There is a fundamental part of cell biology that I haven’t posted about yet, but I have skimmed over it briefly in previous posts here & here. I must admit, the link to Madonna is tenuous (at best), but I will be writing about one of the many ways we all express ourselves. In contrast to the expression of feelings that Madge sung about in the 80’s, this form of expression is not under your conscious control; the fundamental process described in this post is: Gene Expression!
Madonna, expressing herself.
What is it?
Gene expression describes the process by which your cells can convert your DNA (deoxyribonucleic acid) into other molecules or products that have particular jobs/functions within the cell. The most common example of this is the conversion of DNA into proteins, which can then go off and carry out different jobs around the cell. Different cell types (e.g. a heart cell, a blood cell or a neuron) exist because they read different parts of your DNA, and consequently create different proteins that carry out different jobs and functions (see the ‘instruction manual’ metaphor in this previous post!).
So how do your cells convert DNA into proteins?
This is done by a combination of two processes called Transcription and Translation.
But before I get into that, here are the basics of DNA:
DNA is a double stranded molecule. It can be compared to the two sides of a zip (or zipper) that fit together in the middle. Instead of the teeth that fit together on the zip, DNA has individual nucleotides (also called ‘bases’). These are simpler molecules that are the building blocks of DNA. There are four different nucleotides:
Adenine (referred to as A)
Cytosine (referred to as C)
Guanine (referred to as G)
Thymine (referred to as T)
In the DNA ‘zip,’ an A on one strand is always paired up with a T on the other strand, and a C is always paired up with a G. These pairings (perhaps unsurprisingly) make up Base Pairs.
The first process that occurs in gene expression is transcription, and happens in the cell nucleus.
This is the way that DNA is converted (or Transcribed!) into RNA. RNA (ribonucleic acid) is the single stranded equivalent of DNA. Like DNA, it is also made up of four different nucleotides; however while it has the A, C and G nucleotides, it has Uracil (U) in the place of T. The two strands of DNA are first separated by an enzyme called DNA helicase, so that the nucleotides are no longer paired and are exposed ready to be transcribed. Another enzyme, RNA polymerase, then starts ‘reading’ the nucleotides along the open strand of DNA and creates a complementary strand of RNA. This piece of RNA is referred to as the transcript.
Not all of your DNA is transcribed all at once – only the genes that are needed for that cell at that time are transcribed, so the single strands of RNA tend to be much shorter than the double strands of DNA. There are actually many different types of RNA that exist within cells, and they can carry out different functions. However, the form of RNA that goes on to help create proteins is called messenger RNA, or ‘mRNA’ for short.
Translation is the process where mRNA is converted (or ‘Translated’!) into a protein. After its creation, mRNA moves into the cytoplasm of the cell, so this is where translation occurs. To understand how translation works, you need to know these five things:
- 1. Proteins are made up of chains of amino acids.
- 2. Different combinations of amino acids make up different proteins
- 3. A set of three nucleotides in a row will ‘code’ for an amino acid. This sequence of three nucleotides is called a ‘Codon.’
- 4. Different combinations of nucleotides will code for different amino acids
- 5. Therefore the sequence of nucleotides on a strand of RNA provide the instructions for a particular chain of amino acids, which in turn create a particular protein. A different sequence of nucleotides on different strands of RNA will therefore provide the instructions for a different chain of RNA, resulting in a different protein.
So how does translation occur?
A molecule called a Ribosome attaches to the mRNA – ribosomes are like tiny protein-processing factories, and guide the translation process. The ribosome attaches to the first codon – this guides another form of RNA, tRNA (transfer RNA), to that codon. tRNA forms a link between nucleotides and amino acids. On one side it has a codon, and on the other side it has the amino acid that the codon represents or gives the instruction for.
The tRNA needs to have a complimentary codon sequence to the mRNA; if you remember that nucleotides pair together –an mRNA codon made up of CCG would require the tRNA to have a complimentary codon of GGC in order to pair up. This process makes sure the correct amino acids are assembled in the correct order.
The ribosome moves along the mRNA, recruiting tRNA with its attached amino acids as it goes. The amino acids then form a chain, which creates a protein. Once the ribosome has finished moving along the mRNA, the constructed protein is released, and translation is complete!
Transcription and translation are complicated mechanisms, but hopefully I made that clear enough to follow! However, they form a fundamental part of gene expression, which underlies how our bodies (including the best bit – the brain (obviously!) work and grow.
Labyrinth, thinking about gene expression
And although Madonna may have missed the mark, Labyrinth’s more recent version of ‘Express Yourself’ might be little more accurate when he says ‘Being myself is something I do well’ – Yup, thanks to biochemistry and cell biology, we all express ourselves, and generally we do it pretty damn well!