BBC Science The Theory of (nearly) Everything 2019

THE STRUCTURE OF DNA

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Get to grips with the

structure of DNA with

these key terms

DOUBLE HELIX

The two chains of DNA are

coiled together, a bit like a spiral

staircase, in which the paired

bases form the steps and the

sugar-phosphate backbones

form the rails.

NUCLEOTIDE

The ba sic sub-unit of DNA. E ach

nucleotide is made up of a ba se

• the ‘letters’ of DNA: adenine
  (A), guanine (G), thymine (T) or
  cytosine (C) – a sugar and a
  phosphate group. The
  nucleotides form the two
  parallel complementary chains
  of DNA , with adenine matched
  to thymine and guanine
  to cytosine.

PHOSPHATE GROUP

A phosphorous atom

surrounded by ox ygen atoms.

Phosphate groups, along with

deoxyribose sugars, make up

the ‘backbone’ of the long

DNA molecule.

X-RAY

CRYSTALLOGRAPHY

This is the study of the

s truc ture of cr ys t als by f iring

X-rays at them. The X-rays

bounce off the regular

arrangements of atoms in

crystals and the patterns they

make are captured on film. An

equation is then used to work

out the crystal’s structure.

Wilkins went on to share the Nobel

Prize in 1962.

Watson and Crick wrote in their

1953 paper: “It has not escaped our

notice that the specific pairing we

have postulated immediately suggests

a possible copying mechanism for the

genetic material.”

In the years since 1953, researchers

have learned how DNA copies itself

and how its strings of As, Ts, Cs and

Gs provides a template for making

proteins. More recently, analysis of the

human genome has allowed scientists

to glimpse the intricacies of how DNA

orchestrates life.

Cs to Gs, producing a ladder-like helix

with the paired bases forming the

rungs and the sugar-phosphate

backbones the sides.

Model completed, the pair went for

lunch in a nearby pub called The Eagle

and declared that they had found the

meaning of life. When the King’s team

visited this time, they accepted the

model immediately. “Rosy’s instant

acceptance of our model at first

a mazed me,” Watson w rote later.

“Nonetheless... she accepted the

fact that the structure was too

pretty not to be t r ue.”

Crick and Watson’s structure was

published in the journal Nature in

April 1953, along with two articles

from King’s. None revealed the role

that the King’s data had played and

Franklin died in 1958, perhaps never

having k now n. Watson, Crick a nd

James Watson (left) and Francis Crick

(right) pose with their model of DNA that

reveals its double-helix structure. They

won the Nobel Prize for their discovery,

which they shared with Maurice Wilkins

“The instant I saw the picture, my mouth

fell open and my pulse began to race”

James Watson on seeing ‘Photo 51’

NEED TO KNOW

by K A T H E R I N E N I G H T I N G A L E

(@kathnightingale) Katherine is a science writer

with a masters in molecular biology.