THE FUNDAMENTALS OF LIFE

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Gregor Mendel cross-bred different coloured peas

in some of the earliest experiments into heredity

THE STRUCTURE OF

DNA

Before the gene-carrying molecule DNA was discovered, we had no idea

of the mechanics of life. Katherine Nightingale reveals how describing

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T

he yea r is 1869 a nd a young
researcher is toiling away in a
laborator y in a n old castle in
Ger ma ny, on cou rse to ma ke a
remarkable discovery. The lab studies
the composition of cells and Friedrich
Miescher is analysing white blood
cells, which he ext racts f rom t he pus
in a local clinic’s discarded bandages.
Having exhausted his efforts in
classifying the cell’s proteins,
Miescher turns his attention to
another substance in his samples.
He finds it odd – an acid that contains
phosphorus – and declares he has
discovered a completely new type of
substa nce. Nuclein, or DNA as we now
call it, has been found.
Like any good sceptical scientist,
Miescher’s boss Felix Hoppe-Seyler
is wary and waits to repeat the
experiments before, two years later,
allowing publication. But this delay
would turn out to be negligible; it was
many more decades before scientists
saw the importance of DNA. Misecher
went on to find DNA in a variety of
cells, but even he couldn’t believe that
just one substance generated the
enormous diversity of life. As late as
the 1940s, most scientists thought that
proteins – large biological molecules
that come in all shapes and sizes –
were t he only substa nces complex
enough to be t he agents of heredity.

Chromosomes, the coils of DNA and

protein that contain genes, had first

been spotted in cells in the early

1840s. Later that century, researchers

saw them double in number and then

halve again into separate ‘daughter’

cells during cell division. In 1865, the

Aust ria n monk Gregor Mendel used

pea plants to explore theories on

genetic inheritance, proposing that

characteristics are inherited in

discrete units. When his research

was rediscovered in the early 1900s,

a f lu r r y of work deter mined t hat

t hese units, or genes, must be in

chromosomes. But what were they

made of: DNA or protein? And what

did they look like?

A German doctor named Albrecht

Kossel took some of t he f irst steps

towards finding out. Working under

Hoppe-Seyler in t he late 1800s, he

discovered DNA’s ‘bases’ and named

them thymine (T), adenine (A),

cytosine (C) and guanine (G). This

work was continued by Phoebus

Levene, a Lithuanian researcher

d riven to New York in t he ea rly 1890s

because of anti-Semitism in his

adopted home of St Petersburg.

The units of DNA

For three decades from the mid-1890s,

Levene studied the structure of DNA,

identifying its other components: a

sugar called deoxyribose and

phosphate groups. He also discovered

t hat DNA is made up of units he called

nucleotides. Each of these is made up

of a sugar, phosphate group and base,

and are linked by bonds between the

phosphate groups of one nucleotide

and the sugar of the next.

But this was as far as Levene’s

correct findings went. He thought that

each DNA molecule contained only

fou r nucleotides, one wit h each type

of base, linked toget her in a ring he

called a ‘tetranucleotide’.

Levene’s tetranucleotides were too

simple to carry a genetic code and 5