One of the most paradoxically simple and complex things about the structure of DNA is how it
replicates. DNA replication is how the DNA makes an exact copy of itself, which it needs
to do before the cell divides. In this video I’ll focus largely on the DNA replication
in prokaryotes. To understand DNA replication clearly, it helps to understand the structure
of DNA. The nucleotide is the base unit of the DNA molecule, which is made of phosphate,
deoxyribose sugar and a nitrogenous base. This base is adenine but there is also thymine,
guanine and cytosine. The nucleotides covalently bond to other nucleotides at the sugar and
the phosphate. The 5 carbon atoms of the deoxyribose sugar are numbered like this. 1 prime, 2 prime,
3 prime 4 prime and 5 prime. Nitrogenous bases always attach at the 1 prime carbon, and phosphates
attach at the 3 prime and 5 prime carbons of the sugar. Nucleotides can only be added
to the 3 prime end of another nucleotide. Because of this, bases are added in the 5
prime to 3 prime direction of the DNA molecule. So on this bit of DNA, the 3 prime ends have
the sugar, and the 5 prime ends have the phosphate molecule. The two strands of DNA are antiparallel,
so the 5 prime and 3 prime ends are opposite on each strand. Now that we’ve reviewed
some important DNA information, we can look at DNA replication. At the start of DNA replication,
DNA gyrase and DNA helicase work together to unzip and stabilize the DNA. Helicase unzips
the DNA by breaking the hydrogen bonds between the bases. But because DNA is helical and
can also be supercoiled, DNA gyrase is used to help relieve the twisting, torsional strain
of unwinding the DNA. As DNA gyrase and helicase move along the DNA strand to unzip the DNA,
single stranded binding proteins will attach to the unzipped strands to prevent them from
re-annealing. There are now two open strands of DNA to be replicated. In order initiate
replication, DNA primase generates short RNA primers on each strand. These provide a binding
site for DNA polymerase III which will covalently join the free nucleotides on the newly forming
complementary strands. DNA polymerase III can only add nucleotides to the 3 prime end
of a primer so it must move in the 3 to 5 prime direction. DNA polymerase III can work
continuously on the leading strand but works in discontinuous sections, called Okazaki
fragments, on the lagging strand. Now the last thing to fix on the DNA are those RNA
primers that were placed at the beginning by DNA primase. DNA polymerase I removes RNA
primers and replaces them with DNA. There are still unjoined nucleotides between the
Okazaki fragments so DNA ligase will covalently join the fragments together. Now we have two
identical strands of DNA! In summary, DNA gyrase and helicase work to unwind and unzip
the two DNA strands. Single stranded binding proteins keep the strands from re-annealing.
DNA primase adds RNA primers so that DNA polymerase III can bind and join the free nucleotides.
DNA polymerase I removes the RNA primer and replaces it with DNA and DNA ligase joins
the Okazaki fragments by covalent bonds. Scientists can learn the sequence of As Ts Cs and Gs
of DNA by stopping DNA replication. They use dideoxyribonucleic acid, which cannot make
covalent bonds and stops replication. Small amounts of dideoxyribonucleic acid with the
different bases are added to normal PCR mixtures and will be incorporated into the DNA, but
when they are, they stop the replication creating fragments. Gel electrophoresis separates the
fragments to determine the base sequences according to ascending sequence length, and
machines can automatically determine the sequence if the dideoxyribonucleotides are fluorescently
labeled. Thanks for watching this episode of Teacher’s Pet, don’t forget to like
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