Cancer: rogue cells and DNA mutations


The Cancer Genome Project is an attempt to bring the human genome to the understanding of how cancers develop. All our bodies are composed of cells. The human body has about 100 million million
cells within it, and usually those cells behave in a very ordered fashion. They observe certain rules, they divide when they’re told to divide, they’re quiescent when they’re told to remain
quiescent, they stay in a particular position within their tissue, they don’t move out of that.
Occasionally, however, a single cell of those 100 million million
cells decides to behave in a very different
way. That cell keeps on dividing when all its signals around it tell it to
stop dividing. That cell ignores its colleagues around it and pushes them
out of the way. That cell stops observing the rules of the tissue within which it is located and begins to move out of its normal
position, invading into the tissues around it and
sometimes entering the bloodstream and becoming
a metastasis, depositing in another tissue of the body. The reason that the cell behaves in this rogue fashion, is because it has acquired within its genome, within its DNA, a number of abnormalities that cause it to
behave as a cancer cell. Of course all these 100 million million
cells in the human body have got a copy of the human genome, they’ve got two copies, a maternal and paternal copy. And
throughout life all those copies of the genome, in those 100 million million cells, are
acquiring abnormalities, so-called somatic
mutations. These are mutations that are just present in the cell, they’re not transmitted from parents to offspring,
they’re just constrained to that individual cell and its progeny. And those mutations occur in every cell
of the body, normal and abnormal, for a number of different
reasons. They occur because although we have an
extraordinary mechanism for replicating DNA, for those three
thousand million letters of code to be replicated each time a cell divides,
hardly any mistakes are made, despite that extraordinary mechanism,
the occasional mistake is made. Basically every time a cell divides possibly one letter of code out of
the three thousand million is replicated wrongly. So that’s one source of mistakes one
source of somatic mutations. However in addition to that, all these 100 million million cells in the
body are being exposed to a number of different onslaughts. There is
radiation, of various sorts, which is hitting us the whole time, natural radiation; we are a bag of chemicals, chemicals that we generate ourselves, but chemicals that we also ingest or are
exposed to, and those also can bind to DNA and can cause mistakes in replication of DNA to be more common. And finally, we have this mechanism,
these processes that allow DNA to be
replicated in this highly faithful way, this high fidelity mode, but occasionally those mechanisms in a
particular cell may breakdown and then the DNA of that cell begins to
acquire mutations, somatic mutations rather more commonly then other cells. So, you put all that together, every cell
in the body is acquiring mutations throughout a lifetime, and as we get older
we acquire more and more of them. As it turns out, most of those mutations are completely innocuous, they make no
difference at all. They make no difference to the cell itself, and certainly they make no difference to us as multicellular organisms. They’re basically
silent, they sit there, we live with them. Occasionally, however, a mutation
takes place in a particular type of gene, which in an operative way we now call a
cancer gene. What that mutation does it changes that particular gene, and of
course the protein that it makes, such that now that abnormal protein is driving the cell to behave in that unusual rogue fashion that we call a cancer.

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