Genetics, explained

Genetics, explained

What is DNA? What is a gene? What is a mutation?

In brief, DNA is a long molecule that stores information. DNA is located in the centre (nucleus) of every cell and contains all the instructions necessary for the organism to function. Similar to pearls on a necklace, along the DNA strand there are millions of basic structural units called nucleotides. Each nucleotide (pearl) is one of four types: A, T, G or C, as determined by the nitrogenous base they contain, i.e., nucleotide 'A' has Adenine, 'T' has Tyrosine, 'C' has Cysteine and 'G' has Guanine. These four types of nucleotides are arranged along the strand of DNA in such a way that their order spells out a code that can be 'read' and acted upon by cellular components, i.e., if you were to read a segment of DNA, it might look something like 'AATGCTAGCTTTAGGCTCTAGTCTATAG.' This very segment of DNA may be the instructions that, for example, the skin cell needs in order to make the keratin protein that functions to protect the skin from abrasion. The DNA segment, or 'set of instructions,' is called a gene.

A gene is simply a set of instructions that a cell needs in order to make something useful. Genes instruct the making of such things as collagen, pigments for hair colour, enzymes for digestion etc.

A person's DNA is unique to that person, and their DNA is identical across all the cells in their body.

It is the specific function of a cell that determines which genes will be read and which instructions carried out. Using the example of making keratin protein; skin cells are one of the only cell types that use the segment of the DNA containing the instructions that are needed to make keratin proteins. That same gene will be in the same place on all other DNA strands in the other cells of the body, but may not be unpacked or read or used. It will nevertheless be there. If a gene is read and something (e.g. keratin) is made as a result, in scientific language we say that it is being expressed. Cancerous genes express specific proteins that identify the cell as cancerous.

A mutation is simply a mistake in the DNA. For example, perhaps there is a G where there should be an A along the strand. These mistakes can happen when the machinery that copies and checks the DNA isn't doing its job properly. If a mutation happens during embryogenesis, the mistake will be passed on to every cell in the developing embryo and could eventuate in a genetic disorder. If the mistake happens in a single cell, and this happens often due to 'mutagens' in our environment, most of the time it will be corrected by machinery or go unnoticed. If however the mistake happens in a crucial part of a gene that controls the cell cycle, it can be the initiating event for cancer.

Further reading / information:

Patients:  Here is a great video that explains how DNA works ...Watch video

Practitioners: Again, two articles that may be of interest this week
1. Pizon, M et al. 2013, Heterogeneity of Circulating Epithelial Tumour Cells from individual patients with respect to expression profiles and clonal growth (sphere formation) in breast cancer patients, E-Cancer Medical Science ...Link to free full text
2. Babayan, A et al 2013, Heterogeneity of Estrogen Receptor Expression in Circulating Tumor Cells from metastatic breast cancer patients, PLoS One ...Link to free full text