After that brief lesson on evolution where we learnt how life as we know it was reached by cooperation and by increasing complexity via task differentiation both inside the cells and among cells, we started analysing the different cells’ compartments. It was captivating to realise the economy of a system composed by different players cooperating to better achieve the goal of life, that is survive and reproduce. And even among all this diversity both in between cells of the same organism as well as between different organisms, the cell, the building block of life, has always the same compartments. There is only one exception that can be found in eukaryotic cells, that is the compartment involved in the energy production: while animal cells posses mitochondria, that create energy using nutrients and oxygen and release carbon dioxide and water, plant cells have chloroplasts where the process of photosynthesis occurs, with the final release of oxygen in the air.
The first membranous compartment that we studied was the nucleus of the eukaryotic cells. As I already mentioned in my previous post, the nucleus is what differentiate prokaryote and eukaryote cells: having the genetic material that contains the information that the cell needs to survive and reproduce enclosed by a double membrane gives extra protection to it and prevents possible “damages” that can lead eventually to cell death.
All our genetic material is contained in the DNA, that stands for deoxyribonucleic acid: this is basically a really long book written only with four letters (A-C-G-T) called nucleotides, that contains all the information needed to create a living being. The DNA book could be divided in genes, that we could think of as paragraphs of the book, that will then be translated into “concrete” material by other players inside the cells, like in a well organised factory. Our DNA, as humans, is divided into 46 structures called chromosomes, that contain in total an amount of around 6 billions letters: if we could unfold all the DNA contained in all the cells of our body, we would be able to wrap the equator more than 2 millions and a half times!
I just mentioned how our DNA contains all the information to create an exact replica of ourselves, but today for the SRF I would like to talk about a region at the end of the chromosomes, that does not hold information for the expression of any characteristics, called telomere.
“Science Related Fact” (SRF):
The telomeres are regions at the end of each chromosomes with a repetitive nucleotide sequence. Despite not containing information for any characteristics of the individual, they seem to have an important role (yet not fully understood) in preventing chromosomal deterioration and determining the life span of each cell. It has been seen that they become shorter and shorter after each cell division, where the genetic material of one “mother” cell is shared in between two “daughter” cells (see picture below).
This progressive shortening of telomeres has been linked with ageing, ageing-related diseases and, more loosely, mortality. Cells have a limited capacity for cellular division, defined by the Hayflick limit, that is around 40-60 division for a typical human cell: it has been proposed that telomeric shortening could be a reason for this limit, as cells may stop dividing when too much telomere DNA is lost.
On the other hand, telomeres can be lengthened by the so called telomerase, allowing for continued cell division past the Hayflick limit. However, the permanent activation of telomerase can cause cells to become immortal and eventually leads to cancer.
Currently, it was seen that old individuals have usually longer telomeres, but these has been interpreted in the perspective of natural selection that determines a selective mortality of individuals with shorter telomeres, rather than an increase in the actual telomeres’ length. Despite this promising possibility of immortality researchers are very careful in playing with telomeres: as I mentioned above, the chance of living forever via telomeres lengthening could eventually lead to cancer, since the increase of cells’ life span is inherently correlated to the risk of a tumorigenic transformation.