• The epigenome influences which genes can be transcribed in a particular cell.
• The attachment of X to the DNA of a gene prevents transcription to mRNA, by stopping the RNA polymerase binding.
• The modification by addition of X affects how tightly the DNA is wrapped around the histone.
• (DNA in chromosomes is wrapped around proteins called histones. DNA and histones together are called chromatin)
• When wound tightly, the genes are inactive, they cannot be transcribed in mRNA
• The gene therefore cannot make protein, it is ‘switched off’
• During DNA replication, the epigenome markers are copied with the DNA
• RNA polymerase binds to the promoter region on the DNA
• Transcription will only occur when it is attached
• Gene transcription can be prevented by repressor molecules attaching to the promoter region, blocking the site
SIGNALLING ERRORS
• Homeobox (hox) genes are ‘master switches’
• Code for proteins that switch other genes on and off
• Mutations of hox genes cause different signal proteins to be produced, meaning the protein ‘switches’ on the wrong gene
CONTROLLING GENE EXPRESSION
There is enormous variation in organisms, we will be looking at the mechanisms that brings about the variation we see.
There are approximately 25000 individual genes on our chromosomes of our cells.
Typically, a specialized cell will have up to 20,000 genes actively expressed – meaning that different
combinations are expressed in different cells.
Gene expression involves two key stages –
• TRANSCRIPTION- From DNA to mRNA
• TRANSLATION- From mRNA to proteins
During protein synthesis our cells can control which genes expressed.
The different proteins (in their various quantities) controls the type of cell and function in the body.
Furthermore, proteins can be changed once they are synthesized.
TRANSCRIPTION FACTORS
We can easily control gene expression through switching on and off transcription of specific genes.
This is highly effective, because the single mRNA strand may produce lots of different proteins at the
ribosome.
WHAT IS IT
All transcription factors have specific regions that allow them to bind onto specific parts of DNA – promoter region
Transcription factors can either:
Bind directly to promoter sequence of DNA, stimulating transcription.
Bind to regions called enhancer sequences. These change the structure of chromatin, making it more or less exposed to RNA polymerase.
• ‘open chromatin’ – linked to active gene expression
• ‘closed chromatin’ – linked to gene activity
These sites can either be at the site of the gene or they can be found very far away (thousands of base pairs!)
You may have several different transcription factors controlling expression of one gene or A single transcription factor may control activity of several different genes. The advantage of these processes is that we can activate and/or repress different gene(s) at various stages of the development of an organism, in different cell types.
RNA SPLICING
During transcription in the nucleus, the mRNA is made up of all the DNA found in a gene. This includes
exons and introns:
• Intron – DNA/RNA segments containing information that does NOT code for a protein
• Exon – DNA/RNA segments containing information that codes for a protein.
Before reaching the ribosome, several changes to the mRNA take place; this is known as pre-mRNA
The changes to pre-mRNA always involve removing introns, and sometimes even exons.
A group of enzymes called SPLICEOSOMES join together the exons that will be transcribed to make
functional mRNA. Spliceosomes join the same exons in a number of ways (RNA splicing)
Hence, a single gene may produce several types of mRNA, which have been transcribed from the same DNA section.
As different versions of mRNA code for different amino acid arrangements we can make different proteins.
Due to spliceosomes, a single gene gives rise to 576 different proteins that affect the sensitivity of the hairs in the inner ears of chicks
