What does it mean for a gene to be expressed?
If a gene is expressed, transcription and translation of that gene occurs and the protein coded for by that gene is made.
Why do each of your cells look different?
Although each cell in your body is genetically identical, as it is a product of mitosis, each cell looks different.
This is because different cells have different genes switched on and off. Therefore, they produce different proteins, and perform different functions, as well as having unique characteristics.
In which three ways is gene expression controlled?
- regulating transcription
- regulating translation
- epigenetics
What are the 5 different types of stem cells? Describe them.
- totipotent - can differentiate into ANY type of cell
- pluripotent - can differentiate into all cells but NOT PLACENTAL cells
- multipotent - can differentiate into any cell of one type. i.e. bone marrow into all blood cells
- unipotent - can differentiate into only one type of cell
- iPS - induced pluripotent stem cells.
What is the importance of stem cells in day to day life?
Differentiated cells can no longer divide by mitosis. Therefore, you require stem cells of different cell types in order to be able to replace them when they die.
What is an iPS cell?
What are their disadvantages?
induced pluripotent stem cell
These are differentiated adult cells which are engineered into stem cells and can therefore differentiate into any type of cell.
- disadvantages include: this is new technology and therefore long ter,m effects are unknown
Suggest how iPS cells can help alleviate red green colour blindness. (2 marks)
- (In this case, green cones are non functional and green is not differentiated from red).
- iPS cells divide
- iPS cells differentiate into green sensitive cones
What is the role of a promotor region of a gene?
It regulates transcription of that gene. Each gene has a promotor region.
A promotor region is the DNA sequence ‘before’ the gene in the DNA strand.
What are transcription factors?
- these are molecules (usually proteins) which bind to the promotor region, which allow RNA polymerase to bind to the gene and initiate transcription
- transcription factors are activated or inhibited by other molecules
- if a transcription factor has not yet bound to the promotor region of the gene, then RNA polymerase will not bind to that gene.
Describe the regulation of transcription.
- oestrogen diffuses through the phospholipid bilayer of the cell. (Other hormones may be used here, but how they are transported through the membrane depends on the characteristics of the hormone - it is lipid or water soluble etc)
- oestrogen attaches to the INACTIVE transcription factor, changing it’s shape (link to changing shape of protein)
the activated transcription factor ENTERS THE NUCLEUS. It attaches to the promotor region.
This allows DNA polymerase to begin transcription.
How can you inhibit transcription factors?
- a molecule similar in shape to oestrogen and complementary in shape to the binding site on the transcription factor can bind to the transcription factor
- this prevents the ostreogen from binding to the transcription factor, and prevents it from being activated
- therefore the transcription factor cannot bind to the promotor region and stimulate RNA polymerase
Describe the process of regulating translation.
- double stranded RNA (dsRNA) is produced
- An enzyme hydrolyses this dsRNA to produce short lengths of double stranded RNA (siRNA)
- one strand, which is complementary to the target mRNA attaches to a second enzyme
- The single stranded siRNA attaches to the mRNA. The enzyme hydrolyses the mRNA destroying it.
- the mRNA cannot be translated
-the polypeptide/ protein is not produced
When does regulation of translation occur? What is the purpose of it? (not on A level spec)
When the concentration of mRNA in the cytoplasm is too high.
- This could be because transcription is happening at too high of a rate.
The purpose of this is to ensure that only a controlled amount of mRNA is translated at once, and hence only a controlled amount of protein is made.
APPLICATION OF KNOWLEDGE
Why is regulation of translation is this way a successful process?
- ribosomes can only attach to single stranded mRNA
- if a molecule is made into a double stranded molecule, then translation cannot occur
- This is because there are no free bases which can pair up with bases on free tRNA molecules.
What is the definition of epigenetics?
- heritable changes in gene function
- which occur without changes to the base sequence of DNA
Explain the physiological idea of how epigenetics affects gene expression, based off of the structure of DNA.
- chromatin is composed of DNA molecules wrapped around histone proteins
the structure of chromatin can be altered so that the nucleosomes are either closer together (heterochromatin) or further apart (euchromatin). - euchromatin allows a gene to be expressed
(a gene cannot be transcribed if it’s promotor region is wrapped around a histone - the promotor region would be inaccessible to the transcription factor).
What is an epigenome?
An epigenome are all the chemical tags which are attached to the DNA or histone proteins, and affect gene expression
- The two important ones are acetyl groups and methyl groups
What is methylation?
- IMPORTANT EXAM MARK -
- adding of methyl groups to DNA
- (not A level spec) they attach to cytosine.
This is an important mark scheme point. If a question ever asks anything about methylation ALWAYS describe it.
Effect of methylating DNA
- causes genes to be silenced
- this is due to the chromatin becoming more condensed
- this makes it HARDER for DNA to be transcribed
Effect of demethylating DNA
- this causes the genes to be expressed
- this is due to the chromatin becoming less condensed
- this makes it EASIER for the DNA to be transcribed
What is acetylation?
Adding acetyl groups to histone proteins
Effect of acetylating histones
- this causes the genes to be expressed
- this causes the chromatin to become less condensed
- this makes it EASIER for DNA to be transcribed
Effect of deacetylating histones
- this causes the genes to be silenced
- this causes the chromatin to become more condensed
- this makes it HARDER for DNA to be transcribed
How do you remember the role of acetylation vs methylation?
- Methyl groups are like tags added to cytosine groups on DNA. This causes the DNA to become MORE TANGLED, and therefore nucleosomes move CLOSER together. Chromatin is more condensed.
- Acetyl groups are like big spheres added to the histone proteins around which DNA is wrapped around. Therefore, nucleosomes move further away from each other, and become MORE SPACED OUT, as they require more space. Chromatin is less condensed.
