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Fitting Into Your Genes

An Introduction to Genetics

Us humans are a diverse bunch, aren’t we? No two people look the same, not even identical twins. I mean, you can see people who look the image of your best mate’s dad’s brother or find the double of your favourite celebratory, but no two humans are absolutely identical. Next time you are sat on a crowded tube train, take a moment to look up from your copy of the Metro and observe the different attributes surrounding you. You will note different hair colours, skin colours, heights and odours (unfortunately on the tube this one is unavoidable, even if you don’t look up). It is crazy to think that we all descended from one ‘man’ and ‘woman’ millions of years ago.

 

While our exteriors provide us with a unique fingerprint for daily life, this diversity is all constructed within our cells. Internally, our bodies are filled with different proteins working hard to keep you functioning. Most of these proteins function identically in all humans, as the majority have specific jobs vital to human survival. But some proteins can vary in their functionality, making you even more unique than what you can see by eye. You can think of this distinction using the example of eyes and eye colour; you can recognise somebody having a different eye colour to you, but they still have eyes. The eyes are the ‘essential’ trait whereas the colour is variable.

 

This variation from person to person is caused by mutations. Yes, that’s right; we are all mutants of some form or another. Not quite green, slimy, 7-eyed, 23-finger mutants but we all carry mutations involving small changes to your internal coding which results in alterations to how you function. So, let’s dive in (webbed toes not essential) to learn a bit more about why we are unique.

It's in my DNA

You have probably heard of DNA, and you probably associate the term DNA with what makes you individual. DNA is what you inherit from your mum and dad to give your body every instruction it needs on how to make you ‘you’. Your DNA can be used to identify where your ancestors have come from, implicate you in a crime and dictate whether you think coriander tastes like soap or not (it absolutely does). But what actually is it?

 

DNA is an abbreviation of DeoxyriboNucleic Acid. Although a bit of a mouthful, this name tells us a little bit about where you can find your DNA and what it is. DNA is found in every cell in your body which contains a nucleus. At school, you may have been taught the nucleus is the ‘brain of the cell’, and that is because it contains your DNA: every instruction necessary to make the cell function.  

 

“Every Instruction?” I hear you ask. “How long must this DNA code be?” In one cell, the DNA code would be about 2 metres long if stretched out, and considering you have trillions of nucleated cells, you could stretch your total DNA code around the solar system, twice. However, this code is coiled up really tightly within each cell and split into 23 different parts to enable it to fit inside this tiny space. These individual parts are called ‘chromosomes’ and you possess two copies of each: one inherited from your mum (found in the egg) and one from your dad (carried by the sperm). Therefore, you have 46 chromosomes forming 23 pairs, meaning you have two separate copies of the DNA code.

If a DNA code is so long and codes everything about you, it must be super complex, right? Actually, the DNA code is made up of 4 main components. These 4 components are called bases and are denoted A (adenine), C (cytosine), G (guanine) and T (thymine). The DNA code is simply billions of As, Cs, Gs and Ts written over and over again in many different combinations. Each DNA strand has a complimentary strand with each base’s corresponding base (A-T and G-C). This is so when you DNA is copied; no mistakes are made as human DNA is made up of approximately 3 billion bases. 99% of this code is identical in all of us, therefore that 1% difference is what creates our diversity.

 

Specific sequences of bases within your DNA are what forms your genes. Humans have approximately 23,000 genes and they can vary in length from 80 bases to 80,000 bases. As you have two copies of the DNA code (1 from mum and 1 from dad), you have two copies of each of these 23,000 genes. However, your genes only account for 1.5% of your DNA. The other 98% is called ‘non-coding’ DNA, which until recently was thought of as ‘junk DNA’. Some roles for this junk have be identified but that could be a wholeeeee other post. Let’s just stick to our genes and the proteins they form for now.

Follow My Simple Instruction: DNA to Protein

When you hear the word protein, the image of a hench bodybuilder drinking a concoction of raw eggs and powder whilst bench pressing 300kg may come to mind. But eating protein is not all about building huge muscles. Proteins are essential components of the human diet and they can be obtained from many different sources. What dietary protein does is provide your body with building blocks to create new proteins from your genetic code.

 

First and foremost, what are proteins? Proteins are molecules responsible for carrying out every job in your body. If you think of your body as a giant factory, your proteins form the workers, the equipment and the machinery which are constantly working to keep everything running smoothly. This includes clotting blood when you cut yourself to producing a suntan after a day on the beach. Proteins are found either within our cells (intracellular) or outside our cells (extracellular). The genes in your DNA provide the instructions for how to make every protein you need to survive.

The process of making these new proteins is a simple case of following instructions from your DNA code and having all the correct building blocks present; just like piecing together a flat pack from IKEA. But instead of a book containing ambiguous diagrams, the DNA code provides extremely accurate coded instructions, and instead of screws and MDF wood, the raw materials needed to make proteins are amino acids. There are 20 essential amino acids and each protein is formed of different combinations of these 20. Each amino acid is coded by a combination of 3 DNA bases. For example, the sequence ‘TAC’ within a gene would code for the amino acid ‘tyrosine’.

 

The first step in the protein-making process is transcribing the gene of interest from the DNA code into an individual, separate code (called RNA). This small code is then used as a template for the protein to be made. Amino acids corresponding to the template code line-up and join together as a strand and then fold to create the shape of the protein. The initial shape involves small interactions between neighbouring amino acids (secondary structure) which then form more complex interactions across the whole sequence (tertiary structure). When multiple tertiary structures of the same protein join together, this is termed a quaternary-structured protein.

Caution: Mutation Risk

Most mutations which cause the variation we can both see and not see are present in your DNA before the protein production process begins. Most genetic mutations exist as a change, an addition or a deletion of a single DNA base within a gene sequence and the downstream effect of this alteration can have big consequences. This can alter the entire functionality of a protein, independent of its size, meaning it cannot function in the way it is meant to.

 

Let’s take for example a gene which has the sequence TAC coding for tyrosine. If the single A base is changed to a C base, the new sequence (TCC) codes for a different amino-acid; serine. Therefore, a serine amino acid code would be transcribed from the DNA sequence and a serine amino acid would be added to the protein instead of tyrosine. As amino acid interactions are what forms the protein’s shape, this small change may lead to a huge alteration to the final protein structure. As this structure is vital for function, a slight change can alter how the protein works or render it defunct.

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A real-life example of this process can be seen in some individuals with sick-cell anaemia. The coding sequence for the protein called b-haemoglobin, which is responsible for carrying oxygen within your red blood cells, has a CTC (glutamic acid) sequence replaced with a CAC (valine). This causes the red blood cells to change shape from a round cell to an elongated ‘sickle’ cell. These cells are stickier and less flexible than rounded blood cells so are less able to carry oxygen efficiently around the body and can cause vessel blockages.

 

Some mutations are inherited from your parents whereas others can occur ‘de novo’ during development. If the mutation is present from very early on during pregnancy, it will be present in the DNA of all your cells, so all the resultant protein will have an altered function.

That concludes an introduction to your DNA, how proteins are made and how mutations make us all unique. By understanding this fundamental process, you can understand how a lot of genetic diseases occur. There are many different types of mutations which introduce different problems downstream in the protein production line. These will be discussed in future posts, along with the importance of mutations. If we did not have genetic mutations, there would be a much higher abundance of fatal recessive diseases and much less diversity across all species on the planet.

 

This article will be a base for many future posts on genetic disorders, protein abnormalities and DNA architecture so I will be linking back to it frequently. As always, if there are any questions, feel free to contact me or look at the extra reading below. For now, make sure you are getting all 20 of those amino acids in your diet to help your body run smoothly.

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