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Definitions:

Epistasis: Where the expression of one allele blocks the expression of another allele.

Stem cell: A cell which is capable of differentiation and self-renewal.

DNA profiling: Technique used to identify individuals based on what their DNA looks like.

Probe: A short sequence of nucleotides complementary to a specific sequence.

Human genome project: A project which has successfully mapped out the entire human genome, allowing us to track evolution and screen for diseases.

Proteome: All the proteins that a genome can code for (exons)

iPS: Induced pluripotent stem cells, which are made out of unipotent stem cells.

Transcriptional and Translational regulation: Oestrogen and siRNA

-Oestrogen is lipid soluble and therefore can diffuse into cells through the plasma membrane.

-Once inside a cell, oestrogen binds to a receptor site on a transcription factor.

-Binding to the transcription factor changes the shape of the TF so that it is now complementary to DNA.

-TF binds, transcription initiated by RNA pol

-siRNA stands for small interfering RNA. siRNA is single stranded and is complementary to a region of mRNA.

-When it localises to the mRNA, it forms complementary base pairs with the exposed bases on mRNA and therefore generates a short section of dsRNA.

-The double-stranded region is recognised as a termination for transcription, so the gene is chopped up by enzymes.

Mutations and causes: Mutations (see A2 Mutations) are characterised by a change to the genetic code. Mutations can arise randomly due to transcription/replication errors, or can be induced through mutagenic agents, or mutagens such as benzopyrene (cigarette smoke) and UV light (the sun). Mutations can be silent or non-silent, depending on where the mutation happens and what type of mutation it is (addition, deletion,inversion,translocation).

Remember that mutations can occur in introns (non-coding regions) and therefore have no large effect on the phenotype. Mutations can also give rise to the same amino acid that the triplet codon would have originally coded for- owing to the degenerate nature of the code.

Stem cells: Stem cells are special cells which can differentiate (turn into) almost any other type of cell. Stem cells can be adult stem cells (found in the bone marrow) or embryonic (taken from an embryo) stem cells. Embryonic stem cells can differentiate into a wider array of cells (totipotent) compared to adult stem cells (pluripotent). However, taking stem cells from embryos has ethical implications (playing god/destroying life/potential for life).

Stem cells are self-renewing (they make more copies of themselves).

Transcriptional and Translational regulation: Epigenetic changes

DNA methylation: The process whereby -CH3 (methyl) groups are added to DNA, which typically inhibits transcription. Only cytosine bases can be methylated. The methyl group makes it difficult for RNA pol to access the gene and begin transcription.

Histone acetylation: DNA associates with proteins called histones so that it can super coil and form very dense structures containing lots of information. Histone proteins have a positive charge, which is how they bind negatively charged DNA tightly. However, acetylation of histones decreases this positive charge, causing them to hold DNA in a looser configuration and therefore facilitate RNA pol action for transcription.

It is important to note that these epigenetic changes (turning genes on/off etc) are highly regulated and in a constant state of flux to help keep cell division happening but not at an uncontrollable rate.

Genes and cancer: Control of expression & Cancer survival

Oncogenes/Proto-oncogenes:

Proto-oncogenes are genes which produce proteins which are used to stimulate cell division. Mutations in these useful genes turns them to oncogenes, which stimulate uncontrollable cell division. They do so by activating receptors on the cell surface or by producing large amounts of growth factors.

Tumour supressor genes:

These genes, as advertised, stop tumours from forming. They do so by causing cells to stop dividing (either by exiting the cell cycle or by programmed death-apoptosis) If these genes become mutated/abnormally methylated, cancer can arise.

Benign vs Malignant

Tumours can fall under one of two categories depending on their severity. Benign tumours do not proliferate in an invasive manner, do not metastasise, whereas malignant tumours grow quickly and spread. Malignant tumours are  more serious and require chemotherapy or surgical removal of the tumour.

Cancer survival:

Cancer cells have clever mechanisms of staying alive. They give off chemical signals that wounded cells do (VEGF) which stimulates our bodies to grow new blood vessels into the tumour. This is known as angiogenesis, and supplies the tumour tissue with the sugar and oxygen that the cancer demands for continued cell growth, division and proliferation.

Cancer cells can also metastasise, where a piece of the tumour breaks off and enters the bloodstream or lymph. By doing so it encourages that small lump of cells to become wedged somewhere else and continue their proliferation. This is a quality of malignant tumours.

Normal cell abnormally methylated at tumour suppressor genes

Cell begins to divide uncontrollably

Angiogenesis

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