When your paragraphs show a progression in time, use that
natural chronology to link them.
In 1969, when relations
between China and the Soviet Union were at their worst, China provoked
a series of skirmishes, mostly along the Heilongjiang border. Harbin's
government, believing a Soviet invasion to be imminent, set about building
underground corridors, about three kilometres long, that were meant
to house the whole of the city's population in the event of an attack.
These were kept meticulously ready until 1985, when peace broke out.
Now they have a new use. The corridors have been turned into
a thriving temple of free enterprise selling the latest fashions from
Hong Kong. With the shelter the corridors offer from Harbin's -25°C
cold, and with the hundreds of jobs this subterranean market has created,
they must surely be Russia's greatest gift to the chilly city.
The old model was
simple. Information was stored in the DNA of genes. When needed, it
was transcribed into template molecules known as messenger RNAs. Then
a piece of machinery called a ribosome translated the template, constructing
a protein as it went.
Later the model got a bit more complicated.
Genes, it was discovered, consist of lengths of informative DNA interspersed
with apparently meaningless stretches known as introns. Before a messenger
RNA template can be copied into proteins the introns must be removed
from it—a process known as splicing.
Now things are getting more complicated still.
In the past few years a new phenomenon has been discovered. Sometimes,
after the template has been made and the introns removed, the RNA is
edited. Sometimes, indeed, it is edited heavily. In the most extreme
examples known so far, more than half of the information needed to make
a protein has not come from the original gene. Instead, it has been
edited into the messenger RNA template.
