Mech-DNAreplication essential for college activities.pdf

Dna Replication

1. Name : AkshadaNarayan Landge
Class : SY BSC
Roll No : 31
Topic Name : Mechanism of DNA
Replication

2. Introduction Of DNA Replication
•DNA Replication is semiconservative,meaningthat each strand in the
DNA Double helix act as template for a synthesis of a new
complementary strand this process takes us from one starting molecule
to two “daughter” molecule with each newly formed double helix
containing one new & one old strand.
•Basis for inheritance.
•Fundamental process occurring in all cells for Copying DNA to
transfer genetic Information to daughter cells.
•Each cell must replicate it’s DNA before division.

3. DNA Replication

4. Primers and Primase
•DNA polymerases can only add nucleotides to the 3’ end of an existing
DNA strand.
•The problem is solved with the help of an enzyme calledprimase.
Primase makes an RNAprimer, or short stretch of nucleic acid
complementary to the template, that provides a 3' end for DNA
polymerase to work on. A typical primer is about five to ten
nucleotides long. The primerprimesDNA synthesis, i.e., gets it
started.
•Once the RNA primer is in place, DNA polymerase "extends" it,
adding nucleotides one by one to make a new DNA strand that's
complementary to the template strand.

5. Leading and lagging strands :
•DNA polymerase that handles most of the synthesis is DNA
polymerase III. There are two molecules of DNA polymerase III at a
replication fork, each of them hard at work on one of the two new
DNA strands.
•DNA polymerases can only make DNA in the 5’ to 3’ direction, and
this poses a problem during replication. A DNA double helix is always
anti-parallel; in other words, one strand runs in the 5’ to 3’ direction,
while the other runs in the 3' to 5' direction. This makes it necessary
for the two new strands, which are also antiparallel to their templates,
to be made in slightly different ways.
•One new strand, which runs 5' to 3' towards the replication fork, is the
easy one. This strand is made continuously, because the DNA
polymerase is moving in the same direction as the replication fork.
This continuously synthesized strand is called theleading strand.

6. 1. Initiation:
•It involves the origin of replication. Before the DNA synthesis begins, both the parental
strands must unwind and separate permanently into single stranded state. The synthesis of
new daughter strands is initiated at the replication fork. In fact, there are many start sites.
2. Elongation:
•The next step involves the addition of new complementary strands. The choice of
nucleotides to be added in the new strand is dictated by the sequence of bases on the
template strand. New nucleotides are added one by one to the end of growing strand by an
enzyme called DNA polymerase. There are four nucleotides, deoxyribrnucleotide
triphosphates dGTP, dCTP, dATP, dTTP present in the cytoplasm.
3. Termination:
•All the end termination reactions occur. Duplicated DNA molecules are separated from
one another.
•The purpose of DNA replication is to create two daughter DNA molecules which are
identical to the parent molecule.

7. •Replication Enzymes :
•DNA helicase-unwinds and separates double stranded DNA as it moves
along the DNA. It forms the replication fork by breakinghydrogen
bondsbetween nucleotide pairs in DNA.
•DNA primase-a type of RNA polymerase that generates RNA primers.
Primers are short RNA molecules that act as templates for the starting point
of DNA replication.
•DNA polymerases-synthesize new DNA molecules by
addingnucleotidesto leading and lagging DNA strands.
•Topoisomeraseor DNA Gyrase-unwinds and rewinds DNA strands to
prevent the DNA from becoming tangled or supercoiled.
•Exonucleases-group of enzymes that remove nucleotide bases from the
end of a DNA chain.
•DNA ligase-joins DNA fragments together by forming phosphodiester
bonds between nucleotides.