DNA – Introduction to Deoxyribonucleic Acid

Simply, DNA –(transcription)–> RNA –(translation)–> Proteins. (DNA is the template for protein)

Proteins are useful for structure, metabolism, organisation and development…quite essential. What the protein does is dependant on the genes activated and the tissue it is being produced in. A few examples include Keratin which is part of hair (a fibrous protein) and Haemoglobin which is found in red blood cells and binds to Oxygen (a globular protein).

How do we know DNA is the genetic material…and not protein?

In 1928, Frederick Griffith ran experiments with Streptococcus Pneumoniae where he demonstrated how harmless strains could be turned into virulent, harmful strains. He did this by mixing heat killed (therefore protein structure broken) virulent bacteria with live non-virulent forms; resulting in a permanent transformation to a virulant form.

Virulent mixed with non-virulant bacteria

Virulent mixed with non-virulent bacteria

A later discovery in 1944 by Avery was that it was DNA from the virulent cells – not the protein – as DNA is only destroyed by DNase and not by proteases or RNase…which would destroy protein, leaving only DNA.

Later experiments further confirmed this

1. DNA Deoxyribonucleic Acid.

  • Discovered in 1869 in the cell nucleus of Eukaryotes. In Prokaryotes (eg Bacteria) it is not membrane bound and so is ‘loose’ in the cell.
  • Strands of DNA form Chromosomes in Eukaryotes.

– Chromosomes

  • Named Chromosomes because special dyes could pick out AT and CG rich bands – allowing structures to be seen (see Fig 1).
  • Chromosomes are formed from DNA tightly wound round structural proteins such as histone proteins.
  • Cells of a species have the same number of chromosomes. (eg. Humans have 23 pairs in every diploid cell, and 23 in every hapoid cell.)
  • All cells apart from gametes (sex cells) are diploid…aka have a two of each chromosome (so in humans, 46 total). Gametes are haploid and only have one copy of each chromosome (in humans, 23 total).
  • A pair or set of identical chromosomes are called homologous chromosomes.
  • The full set of chromosomes (in humans, all 23) is referred to as the karyotype.

Whilst the number of genes = the complexity of species, the same rule is not true for the number of choromosomes as any particular chromosome may have a greater amount of DNA in it, and so any number of genes on it.

  • Also worth noting is that DNA contains sections of junk DNA which has a purely structural role – in humans only 9-27% is coding DNA (so only 9-27% can be used to synthesise proteins, the rest of the DNA strand is there to make sure the DNA can be stored correctly in chromosomes etc).

Fig 1 - high resolution image of a Chromosome showing different areas.

A Chromosome is made up of 2 chromatids joined at the centromere (also the point where spindle fibres attach during cell replication & division). There are 46 chromosomes (23 pairs) in a human; as shown.

Fig 2 - Showing Karyotype of Human Male

Note how there is no chromosome pair 23 on Fig 2. This is because the X and Y (or the sex chromosomes) form the final pair. Having 1 X and 1 Y chromosome results in a Male while 2 X chromosomes result in a female.

Offsite reading:

  1. http://www.ebi.ac.uk/2can/bioinformatics/dna.html – More detailed history of DNA discovery from the European Bioinformatics Institute.