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We have all heard of DNA but do we really understand what it is and what it does?
DNA (deoxyribonucleic acid) is found in all living organisms from bacteria to humans and everything in between. It is our genetic material and contains our blueprint.
Find out here how DNA shapes our future.
THE DOUBLE HELIX
In 1953 James Watson and Francis Crick, along with a team of eminent scientists at the Cavendish Laboratories in Cambridge, discovered the structure of DNA.
Imagine a ladder that is twisted into a spiral – this is the basic ‘double helix’ structure that Watson and Crick discovered. The ladder is made up of building blocks called nucleotides; the outside posts consist of a sugar phosphate while the rungs are made from pairs of four special chemicals called bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C), which always pair up in the same way, A with T and G with C.
The way these letters pair up, rung after rung, is what makes each of us unique. The order of these letters forms an instruction manual for every cell in our bodies.
CHROMOSOMES AND GENES
There are billions of cells in the human body, each one crammed with DNA. Inside every cell (except red blood cells) are 46 chromosomes (these are the DNA ladders). Chromosomes come in pairs – 23 from the mother and 23 from the father.
If you consider that each chromosome (or DNA ladder) has its own language made up of thousands of words (which are all three letters long), then it is in fact the ‘sentences’ within the chromosome that make up our genes – the most important sub-units of DNA. Genes carry the instructions or ‘recipe’ for making proteins; these proteins control the growth and function of every cell in the body.
*In every 23 pairs of chromosomes is a pair of sex chromosomes. Females will have two ‘X’ chromosomes and males will have one ‘X’ and one ‘Y’ chromosome".
GENETIC EXCHANGE
The number of cells in your body is not fixed: cells are constantly reproducing themselves to allow for human growth and repair. New cells are made to combat damage, due to injury and general wear and tear, and to replace dying cells, some of which have a very short life span,
When a somatic cell (all cells of the body that are not sex cells) divides, the DNA uncoils and separates into single strands like a zip; then a new replica of the missing strand is made, so that each new cell has the same genetic make-up.
When a sex cell divides to become an egg or sperm, it replicates its DNA in the same way as a somatic cell, but then divides again and this time each new cell retains only a single chromosome. As a result, when egg meets sperm, the two chromosomes pair up, one from each parent, to give the full complement of chromosomes.
Sometimes during a normal cell division, bits of chromosomes “jump” across to the opposite strand in a process called recombination. This is how genetic material is exchanged between individuals and how the gene pool is constantly being “shaken and stirred”.
Sperm and eggs have only one sex chromosome so the sperm can donate either X or Y whereas the egg can only donate an X chromosome, so it is in fact the male that determines the sex of the baby.
GENOMES AND THE GENE POOL
We have around 30,000 – 50,000 genes in our chromosomes and these make up our genome, which is all the genetic material required to make a complete person. The human genome project was set up in 1988 to map all the genes in our chromosomes to provide a better understanding of diseases for advancements in medicine.
The location of a gene on the chromosome is called the locus and a variable of that gene is called an allele. Paired chromosomes carry the same genes at the same location on the DNA ladder, but because one comes from the mother and one from the father they may carry different instructions.
The gene pool is the sum total of all the alleles for all genes in a population. ‘Mutations’ to genes, caused by radiation, sunlight, smoking and chemicals or by an invading organism (such as a virus) can result in further alleles at a specific point on the chromosome, thus altering the gene pool.
Natural mutations occur all the time, particularly during the process of DNA replication. Most are of little use, many are harmful and do not continue in the gene pool; but few convey benefit and remain. This is how genetic evolution happens and over millions of years can make significant inheritable changes.
INHERITANCE AND HOW IT WORKS
Long before the structure of DNA was discovered, a monk called Gregor Mendel studied the patterns of inheritance of a wide range of characteristics in plants, insects and people. In 1865 Mendel worked out a theory to explain how characteristics segregate and are inherited, which formed the basis for what we call genetics and it still works today, although it is a bit more complex than Mendel anticipated.
Our characteristics are determined by the instructions provided by the genes we inherit from our parents. If we inherit the same allele of a gene from both parents, this is known as being homozygous for that gene; however if we inherit different alleles we are then heterozygous.
Some characteristics are expressed as a mix between the two parents; others are the result of either one or the other. For some characteristics, one allele of a gene is dominant over the other recessive allele, and the dominant instructions override the recessive instructions. This is how eye colour is determined, for example, and the same principle applies to inherited diseases. If a gene is dominant, only one copy of the gene is needed to pass on the characteristic or disease (such as brown eyes or Huntington’s disease). If a gene is recessive, then both copies are needed to manifest the characteristic or disease. With one copy of a recessive gene, a person will be a carrier but not have the characteristic or disease (such as blue eyes or albinism). Therefore there can be a difference between the message your genes contain (genotype) and how the message is manifest (phenotype).
Patterns of inheritance also define whether genes are inherited independently or in groups, in which case they are called linked genes. The closer together genes are on the chromosome, the more likely they are to be inherited together. An unrelated gene is sometimes found in people who share certain characteristics, in which case that gene becomes the “marker” for that characteristic, which is particularly helpful when looking for “markers” for diseases.
GENES AND DISEASE
DNA has its own repair mechanism so when mutations happen, they can be fixed without any damaging effects. Sometimes this doesn’t work and the result is a malfunction of the DNA. A mutation in a gene is how many diseases originate (thanks to abnormal or altered proteins being produced).
Some diseases, such as cystic fibrosis, Huntington’s disease and muscular dystrophy are caused by a single gene mutation; others may be caused by mutations of multiple genes, such as Alzheimer’s disease. ‘Linked genes’ associated with a disease can increase the risk of developing that disease, such as in type 2 diabetes, where mutations in at least nine genes have been found to increase risk.
Cancer cells are essentially cells that no longer obey the rules of normal cell growth and are caused by mutations of one or more important genes that control cell division. Again there are genetic “markers” for some cancers, such as breast cancer, and these are inherited.
