Biology Key Skills
Poodles make more poodles. Sheep make more sheep. Replication is a
basic fact of life. All living things make other living things that
are to one degree or another duplicates of themselves. What is the
mechanism behind all this? The answer lies in a molecule called DNA.
In 1869, Friedrich Miescher extracted a substance, which he called
nuclein from the nuclei of white blood cells. Nuclein later became
known as nucleic acid. Living cells contain two kinds of nucleic
acids-ribonucleic acid (RNA) which contains the sugar, ribose and
deoxyribonucleic (DNA) which contains the sugar, deoxyribose. Nucleic
acids are found in all living ...view middle of the document...
This unwinding is accomplished
by an enzyme known as DNA helicase. This unwound section appears under
electron microscopes as a "bubble" and is thus known as a replication
As the two DNA strands separate (unzip) and the bases are exposed, the
enzyme DNA polymerase moves into position at the point where synthesis
The start point for DNA polymerase is a short segment of RNA known as
an RNA primer. The term "primer" is indicative of its role, which is
to "prime" or start DNA synthesis at certain points. The primer is
"laid down" complementary to the DNA template by an enzyme known as
RNA polymeraseor Primase.
The DNA polymerase then adds nucleotides one by one in an exactly
complementary manner, A to T and G to C.
DNA polymerase is described as being "template dependent" in that it
will "read" the sequence of bases on the template strand and then
"synthesize" the complementary strand. The template strand is always
read in the 3' to 5' direction. The new DNA strand must be synthesized
in the 5' to 3' direction. DNA polymerase catalyzes the formation of
the hydrogen bonds between each arriving nucleotide and the
nucleotides on the template strand.
In addition to catalyzing the formation of Hydrogen bonds between
complementary bases on the template and newly synthesized strands, DNA
polymerase also catalyzes the reaction between the 5' phosphate on an
incoming nucleotide and the free 3' OH on the growing polynucleotide.
As a result, the new DNA strands can grow only in the 5' to 3'
direction, and strand growth must begin at the 3' end of the template.
Because the original DNA strands are complementary and run
antiparallel, only one new strand can begin at the 3' end of the
template DNA and grow continuously as the point of replication (the
replication fork) moves along the template DNA. The other strand must
grow in the opposite direction because it is complementary, not
identical to the template strand. The result of this side's
discontiguousreplication is the production of a series of short
sections of new DNA called Okazaki fragments. To make sure that this
new strand of short segments is made into a continuous strand, the
sections are joined by the action of an enzyme called DNA ligase,
which ligates the pieces together by forming the missing
The last step is for an enzyme to come along and remove the existing
RNA primers and then fill in the gaps with DNA. This is the job of yet
another type of DNA polymerase, which has the ability to chew up the
primers and replace them with the deoxynucleotides that make up DNA.
This form of replication has been refered to as semi-consravtive
replication, because each newly formed double helix contains one of
the polynucelotide chains of the original double helix.