With Freshers week upon us for many students across the country, we thought it might be a nice idea to recap some of the basics of “the cell” for those relatively new to the subject and to introduce you to a few new things you might find interesting.
So, where to start? The majority of our cells contain DNA held within a protective body called the nucleus, surrounded by a nuclear membrane containing a number of proteins, lipids and other molecules, acting as a barrier to stop any damaging agents from reaching our genome. Bearing in mind we encounter a lot of agents from outside of our bodies that can cause DNA damage, would you be surprised to hear that we actually generate some of these molecules ourselves? Reactive oxygen species are a bi-product of the metabolic processes in our cells that we use to produce ATP (the energy source of our body), which if left to their own devices would wreak havoc on our genome.
But what about our DNA itself? Many of you will already know a little about this amazing genetic code, the strings of “letters” that decide whether we have blue eyes, or brown hair, two arms and two legs, whether we’re human or something altogether different. These “letters” are actually chemical bases, and in DNA we have four: Adenine, Threonine, Guanine and Cytosine. As our DNA is made up of two strands (with the backbone of these strands containing sugars and phosphates), bases will pair up across each strand via hydrogen bonds that are protected in the centre of DNA’s characteristic helical shape. A will only pair with T and G will only pair with C. So how can a code with only four letters make a human, an organism so complex that there are still things we don’t understand about our own bodies? The answer lies in the length of DNA- which is very long indeed. In fact, the only reason we have enough room for it in our cells is because it exists as a structure called chromatin, where DNA is coiled into filaments which are then wound around proteins called histones, which essentially render most of the DNA inaccessible to other molecules. Both the histones and the DNA can be modified by the addition of small chemical groups which are defined as “post translational modifications”, which in turn can cause a gene to be turned on or off, or can recruit other proteins which will change the chromatin structure (allowing more access to the bases), help to repair it, etc.
Having access to the bases becomes really important when it comes to replication of DNA which is required when our cells divide (a process called mitosis) as well as when we use this DNA code to create RNA which in turn is used as a template to create all the proteins a cell requires. RNA is really important as it can be exported from the nucleus into the cytoplasm of the cell, where it can be targeted to its final destination and interact with proteins that could cause damage if they were in the nucleus.
There are a number of different RNAs, each of which has its own role in the cell. If you’ve heard of RNA, you’ve probably looked into messenger RNA (mRNA) which is the type of RNA used in protein synthesis (translation). mRNA attaches to ribosomes (a 2 subunit complex which is itself made up of a mix of RNA and proteins) which are then targeted to the endoplasmic reticulum (ER). As the mRNA is fed through the ribosome, it in turn assembles strings of amino acids which are linked by peptide bonds to form a backbone which is fed into the lumen of the ER, with each of the amino acids being brought to the ribosome by tRNAs. These tRNA will only bind and deposit their amino acid cargo if the mRNA code (and thus the original DNA code) is complementary to the tRNA code, with 3 bases coding for one amino acid. Each amino acid is unique and has distinct properties which will cause the final protein to fold into a particular shape and have different properties. The process is actually a lot more complex than this, with certain portions of the RNA not coding for protein and instead being required for attachment of other molecules which will interact with it and influence how long it exists before it is degraded, whether the RNA will be translated at all (useful if you only need a protein at specific times), whether it will be edited (another way our genome becomes more and more complex!), etc. As you can see, the cell is not as simple as it seems!
But the complexity doesn’t end here! The proteins will start to fold when they are inside of the ER lumen, aided by chaperone proteins that stop them from forming an incorrect structure- if proteins misfold they are said to form “aggregates” which are often degraded. This is the basis of a lot of genetic diseases, where a protein is formed incorrectly so it is destroyed and cannot carry out its normal process in the cell.
Before and after folding has occurred, the amino acids within the protein can also be modified by other proteins present in the ER as well as in the Golgi Apparatus, which is where the protein is next sent. The Golgi Body is essentially a protein sorting office, helping proteins to become packaged up and sent off to their final destination, to where they are targeted by certain amino acid sequences which are often found in the N or C terminal of the protein, which simplistically is each end of the protein string. Proteins can be inserted into the cell membrane (transmembrane proteins), secreted into the blood, sent to other organelles within the cell… there are a whole host of possibilities.
And there you have it. The not so simple cell. There is obviously a lot more going on here than I’ve explained, I haven’t really mentioned the mitochondria and its generation of ATP, the lysosome where proteins are sent to “die”, the cytoskeleton that holds tho whole thing together… and that is just the tip of the iceberg! Our cells are complex machines capable of carrying out extraordinarily complex processes and functions. Bear in mind there are lots of different types of cell, and that each of these has its own internal structures, creates different proteins and carries out different functions, and you’ll come to realise that you could read about the blueprints of our body for a lifetime and still not completely understand it. If you’re about to start a biomedical/biology based degree, I salute you.
Welcome to the wonderful world of science.
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Image ref: http://upload.wikimedia.org/wikipedia/commons/a/ae/Cell_Structure_,_Cell_Diagram.png