82 JUNE2013|COMPUTERSHOPPER|ISSUE304
RICHARDIII
82 JANUARY2021|COMPUTERSHOPPER|ISSUE395
Genetic information is encoded in the two strands of the DNA
molecule’s double-helix structure by so-called nucleobases,
often referred to as just bases. There are four bases: guanine,
adenine,thymine andcytosine,orG,A,Tand C. The order in
which theyappear along the strands provides the genetic
information, and it’s this order that DNA sequencing reveals.
Between the two strings, the bases formpairs, but Gs will
only pair with As, and Ts with Cs. This is the secret to passing on
genetic information. When cells divide,the double helix unzips,
and because each base type is selective about which base it
pairs with, the double helix is automatically reconstructed as
two identical copies, as each single strand attracts the necessary
bases fromapool of Gs,Ts, As and Cs to createthe new strand.
Asimilar chemical reaction is the keytoDNA sequencing.
DNA profilingrequires lots of copies of the DNA molecule.
Forclarity,the DNAinthe diagram(see 1) is much shorter than
in reality and the double helix has been unwound. The first
step is to amplify the DNA, which means making lots of copies.
This involves achemical process called the polymerase chain
reaction (PCR), which works in asimilar waytoDNA duplication
during cell division. During the reaction, every DNA molecule
becomes two,soperforming the process repeatedly increases
the number of molecules exponentially (see 2).
Once you have lots of copies, the sequencing can begin. This
also uses aprocess similar to that involved in cell division. First
the double helix is heated so that its strands separate. (3).
Then the bases are added, along with aso-called primer that
causes only one of the two strands to start reconstituting.
We’re using the strand shown in (4) in our example,
although either could be sequenced as the order of one
determines the order of the other.
As well as the ordinary bases, asmall percentage of
modified versions of the four bases is added to the mix
(represented by the hollow circles in our diagram). The
chemical structure of these modified bases ends the
replication process prematurely.Because there are lots of
copies of the DNA and the mixture contains just afew
modified bases, prematurely terminated strings of many
different lengths will be produced (5). These will eventually
separatefromthe strand on which theywere grown.
As well as terminating the string, the modified bases
have afluorescent element that fluoresces adifferent colour
foreachbase. The next step is to draw the assorted strands
through atube filled with gel while subjecting themto an
electrostatic field. This sorts the strands by size (6) with
the shorter strands emerging first. As theyemerge,the
florescent base that terminated the reaction is detected and
the colour recorded (7). The order in which the bases are
detected corresponds to the order of the bases in the
strand, letting you read the sequence.
DNA SEQUENCING
⬆The Sanger method of DNA seqencing interrupts the usual process of DNA replication with fluorescent versions of thebases that make up the
genetic code.Thisproduces strands of various lengths, and sorting these by size enables scientists to read the sequence
HOWSANGERDNA SEQUENCING WORKS
