to the start of the target gene, preventing RNA polymerase from binding which stops transcription
What does oestrogen do to target genes
oestrogen can initiate the transcription of target genes
what is oestrogen
a steroid hormone
how does oestrogen initiate the transcription of target genes
It is a steroid hormone that bonds to a transcription factor called an oestrogen receptor
This forms an oestrogen- oestrogen receptor complex
The complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene
The complex can act as an activator of transcription (helping RNA polymerase bind to the start of the target gene)
what is RNAi and what does it do
RNA interference
Small double stranded RNA molecules stop mRNA from target genes being translated into proteins
What are the 2 types RNAi
siRNA (small interfering RNA)
miRNA (microRNA)
How does RNAi work in plants
Once mRNA has been transcribed it leaves the nucleus for the cytoplasm
In the cytoplasm double stranded siRNA associates with several proteins and unwinds
A single strand then binds to the target mRNA
The base sequence of the siRNA is complimentary to the base sequence in sections of the target mRNA
The proteins associated with the siRNA cut the mRNA into fragments- so it can no longer be translated
the fragments then move into a processing body which contains tools to degrade them
How does RNAi work in mammals
the miRNA isn’t fully complimentary to the target mRNA
Its makes it less specific than siRNA, so it may target more than one mRNA molecule
Like siRNA it associates with proteins and binds to target mRNA in the cytoplasm
the miRNA-protein complex physically blocks the translation of the target mRNA
mRNA moved into a processing body where it can either be stored or degraded
If stored it can be returned and translated at another time
describe the events of the lac operon either stopping or enhancing transcription
E.coli is a bacteria that respires using glucose but if that is not available it can use lactose
If lactose is present E.coli makes an enzyme (galactosidase) to digest it
If there’s no lactose it doesn’t make the enzyme
Production of enzyme controlled by transcription factor (lac operon)
When there’s no lactose, the lac operon binds to the DNA at the start of gene and stops transcription
When lactose is present it binds to the lac repressor, stopping it binding to the DNA, so the gene is transcribed
how does epigenetic control determine whether or not a gene is expressed
It works through the attachment or removal of chemical groups (epigenetic marks) to or from DNA histone proteins
These epigenetic markers don’t alter the base sequence of DNA
Instead they alter how easy it is for the enzymes and other proteins needed for transcription to interact with and transcribe the DNA
how do epigenetic changes get inherited by offspring
Organisms inherit base DNA sequences from their parents
Most epigenetic marks on the DNA are removed between generations but some escape the removal process and are passed onto the offspring
This means the expression of some genes in the offspring can be affected by environmental changes that affected their parents or grandparents
How does increased methylation switch off a gene
When a methyl group is attached to the DNA coding for a gene
The group always attaches at the CpG site, which is where a cytosine and guanine bases are always next to each other in the DNA
Increased methylation changes the DNA structure so that the transcriptional machinery can’t interact with the gene- so the gene is not expressed/ switched off
How does decreased aceytlation of histones switch off a gene
When histones are acetylated the chromatin is less condensed
This means that the transcriptional machinery can access the DNA allowing genes to be transcribed
When acetyl groups are removed from the histones the chromatin becomes highly condensed and genes in the DNA can’t be transcribed because the transcriptional machinery can’t physically access them
Histone deacytlase enzymes (HDAC) are responsible for removing the acetyl groups
