An illustration to show the double helix structure of DNA.
DNA or deoxyribonucleic acid is a long molecule that contains our unique genetic code. Like a recipe book it holds the instructions for making all the proteins in our bodies.
- Your genome is made of a chemical called deoxyribonucleic acid, or DNA for short.
- DNA contains four basic building blocks or ‘bases’: adenine(A), cytosine (C), guanine (G) and thymine (T).
- The order, or sequence, of these bases form the instructions in the genome.
- DNA is a two-stranded molecule.
- DNA has a unique ‘double helix’ shape, like a twisted ladder.
- Each strand is composed of long sequences of the four bases, A, C, G and T.
- The bases on one strand of the DNA molecule pair together with complementary? bases on the opposite strand of DNA to form the ‘rungs’ of the DNA ‘ladder’.
- The bases always pair together in the same way, A with T, C with G.
- Each base pair is joined together by hydrogen bonds.
- Each strand of DNA has a beginning and an end, called 5’ (five prime) and 3’ (three prime) respectively.
- The two strands run in the opposite direction (antiparallel) to each other so that one runs 5’ to 3’ and one runs 3’ to 5’, they are called the sense strand and the antisense strand, respectively.
- The strands are separated during DNA replication.
- This double helix structure was first discovered by Francis Crick and James Watson with the help of Rosalind Franklin and Maurice Wilkins.
- The human genome is made of 3.2 billion bases of DNA but other organisms have different genome sizes.
There is a fundamental difference between invention and discovery. Humans invent things which are an infinitesimally small subset of nature’s work. DNA was discovered by Watson and Crick.It is a common misconception that James Watson and Francis Crick discovered DNA in the 1950s. In reality, DNA was discovered decades before. It was by following the work of the pioneers before them that James and Francis were able to come to their ground-breaking conclusion about the structure of DNA in 1953.The discovery of DNA took place long before Watson and Crick. The big question was: Is protein the basis of inheritance, or is DNA At that time, it was possible to differentiate between proteins and DNA because proteins contain sulfur (S) where as DNA does not. DNA however, contains phosphorus (P), an element that is fundamental to the “sugar-phosphate backbone.”
The story of the discovery of DNA begins in the 1800s…
The molecule of life
The molecule now known as DNA was first identified in the 1860s by a Swiss chemist called Johann Friedrich Miescher. Johann set out to research the key components of white blood cells, part of our body’s immune system. The main source of these cells was pus-coated bandages collected from a nearby medical clinic.
Johann carried out experiments using salt solutions to understand more about what makes up white blood cells. He noticed that, when he added acid to a solution of the cells, a substance separated from the solution. This substance then dissolved again when an alkali was added. When investigating this substance he realised that it had unexpected properties different to those of the other proteins he was familiar with. Johann called this mysterious substance ‘nuclein’, because he believed it had come from the cell nucleus. Unbeknown to him, Johann had discovered the molecular basis of all life – DNA. He then set about finding ways to extract it in its pure form.
Johann was convinced of the importance of nuclein and came very close to uncovering its elusive role, despite the simple tools and methods available to him. However, he lacked the skills to communicate and promote what he had found to the wider scientific community. Ever the perfectionist, he hesitated for long periods of time between experiments before he published his results in 1874. Before then he primarily discussed his findings in private letters to his friends. As a result, it was many decades before Johann Friedrich Miescher’s discovery was fully appreciated by the scientific community.
For many years, scientists continued to believe that proteins were the molecules that held all of our genetic material. They believed that nuclein simply wasn’t complex enough to contain all of the information needed to make up a genome. Surely, one type of molecule could not account for all the variation seen within species
The four building blocks of DNA
Albrecht Kossel was a German biochemist who made great progress in understanding the basic building blocks of nuclein.
KEY FACTAlbrecht Kossel isolated the five nucleotide bases that are the building blocks of DNA and RNA: adenine, cytosine, guanine, thymine and uracil.
In 1881 Albrecht identified nuclein as a nucleic acid and provided its present chemical name, deoxyribonucleic acid (DNA). He also isolated the five nucleotide bases that are the building blocks of DNA and RNA: adenine (A), cytosine (C), guanine (G), thymine (T) and uracil (U).
This work was rewarded in 1910 when he received the Nobel Prize in Physiology or Medicine.
The chromosome theory of inheritance
In the early 1900s, the work of Gregor Mendel was rediscovered and his ideas about inheritance began to be properly appreciated. As a result, a flood of research began to try and prove or disprove his theories of how physical characteristics are inherited from one generation to the next.
In the middle of the nineteenth century, Walther Flemming, an anatomist from Germany, discovered a fibrous structure within the nucleus of cells. He named this structure ‘chromatin’, but what he had actually discovered is what we now know as chromosomes. By observing this chromatin, Walther correctly worked out how chromosomes separate during cell division, also known as mitosis.
The chromosome theory of inheritance was developed primarily by Walter Sutton and Theodor Boveri. They first presented the idea that the genetic material passed down from parent to child is within the chromosomes. Their work helped explain the inheritance patterns that Gregor Mendel had observed over a century before.
Interestingly, Walter Sutton and Theodor Boveri were actually working independently during the early 1900s. Walter studied grasshopper chromosomes, while Theodor studied roundworm embryos. However, their work came together in a perfect union, along with the findings of a few other scientists, to form the chromosome theory of inheritance.
Building on Walther Flemming’s findings with chromatin, German embryologist Theodor Boveri provided the first evidence that the chromosomes within egg and sperm cells are linked to inherited characteristics. From his studies of the roundworm embryo he also worked out that the number of chromosomes is lower in egg and sperm cells compared to other body cells.
American graduate, Walter Sutton, expanded on Theodor’s observation through his work with the grasshopper. He found it was possible to distinguish individual chromosomes undergoing meiosis in the testes of the grasshopper and, through this, he correctly identified the sex chromosome. In the closing statement of his 1902 paper he summed up the chromosomal theory of inheritance based around these principles:
- Chromosomes contain the genetic material.
- Chromosomes are passed along from parent to offspring.
- Chromosomes are found in pairs in the nucleus of most cells (during meiosis these pairs separate to form daughter cells).
- During the formation of sperm and eggs cells in men and women, respectively, chromosomes separate.
- Each parent contributes one set of chromosomes to its offspring.
Image credit: Genome Research Limited
Knowledge source: https://www.yourgenome.org/facts/what-is-dna