X-ray crystallography. X-rays are unique in that their wavelength is about the length of bonds within molecules. When X-rays hit a crystallized molecule, the electrons surrounding each atom cause the beam to bend. Because there are many atoms the result is that when the X-rays exit the crystal and fall onto a photographic plate, they produce a series of light and dark patches. Measuring the intensity and relative position of each patch indicates the relative positions of atoms within the crystal.
With her co-worker John Desmond Bernal (1901-1971) Hodgkin produced the first diffraction patterns for proteins.
‘Description of the basic helical structure of the DNA molecule’
Her work is used, unaccredited, in Watson & Crick’s Nobel Prize-winning paper, from information ‘secretly’ leaked from Maurice Hugh Frederick Wilkins.
Through her work on X-ray diffraction, she realised that the ‘backbone’ of the DNA molecule was on the outside.
By 1952, Franklin had taken the clearest pictures of the molecules to date, which provided evidence of a helical, or spiral structure.
Watson & Crick would eventually articulate a ‘double-helix’ construction.
‘The self reproducing genetic molecule DNA has the form of a double helix’
WATSON & CRICK
The structure explains how DNA stores information and replicates itself.
The helical strands of DNA (deoxyribonucleic acid) consist of chains of alternating sugar and phosphate groups. Four types of base – adenine (A), cytosine (C), guanine (G) and thymine (T) – form the rungs of the DNA ladder, which can only be linked by hydrogen bonds in four combinations: A-T, C-G, T-A, G-C.
The DNA code is based on the order of these four bases and is carried from one generation to the next. The sequence of base pairs along the length of the strands is not the same in DNAs of different organisms. It is this difference in the sequence that makes one gene different from another.
‘All humans have evolved from a single woman – dubbed Eve – or, more likely from a small group of women, who lived about 200,000 years ago in Africa’
This hypothesis has been proposed after examining the mitochondrial DNA from 147 individuals from Africa, Europe, Australia and Papua New Guinea.
The mtDNA is passed to the next generation only in the mother’s egg cell – with no contribution from the father because the sperms’ mitochondria do not survive fertilisation. The mtDNA of an individual is thus inherited from the female line.
133 different mtDNA types were used to draw an evolutionary tree that relates these types to each other and to a derived ancestral mtDNA tree. Their mtDNA tree had a common primary root of descent.
Not all scientists support the hypothesis. They argue that humans originated about one million years ago in different regions of the world.