Virtual Medical Centre

DNA (Deoxyribonucleic Acid)

• Introduction to DNA
• What is DNA?
• Where is DNA found?
• What does DNA look like?
• Where does DNA come from?
• Why do cells divide?

Introduction to DNA

Humans are made up of billions and billions of cells, all these cells working together in order maintain life. All our organs are made up of different types of cells, for example our largest organ, the skin, is made up of skins cells that are all tightly adhered together to function as a protective barrier for the rest of our body. The cells of the skin have different functions and characteristics than cells that make up our heart, or the ones that line our stomach. It is the specific functions of cells that allow us to live.

So how do these cells perform certain functions? How does a new cell “know” that it is a skin cell and not a brain cell? And once it knows it is a skin cell how does it know how to perform functions that skin cells need to do? And how does it know if it has been damaged by the sun and needs to be replaced by another skin cell?

Well cells do not have a brain, so they do not “know” anything, they instead are controlled by a very important chemical; deoxyribonucleic acid, or as it is better known, DNA. Every characteristic, quality, function, appearance and location of a cell is determined by DNA, which means that every characteristic, quality, function and appearance of our body is also determined by our unique DNA.

What is DNA?

DNA is very complex and very tightly regulated chemical sequence that contains all the information our cells require to grow, perform functions and replicate. The information is contained in gene sequences, which are particular stretches of chemical patterns within the DNA that make up our genes.

So firstly, what are genes? Genes carry our hereditary characteristics. One gene is responsible for one function in one type of cell. For example one gene is responsible for our eye colour. This eye colour gene sits on the same part of DNA in every cell in the body however it is only the in the cells that make up the iris that that gene will be expressed. The same way that the gene that controls your heart beat will only be expressed in the heart, although the cells in the iris will also carry these genes.

All the genes and genetic information required for life contained in a cell is called the cell’s genome. So you can imagine if each function of our cells and body is controlled by a separate gene then each of us needs a lot of genes. The Human Genome Project set out to determine just how many genes there are in our DNA and what these genes are responsible for. In 2001 the first draft of the Human Genome Project was released; 30,000 genes were identified! The Human Genome Project or “mapping of human DNA” has been one of the most informative biological investigations. Knowing where genes exist within the millions of chemicals that make up DNA has helped to characterise genetic defects and diseases and how these diseases can be treated or cured.

The genome is important in every aspect of cellular function, including cell structure, growth, movement and division. Though we all have a very similar genetic make up that functions in similar ways, there are lots of very small differences in our DNA and this is what makes each and every one of us unique.

Where is DNA found?

The same set of DNA is found in the nucleus of every cell in our body. The DNA is so tightly coiled and packed that it is estimated the nucleus of each human cell can hold about 2 meters of DNA. The DNA exists in 46 separate segments within the nucleus; these segments are known as chromosomes.

Each chromosome has a partner that contains the same genes in almost the same sequence of DNA, one member of the chromosome pair or homolog came from the father and one came from the mother during fertilisation. The chromosomes are what carry genes. An example of the genes carried by chromosomes are the sex genes, how the chromosome combination determines which gender you will be will be discussed later on.

What does DNA look like?

The shape of DNA at the molecular level is thought to look like a gently twisting ladder. Each of the rungs on the ladder represents a chemical bond between the chemicals that make up the DNA molecule. These chemicals are called nucleotides and include:

• Adenine (A);
• Thymine (T);
• Cytosine (C); and
• Guanine (G).

DNA is said to be “double-stranded” because it is made up of two sequences of nucleotides that are tightly connected together by chemical bonds. The chemical bond between the nucleotides always exists between A and T and G is always bound to C. For example;

• Strand 1: C – A - G – C - A – T – T – G
                  
• Strand 2: G – T - C - G - T - A - A – C

Chemical bonds exist between strand 1 and strand 2 that connect the nucleotide pairs The strands are complimentary to one another, by knowing the sequence to one you can figure out the other because of the specific chemical bonds. The very long DNA strands are coiled very tightly into chromosomes.

Where does DNA come from?

The sperm contains half the amount of chromosomes that exist in other cells of the body and so does the egg. Because there are only 23 chromosomes in the sperm and the egg, they are known as haploid cells whereas every other cell in the body contains 46 chromosomes and are known as diploid cells.

When the sperm fertilises the egg half of our chromosomes from our mother, maternal chromosomes, and half from our father, paternal chromosomes, are combined in the one cell (the egg) this is why we share certain traits with either parent. For example, you may have blue eyes like your father, but have your mother's blonde hair.

23 chromosomes from the father have a chromosome pair with each of the 23 chromosomes from the mother. These pairs are referred to as homologs. The homologs contain a very similar DNA sequence and therefore they contain the same set of genes. However the paternal chromosome that contains the hair structure gene will contain an alternative version of this gene than the maternal chromosome; one may be a curly hair gene whereas the other chromosome may contain a straight hair gene. An allele is the name given to one of these alternative sets of genes.

Our DNA is even further mixed up by a process called 'independent assortment' or 'chromosomal crossover' which change our genetic make up slightly and explain why siblings do not have the same genetic make-up, despite sharing the same parents. Cross-over occurs where segments of the one chromosome are replaced with the corresponding segment from its homolog and vice versa. Once all 46 chromosomes have paired up and cross-over has occurred then the cell divides.

Why do cells divide?

Cells divide for a number of reasons. Firstly cells must divide in order to create life. We all start out as a single cell, this cell must divide, then divide again then divide again and again, to become the millions of cells that make up the human body. As cells continue to divide they become the different cells in our body through a process called cell differentiation which will be discussed later on.

Even after we have been born, cells in our body are continually dividing, and hence we grow and change. For example our skin cells have quite a rapid life cycle and are continuously being replaced by new cells. Cells need to be continually replaced because all cells have a life cycle and they will eventually die. Cells die either because they only have a short life cycle and are programmed to die or because they have become damaged and need to be replaced in order to avoid cancer formation.