Download Question 1: Decribe the inding of Hershey Chase Experiment and

Question 1: Decribe the inding of Hershey Chase Experiment and indicate its findings?
Answer
Hershey- Chase in 1952 did an extraordinary experiment to prove that the genetic
information is transferred to the next generation through DNA and not proteins. Experiment
was carried in 2 phases:
a)The bacteriophage were grown in S35 media so that the radioactivity is incorporated in
sulfur containing amino acids namely methionine and cysteine in phage coat proteins. This
phage was used to infect the E. coli cells which was followed by vigorous shaking and
centrifugation. The radioactivity was found in the supernatent that contained empty phage
coat proteins and no radioactivity was found in the pellet (Fig.1).
b) The bacteriophage were grown in P32 media so that the radioactivity is incorporated in
phosphorus contained in the DNA. This phage was used to infect the E. coli cells which was
followed by vigorous shaking and centrifugation. The radioactivity was found in the the cells
(pellet) that contained assembled new phage cells (with radiolabelled DNA) and no
radioactivity was found in supernatent (empty protein coats of parent phage) (Fig.1).
This experiment proved that it is the DNA that contains genetic information that is
transferred from the parent to next generations.
Fig.1: Steps involved in Hershey and Chase Experiment
Question 2: Differentiate between Z and B DNA.. Which form is amendable for binding to
transcription factors? How might it affect gene expression?
There are 3 helical forms of DNA: A DNA, B DNA, and Z DNA. B DNA is the most
commonly occurring helical form of the DNA. A comparison between B and Z DNA are
specified in table 1.
The transcription factors are known to bind to the major groove of B DNA that promotes
transcription and hence enhances gene expression. On the other hand, CpG islands tend to
form Z DNA upon methylation which prevents the binding of transcription factors and
promotes the recruitment of transcription repressors, hence downregulating the gene
expression.
Question 3: Outline the process of DNA replication and explain the roles of each enzyme
involved.
DNA replication is a process by which double stranded DNA makes identical copy of itself.
It involves a number of proteins/ enzymes namely helicase, single strand binding proteins
(SSB), RNA primase, DNA polymerase, topoisomerase, and DNA ligase. The process of
DNA replication always occurs in 5’ to 3’ direction.
The DNA replication involves a number of steps which are explained below (Fig. 2):
a) DNA helicase binds to the double stranded DNA to initiate the process of DNA
replication. As a result of unwinding of 2 parent strands, a Y shape fork is formed which is
referred as replication fork. As the double stranded unwinds, it induces topological strain in
DNA strands. Topoisomerase releases the strain so that DNA helicase can further unwind the
DNA and process continues.
b) Unwinding of DNA is followed by the binding of single strand binding proteins (SSB)
which binds to single strands of DNA and prevents them to form double stranded DNA.
c)RNA primase adds RNA primer to single stranded DNA which is followed by recruitment
of DNA polymerase. DNA polymerase adds the nucleotides always in 5’ to 3’ direction. In
DNA replication, the 2 new DNA strands are formed which are referred as leading and
lagging strand. In leading strand, the synthesis of complementary daughter strand is
continuous while in lagging strand, the synthesis of DNA is discontinuous and is
accompanied by formation of Okazaki fragments.
d)In the final step, DNA ligase seals the gaps between the Okazaki fragments leading to the
formation of complete and continuous lagging strand.
After DNA replication process is complete, two identical double stranded DNA is formed.
Since each of the newly formed double stranded DNA has one strand from parent and other
newly synthesised one, hence it is called semi- conservative DNA replication.