Control of gene expression: DNA Flashcards
How are differences between cells brought about when they all contain the same genes?
Differential gene expression - only a fraction of the genes in the cell are expressed
What is the most important change in a cell in a diseased state?
Expresses different set of genes
What causes cells to change gene expression?
In response to signals and other cues in the environment
What regulates the level of expression of a gene in the cell?
The level of the transcript, regulated by transcription factors
(Lots of RNA –> lots of proteins)
What charge do DNA binding proteins have?
Positive (as DNA is negatively charged)
Where do DNA biding proteins reach into on DNA?
The major groove
What are the 3 positively charge amino acids which help DNA binding proteins to be attracted to DNA?
- Arginine (Arg)
- Leucine (Leu)
- Histadine (His)
Where do GENERAL transcription factors bind to DNA?
Response element - short stretch of DNA within a promoter
Cis
Out of DNA and proteins, which has a limited topology?
DNA
How do GENERAL transcription factors recognise response elements in promoter?
- Proteins reach into the major groove and interact with groups which decorate the outside of the double helix, is a specific pattern
- H-bond acceptors and H-bond donors, as well as methyl groups (hydrophobic)
ALSO
- Amino acid side chains from a-helice/b-sheet on the TF dangle into the major groove and make contacts with he DNA
- Can form H bonds with BASES
- Binds to many places on the DNA to hold the transcription factors down
What group in DNA is hydrophobic?
Methyl group CH3
What is needed to stabilise the transcription factor on the DNA?
Multiple connections between the bases on DNA and the amino acids on the transcription factor
What is Rox1 and where does it bind?
Rox1 is a transcription factor from yeast
Binds to 8 DNA sites in 3 different yeast GENES
Where are the binding sites for Rox1?
In yeast:
3 sites in HEM13 gene
4 sites in ANB1 gene
1 sites in ROX1 gene
Why does Rox1 transcription factor bind to its own gene?
So it can regulate itself
Negative feedback loop
What is the structure of the Rox1 binding site in genes?
All are different but all contain GTT in the centre of the site
What are 3 ways to compare the binding sequences of a transcription factor?
1) Compare the frequency at each position
2) Make a consensus sequence
3) Make a sequence logo
What is a consensus sequence?
A way of comparing the biding sequence of sites for a specific transcription factor:
- Write a sequence with the most COMMON base at each position
- Where unsure - put a different letter
What is a sequence logo?
A way of comparing the biding sequence of sites for a specific transcription factor:
- Draw the letters of the bases different sizes, depending on how common they are at that position
What can a consensus sequence be used for? (2 things)
1) To identify more genes regulated by the transcription factor
2) Identification of the sequence in other organisms and cells - homologues
Describe the affinity of the DNA biding sites for a transcription factor
Have DIFFERENT affinities, but when the transcription factor increases to a certain level - all become saturated
Site with the highest affinity is the FIRST one that transcription factor binds to - will also be the site which is occupied most of the time
However, binding is TRANSIENT
Why is the binding between a transcription factor and the binding site on DNA not perfect?
Ideal to have TRANSIENT binding to be able to MODULATE the levels of gene expression within the cells
In the DNA sequence, what is conserved between closely related species?
- Coding sequence (exons)
- Regulatory sequences in non-coding regions (introns)
What did alignment of the genes in closely related species identify?
Conserved sequences in introns, which are important regulatory sequences
Proteins which bind to these sequences
What are motifs which allow proteins (such as transcription factors, activators and repressors) to BIND TO DNA?
(4 of them)
1) Helix-TURN-helix
2) Zinc fingers
3) Leucine zippers
4) Helix-LOOP-helix
What is the structure of a helix-TURN-helix?
2 helices:
- RECOGNITON helix - inserts into the major groove and makes specific contacts with the DNA with its amino acids
- Other - stabilises the position on DNA
How does a helix-TURN-helix motif bind to DNA?
As DIMERS to 2 consecutive major grooves on DNA