• Mon, Jul 23 2007

Tutorial 1: Genes and how they work

I’ve been asked to provide ’mini tutorials’ on various genetic topics  My tutorials are directed to those wishing to bring themselves up to speed on the basics of genetics.  So apologies to all genetics specialists who have spent lifetimes researching, publishing and commenting on this subject.  That said, I would welcome your comments, views and updates!


TUTORIAL 1: Genes and how they work

What is a gene?

Genes are the biological information inherited from your parents. They affect the way your body grows, works, and looks. Every single cell of your body is a tiny building block and contains all the information you inherited. The information is contained in 46 chromosomes (23 pairs) within the core (nucleus) of each cell. Chromosomes are like containers or filing cabinets filled with all the genetic information your body needs to work properly. Chromosomes can actually be seen through a strong microscope.

Genes are the actual ‘files’ contained in these chromosomal filing cabinets. It is estimated that each human cell contains around 30,000 genes. Different genes have different functions. Some genes clearly determine different things about us: for example, eye colour seems to be linked to a particular gene.

However, other things about us are the outcome of several genes and our environment interacting together. For example, a person’s weight and height are linked to the genes they have inherited (someone with tall parents is likely to be tall) and also to diet, exercise, childhood illnesses and so on. It seems that cancer, even if there is a strong family history, is almost always the outcome of an interaction between genes and the environment.

The actual ‘words’ inside our genetic ‘files’ are written in a ‘chemical language’ or ‘code’ that consists of four chemicals (bases), which are abbreviated to four letters: A, T, C and G. (A=Adenine, T=Thymine, C=Cytosine and G=Guanine).  These four letters, repeated over and over again in different combinations in our cells, contain all the information our body needs to function. This coded information is also called the DNA (this is short for deoxyribonucleic acid).

A genetic mutation is a ‘spelling’ mistake 

A genetic or DNA test looks at the order in which the chemical ‘letters’ of the genetic code are found within a gene. This is a bit like trying to read a big file or book – and checking it for any spelling mistakes. For example BRCA1, the first gene identified as playing a role in hereditary breast cancer (and therefore called (BReast CAncer one), consists of around 100,000 ‘letters’.

Each cell carries out a specific function for the body and it uses the information contained in its DNA to know what to do. So your genes work a bit like an instruction manual. Therefore, if there is a spelling mistake contained in any of the ‘files’ stored in your body cells, it can make things go wrong with the cell. A cell could die or could grow into tissue that is not quite right (like a cyst) or it could grow and multiply out of control and develop into a cancer.

A genetic test tries to find these spelling mistakes or genetic changes (mutations). A genetic mutation is not in itself a cancer or a disease, but it might mean that you are more likely to develop cancerous cells or develop other diseases like cardiovascular disease, than other people in the population.

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  • Derek Pretorius

    DNA Question
    I understand that the billions of DNA instruction containing ATC&G codons required are stored in the 46 chromosomes that are contained in the nucleus of every cell.

    My 3 question are;

    Does this mean that this string of DNA instruction is not one continuous string but starts in one chromosome then is broken off and then continues again in the next chromosome. In other words it the one string divided into 46 separate parts?

    Are the chromosomes all the same size, or do some contain more DNA than others?

    Can the chromosomes then be broken up into some sort of very basic functions, example; Could chromosome 1 be responsible for the protein required for internal organs, chromosome 2 for growth and reproduction, etc etc?


  • Grace Ibay

    hi Derek,
    Thanks for the thoughtful questions! You inspired me to prepare another tutorial, with more detailed answers about the chromosome and our genes. However, here are the quick and short answers to your questions.

    The chromosomes are different sizes and some do contain more DNA than others.
    The DNA string is not continuous from one chromosome to the next. A gene may be made up of a string of DNA found in a chromosome, but the same gene may be modified, controlled or activated by another set of DNA string (or genes) on some distance away in the same chromosome, or another chromosome.
    So, the chromosomes are not specific to any set trait or function.
    The genes are scattered throughout the genome and that’s why scientists are “looking” for genes (what they are and where they are located on the chromosomes) that control the different human traits.

  • Derek Pretorius

    Thanks for your response to my request for help.

    Coming from an engineering background I need to see the overall picture before getting bogged down in the details. I find it very hard to understand something if I can’t envisage it, therefore I try and break it in to simple pictures.
    With the help of your explanation I now see the nucleus of the cell, (in my oversimplified terms as) as a transparent plastic bag full of water. Inside that plastic bag I have 46 pieces of soft cooked spaghetti of different lengths floating around. This spaghetti nucleus contains the story of the building instruction of the cell.
    On the sides of these pieces of spaghetti a total story has been printed. The story is made up of 46 separate sentences. The story starts on spaghetti one, and like any good sentence the first letter is upper case to mark the start, the sentence then ends with a period to indicate the pause of the DNA. Then follows a postscript, which states the reader must go onto spaghetti two for the continuation of the story. Like any good story to read it one must start on sentence one and finish on the end of sentence 46.

    Great, this simplistic vision is making sense to me, so the next thing that I do is to open that plastic bag and lay all the spaghetti in a straight line, end to end starting from spaghetti 1 and ending with spaghetti 46. In my mind this is now stretched out for 20 metres in front of me.

    Now, the first thing that I start to look for in and story or set of building plans is an index. I would have though that by now science would have mapped some sort of plan on which DNA is doing what?

    I am told that the human code contains 3,000,000,000 base pairs of DNA and that these base pairs make up the 25,000 different genes.

    Right, now my next problem is what constitutes a gene?

    I understand that each cell has 25,000 genes but each cell only uses a select few genes, how does it know what to use?

    How would they know when one gene starts and ends, then the next one starts?

    I am told that an embryo receives 25,000 genes from each parent, making a total of 50,000 genes, but as the child will only have 25,000 genes what happens to the others?

    If Genes are made up of protein which is bonded amino acid, then by eating meat it would provides the protein the body requires, however eating plants does not provide any protein as the plants do not contain any. If that is true then how is the DNA of a plant bonded?

    You will see from my questions that the problems I have is in understanding the overall concept of cells, not in the detail, well not yet, I have not got to those questions yet..

    Regards Derek