Control of gene expression

What is a gene mutation?

• The alteration of a base in the sequence of bases for one gene.

• Likely to happen during DNA replication.

• Spontaneous but mutagenic agents increase frequency.

Mutagenic agents

• High energy and ionising radiation e.g. X-ray

  • Disrupt structure of DNA

• Carcinogens e.g. cigarettes

  • Interfere with transcription

Addition mutations

• Frameshift

• When one base is added

• Altered codons could potentially code for different amino acids→ different 3D tertiary structure → non-functioning protein.

Deletion mutation

• Deletion of a base in a sequence

• Frame shift

• Also can lead to non-functioning protein.

Substitution mutation

• Base has been changed for another base but number of bases remains the same.

• No frame shift, only one codon changing and due to degenerate nature of genetic code, same amino acid and therefore have no impact.

Inversion mutation

• A section of bases detach from DNA, but when they re-join they are inverted, so this section of code is back to front.

• This results in different amino acids being coded for in this.

• No frameshift but could still affect polypeptide as it can affect multiple codons.

Translocation

• A section of bases on one chromosome detaches and attaches onto a different chromosome.

• This is a substantial alteration and can cause significant impacts on gene expression and therefore the resulting phenotype.

What are stem cells?

• Multicellular organisms have a diverse range of specialised cells that all originate as undifferentiated stem cells.

• Stem cells are undifferentiated cells that can continually divide and become specialised (differentiation).

Totipotent

• Stem cells that can divide and produce any type of body cell.

• Found during early development of embryo for limited time.

• During development, totipotent cells translate only part of their DNA, resulting in cell specialisation.

Pluripotent

• Stem cells are found in embryos and can become almost any type of cell.

• They are used in research with the prospect of using them to treat human disorders.

• These stem cells could be used to regrow damaged cells in humans, such as replace burnt skin cells, or neurones in Parkinson’s disease sufferers.

• There are issues with this as sometimes the treatment doesn’t work or stem cells continually divide to create tumours.

• Additionally, ethically there is debate on whether it is right to make a therapeutic clone of yourself, to make an embryo to get a stem cells and then destroy it.

Multipotent and unipotent

• These stem cells are found in mature mammals and can divide to form a limited number of different cell types.

• Multipotent cells such as in bone marrow can differentiate into a limited number of cells whereas unipotent cells can only differentiate into one type of cell.

Sources of stem cells in mammals:

• Up to 16 days after fertilisation contains pluripotent stem cells.

• Umbilical cord contains multipotent stem cells.

• Placenta also has multipotent.

• Adult stem cells such as in bone marrow are a source of multipotent cells.

Induced pluripotent stem cells

• Pros and cons of using stem cells to treat human disorders.

• Induced pluripotent stem cells can be produced from adult somatic cells using appropriate protein transcription factors to overcome some of the ethical issues with using embryonic stem cells.

• Turning on all genes in a cell means it’s no longer specialised.

How are pluripotent stem cells induced?

• Adult unipotent cells and used and can be from almost any body cell.

• Altered in the lab by turning off genes to make the cell specialised, so it is no longer specialised, using transcriptional factors, returning them to a state of pluripotency.

What is one way gene expression is controlled?

• Epigenetics

What is meant by gene expression?

• A protein is created: includes transcription and translation

What is epigenetics?

• The heritable change in gene function without changing the DNA base sequence.

• Changes are caused by environmental changes and can inhibit transcription.

Control of gene expression

• Factors such as diet, stress and toxins can add epigenetic tags to the DNA, and this can control gene expression in eukaryotes.

• A single layer of chemical tags on the DNA is called the epigenome and this impacts the shape of the DNA-histone complex and whether the DNA is tightly wound so won’t be expressed or unwound so it will still be expressed.

• If the DNA is tightly wound, then transcription factors cannot bind and therefore the epigenome, which is due to changes in the environment can inhibit transcription.

What is heterochromatin?

• When the histone is tightly coiled and transcription is inhibited due to increased methylation and decreased acetylation.

• Euchromatin on the other hand is when genes will be expressed as there is decreased methylation or increased acetylation, making the DNA less tightly coiled.

Methylation of DNA

• Increased methylation of DNA inhibits transcription.

• When methyl groups are added to DNA, they attach to the cytosine base.

• This prevents transcriptional factors from binding and attracts proteins that condense the DNA-histone complex.

• In this way, methylation prevents a section of DNA from being transcribed.

Acetylation of histone proteins

• Decreased acetylation of associated histone proteins on DNA inhibits transcription.

• If acetyl groups are removed from the DNA then the histone become more positive and are attracted more to the phosphate group on DNA.

• This makes the DNA and histones more strongly associated and hard for transcription factors to bind.

Epigenetics and cancer

• Tumour Suppressor genes

• These genes produce proteins to slow down cell division and to cause cell death if DNA copying errors are detected.

• If a mutation results in the tumour suppressor gene not producing the proteins to carry out this function, then cell division could continue, and mutated cells would not be identified and destroyed. BRCAI and BRCA2 are two known mutated tumour suppressor genes that are linked to breast cancer.

• This leads to abnormal methylation so tumour suppressed could become hypermethylated and this could result in the gene being inactivated and turned off.

• The opposite could occur in oncogenes, as they may be hypomethylated, reducing the number of methyl groups attaches, meaning the gene is permanently switched on and is continually producing proteins that trigger DNA replication.

Where to transcription factors move from and to?

• Cytoplasm into nucleus to turn genes on and off.

Where does transcription control occur?

• In the nucleus

• Transcriptional factors are proteins that can bind to different base sequences on DNA, initiating transcription of genes.

• One part binds to DNA, and other part binds to a receptor.

• Once bound, transcription begins, creating the mRNA molecule for that gene which can then be translated in the cytoplasm to create the protein.

• Without this binding, the gene is inactive and the protein wouldn’t be made.

Oestrogen

• Oestrogen is a steroid hormone that can initiate transcription by binding to a receptor site in the transcriptional factor.

• When it binds to the transcription factor, it causes it to change shape slightly, and this makes it complementary and able to bind to DNA to initiate transcription as RNA polymerase attaches after transcriptional factor attaches and the active site of RNA polymerase is only complementary to the DNA and transcriptional factor together.

• Steroid means lipid soluble, can diffuse through plasma membranes simply into cytoplasm.

• After binding, it then diffuses into nucleus.

What is RNA interference?

• When an mRNA molecule that has already been transcribed gets destroyed before it is translated to create a polypeptide chain.

• This is done by using small interfering RNA, siRNA.

• This is translational control.

Translation control

• Occurs in cytoplasm

• An enzyme can cut the mRNA into siRNA and another enzyme can make it single stranded.

• One strand of siRNA then combines with another enzyme.

• This siRNA-enzyme complex will bind via complementary base pairing to another mRNA molecule.

• Once bound, the enzyme will have cut up the mRNA, so it cannot be translated, turned off.

• Therefore, the polypeptide chain is not made.