DNA Science Basics
The DeoxyriboNucleic Acid (DNA) molecule contains everyone’s hereditary material and is the blue-print for all life. DNA is found within every living cell, wrapped up into chromosomes and comprising the genes that give us all of our physical features and all hereditary characteristics. The information is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
Cells are generally known as the basic building blocks of every living system (Fig. 1). In the 1600’s Robert Hooke noticed small cavities in plant material. He named them ‘cells’ meaning ‘little rooms’. With the advent of microscopy in the 1800’s, the cell theory was formulated. It states that all living matter is composed of cells. The human body is made up of billions of cells that each performs different functions. For example, every hair cell produces a single strand of hair. Each cell knows what to do because they are controlled by their specific DNA. This means that our body’s appearance and function is controlled by our unique DNA. All cells arise from other cells and the hereditary genetic material (DNA) contained within the nucleus is passed from generation to generation.
Figure 1: The structure of a typical animal cell showing all cellular features and structures.
The DNA of a human cell is enclosed within a nuclear membrane, called the nucleus, which is a large often spherical structure within the cell. There is approximately 2 metres of DNA in each of our cells. The DNA packs tightly together into the form of lateral structures known as chromosomes, 23 pairs in each cell (Fig. 2). Walter Flemming in the late 1800’s observed the ‘dance of the chromosomes’. This happens when a cell divides into two new cells and the chromosomes unwind and separate. This process is called mitosis. Mitosis gives rise to two new cells with the same number of chromosomes (same amount of DNA).
Figure 2: A diagrammatic representation of a chromosome’s structure.
Deoxyribonucleic acid (DNA):
The questions still remained, what was the structure of DNA within the chromosomes and how did it work? In 1953, Watson and Crick, working off the X-ray diffraction images of DNA (Photo 51) from their colleagues Franklin and Watkins, presented their model of DNA as a double stranded helical structure; which still holds true today (Fig 3).
DNA is considered to have the following features:
- Long double stranded helix.
- Contain four nucleotides, adenine (A), guanine (G), thymine (T) and cytosine (C). These four letters spell out the genetic code.
- Three nucleotides make up a codon encoding for one or more amino acid (Table 1).
- In DNA, G only pair with C and T only pair with A.
- Each nucleotide contains a sugar (deoxyribose), a phosphate group and a purine (A and G) or pyrimidine (T and C) base.
- The G bonds to C with three hydrogen bonds.
- The T bonds to A with two hydrogen bonds.
DNA is a very complex molecule that stores all the information that we require to live and reproduce. This information is stored in the form of genes, long sequences of nucleotides, which code for different functions. It is estimated that the complete human genome contains approximately 3 billion (3 000 000 000) nucleotides, giving rise to 30, 000 genes that determine our appearance. The genetic makeup, the combination of G, C, T, A’s, of an organism is known as its genotype. This genotype, in combination with environmental factors gives rise to the individual’s phenotype. Most scientists accept the rule;
Phenotype = Genotype + Environment
For more information and visualisation of human chromosomes see the Human Genome Project at http://www.ncbi.nlm.nih.gov/genome/guide/human
The DNA language:
Nucleotides (G, C, A, T) come together in triplets of bases, known as codons. There are a possible 64 combinations of codons using G, C, A and T. Each codon produces one amino acid. One amino acid can also be produced by one or more codons (Table 1). Chains of amino acids are the major components of proteins. There are three major types of proteins; enzymes, such as lactase that breaks down milk, hormones such as insulin or structural proteins such as collagen.
Figure 4: The crazy scientist showing how a chromosome is made up of a long strand of DNA. The DNA codes for chains of amino acids which make up the structure of proteins.
Crazy scientist in figure 4 shows how the chromosome unwinds into a long strand of DNA. We can read the DNA nucleotide sequence as GGA GTC GAC GAT GTC. Using Table 1 as a reference, we can convert the DNA sequence into a chain of amino acids Gly-Val-Asp-Asp-Val. This chain of amino acids then folds into a protein molecule.
The DNA Strand;
gag ttt gat cct ggt tca gga cta cgc tgg cgg cgt
codes for the protein strand;
Ala=alanine Arg=arginine Asn=asparagine Asp=aspartic acid Cys=cysteine Gln=glutamine Glu=glutamic acid Gly=glycine His=histidine ile=isoleucine Leu=leucine Lys=lysine Met=methionine Phe=phenylalanine Pro=proline Ser=serine Thr=threonine Trp=tryptophan Tyr=tyrosine Val=valine
Table 1: The genetic code: from this table showing all 64 codons, you can see which codon encodes for which specific amino acid. For example GCT codes for Alanine (Ala), TAT codes for Tyrosine (Tyr)