IB Biology Year 1
Cairns' Experiment
John Cairns' 1963 experiment showed that replication of deoxyribonucleic acid was a highly coordinated
process where the DNA strands are unwound when replicated, and replicated semi-conservatively.
Cairns made E.coli DNA radioactive by growing cells in a medium containing thymidine (that's what we
call the DNA nucleotide that contains the base thymine) labeled with tritium 3H. At first he allowed just
one replication cycle to occur. When the DNA was carefully isolated, spread on a slide, and overlaid with
a photographic emulsion for several weeks (autoradiography – remember Calvin's lollipop experiment?),
the radioactive thymidine residues generated "tracks" or rings of silver grains in the emulsion. He looked
at these rings under a light microscope. They revealed that the intact chromosome of E. coli is a single
huge circle. (Other scientists had hypothesized a circular DNA molecule for E. coli from genetic analysis).
Cairns interpreted each ring as having one labeled ("hot") strand and one unlabeled ("cold") strand. In
figure 1, the cold strand is dark blue and the hot strand, light blue.
Figure 1
Hot DNA isolated from the cells during a second replication showed an extra loop (see the second figure).
Cairns concluded that the loop resulted from the formation of two radioactive daughter strands, each
complementary to a parent strand. Both ends of the loop are dynamic points, termed replication forks,
where parent DNA is being unwound and the separated strands quickly replicated. The density of the
silver grains is consistent with a semi-conservative mechanism of DNA replication; one strand was ½ and
½, labeled and not; the other had twice as much silver indicating that both strands were labeled.
Figure 2
Observing many molecules corresponded with the progressive movement of the replication forks around
the ring.
Fun replication fact from Dr. John Locke, Dep't of Biological Sciences, U. of Alberta:
The E. coli chromosome is replicated in about 40 minutes. It is about ~ 4000 kilobases (kb) or in size. (1
kb is equal to 1000 base pairs of DNA, so 4000 kb is about 4 million base pairs.) Therefore, each fork
replicates 2000 kb in 40 min. or ~ 50 kb/min or ~ 800 bases/sec!
Sources consulted
Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H.
Freeman;
2000. Replication of DNA. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21790/
Locke, J. Biology 207, Fall 2011. Lecture # 2, "DNA Structure and Replication." Available from:
http://www.biology.ualberta.ca/courses.hp/bio207.hp/locke/lec_02_F11_OHL.doc.