Control of gene expression in Eukaryotes - week 8 Flashcards
in a human eukaryotic cell, how much of the DNA is encoding for protein/ polypeptide
A single human cell, a eukaryotic cell, contains enough DNA (6 billion bp) to encode several million different polypeptides.
but most of this DNA does not actually code for proteins, mammalian genomes contain ~35,000 protein-coding genes but a typical mammalian cell may only make ~5000 different polypeptides at any given time. Most DNA doesn’t code for proteins because they are housing genes.
which cells are expressed all the time and not always, with example
- Some genes are expressed in all cells all the time (housekeeping genes)
Some genes are always expressed in particular cell types but not in others. For example, a plasma cell continuously expresses the gene for the antibody it synthesizes - Some are expressed only as conditions around and in the cell change. For example, a hormone’s arrival may turn on/off certain genes in that cell.
where does regulation of gene expression occur in eukaryotes and what does each step regulate
regulation of gene expression in eukaryotes can occur at any of the steps in the process (the process that produces a functional protein) of gene expression.
Gene expression can occur at
-the DNA (genome),
-Transcription,
>Transcriptional Control is turning on and off, of mRNA formation.
-RNA processing,
>Regulation of the processing of a pre-mRNA into a mature mRNA
-Translation
>Regulation of the rate of Initiation
-Post-translation (after translation when the protein is already completed)
>Regulation of the modification of an immature or inactive protein to form an active protein
What is chromatin remodelling
In eukaryotes, DNA is associated with proteins to form a structure which is called chromatin.
The first potential point of the control of gene expression is chromatin remodelling.
Chromatin Remodelling is the region of the chromosome that must be opened up for enzymes and transcription factors to access the gene.
what are the 2 forms of chromatin
Chromatin exists in 2 forms:
* Euchromatin
* Heterochromatin
Euchromatin is less condensed, Euchromatin is the open conformation form, and Euchromatin is associated with the activation of the transcription.
Heterochromatin is highly condensed, and it is usually associated with the inhibition of transcription.
what is nucleosome and how does it affect transcription
Chromatin looks like beads on a string, each bead is called a nucleosome. Each nucleosome is made up of DNA wrapped around histone proteins.
chemical modifications to histones and DNA can affect the conversion between Heterochromatin and Euchromatin. Essentially, chemical modifications to histones and DNA of the nucleosome can affect transcription.
what domain of histone is subject to chemical modifications and what do these modifications affect
A particular protein domain, that is the main terminal domain of histones is the N-terminal domain of the histones, this is usually subject to chemical modifications.
These potential modifications are:
* Acetylation
* Methylation
* Ubiquitination
* Sumohylation
* Phosphorylation
These chemical modifications that can affect the amino acid of the amino ‘tail’ domain, which is usually Lysine’s, can directly affect the interaction between DNA and the histone proteins.
what is acetylation and what does it affect
Acetylation is the covalent attachment of the acetyl group to the Lysine but acetylation of the Lysine eliminates the positive charge of the Lysine.
The acetylation affects the electrostatic interaction between the DNA and the histone protein and because of this interference affects the transcription.
what is methylation
Lysine methylation retains the positive charge whether mono-, di-, or trimethylated. The addition of methyl groups can condense chromatin. It is associated with reduced transcription.
What is the HAT enzyme and how does it activate transcription
HAT are enzymes that can attach an acetyl group to the histone.
The activity of enzymes that are called Histone Acetyl Transferases (HAT) results in the decondensation of the chromatin. As the decondensed (loosened) chromatin is associated with the activation of the transcription thus histone acetyl transferases (HAT) enzymes are activators of the transcription.
how does HDCA enzyme inhibit transcription
Histone deacetylases (HDCA) enzymes condense the chromatin, so they are associated with the inhibition of transcription.
what enzymes are examples of how gene expression is regulated
HAT enzymes activate transcription whereas HDCA inhibits transcription.
There is an example of how eukaryotes can regulate the expression of a given gene by affecting the DNA, and the modification of the chromatin.
where the core promoter and control elements found
A typical eukaryote gene is organised in a eukaryotic cell. In eukaryotes, proteins-encoding genes have a core promoter and control elements (regulatory elements)
what is a core promotor and what does it consist of
The core promoter is the binding site of the general transcription factors and RNA polymerase. The core promoter has a sequence that is common to most genes of eukaryotes.
The core promoter contains the TATA box, which is the binding site of RNA polymerase, and it contains the transcriptional start site, the nucleotide is the start site, where the transcription begins.
what are control elements
The core promoter has a sequence that is common to most genes of eukaryotes. But in contrast, control elements (also called regulatory elements) are regions within the DNA located upstream of the core promoter, and they have a sequence that is unique to a specific gene.
