Removal of primer happens with the help of two processes –
1. Removal by RNA H enzymes:
a. Primer
RNA Removal by RNase H Enzyme: At the start of DNA
replication, a small piece of RNA called a primer is used to provide a starting
point for DNA synthesis. After the copying is done, an enzyme called RNase H
recognizes the RNA parts of the DNA-RNA hybrid (the primer) and chops it into
smaller pieces. This is like cutting out the used-up starting guide.
b. Gap Left
Behind: Once the primer RNA is cut out, a gap is left where the RNA
used to be. This gap needs to be filled with DNA to complete the replication.
c. DNA
Polymerase 1 Activity: Here’s where DNA Polymerase 1 comes into play.
This enzyme helps to fill in the gap left behind with the right DNA bases. It
starts adding the correct DNA bases one by one, creating a new DNA strand.
d. DNA
Ligase Sealing: After DNA Polymerase 1 has filled in the gap,
there are still tiny breaks between the new pieces of DNA. DNA Ligase is
another enzyme that comes in to seal these breaks. It acts like a “DNA glue,”
connecting the pieces together into a continuous strand.
So, to sum it up, the process
involves RNase H cutting out the old primer RNA, DNA Polymerase 1 filling in
the gap with new DNA bases, and DNA Ligase sealing the final DNA strand. All of
these steps ensure that the DNA replication is accurate and complete, giving us
two identical DNA molecules that can be passed on to new cells.
Original DNA Strand:
5'
---- A ---- T ---- G ---- C ---- 3'
RNA Primer (a short piece of RNA):
5'
---- A ---- T ---- G ---- C ---- 3'
|
3' ---- 5'
RNAase H removes the RNA Primer:
5'
---- A ---- T ---- G ---- C ---- 3'
DNA Polymerase I fill the gap:
5'
---- A ---- T ---- G ---- C ---- 3'
5'
---- A ---- T ---- G ---- C ---- 3'
DNA Ligase seals the DNA strand:
5'
---- A ---- T ---- G ---- C ---- 3'
5'
---- A ---- T ---- G ---- C ---- 3'
Final Replicated DNA Strand:
5'
---- A ---- T ---- G ---- C ---- 3'
5'
---- A ---- T ---- G ---- C ---- 3'
2. Certainly!
The “flap” model refers to a step in the
process of removing RNA primers and filling gaps during DNA replication. Let’s
break it down:
a. RNA
Primers: During DNA replication, a short piece of RNA called a primer
is used as a starting point for DNA synthesis. It guides the DNA polymerase
enzyme to begin copying the DNA strand.
b. DNA
Replication: As the DNA strand is copied, the primer RNA
needs to be removed because DNA doesn’t contain RNA. This is where the “flap”
model comes in.
c. Flap
Formation: After most of the RNA primer is removed, a small
piece of single-stranded DNA called a “flap” can remain. This flap is a segment
of DNA that is displaced or “flaps” away from the rest of the newly synthesized
DNA strand.
d. Exonuclease
Activity of DNA Polymerase: DNA Polymerase, the enzyme
responsible for adding new DNA bases, has a special activity called
“exonuclease.” It can remove nucleotides from the end of a DNA strand. In the
flap model, the displaced flap of single-stranded DNA is recognized by DNA
Polymerase, and the exonuclease activity helps to “chew away” the flap.
e. Flap
Removal: As the DNA Polymerase’s exonuclease activity proceeds, it
removes the flap of single-stranded DNA. This allows the DNA Polymerase to
continue filling in the gap with the correct DNA bases.
f. Ligase
Sealing: Once the gap is filled and the flap is removed, the resulting
DNA strand is continuous, and any small nicks or breaks in the DNA backbone are
sealed by DNA Ligase. DNA Ligase acts as a “molecular glue,” joining the DNA
segments together.
So, the “flap” model is a way of
explaining how DNA Polymerase removes the RNA primer and the short
single-stranded DNA flap and then fills in the gap to create a complete,
continuous DNA strand. This process ensures accurate and faithful DNA
replication.
Original DNA Strand:
5' ---- A ---- T ---- G ---- C ---- 3'
RNA Primer (a short
piece of RNA):
5' ---- A ---- T ---- G ---- C ---- 3'
|
3' ---- 5'
RNAase H removes
the RNA Primer:
5' ---- A ---- T ---- G ---- C ---- 3'
|
3' ---- 5'
A gap in the DNA:
5' ---- A ---- T ---- G ---- C ---- 3'
DNA Polymerase I
fills the gap:
5' ---- A ---- T ---- G ---- C ---- 3'
|
3' ---- 5'
5' ---- A ---- T ---- G ---- C ---- 3'
DNA Ligase seals
the DNA strand:
5' ---- A ---- T ---- G ---- C ---- 3'
|
3' ---- 5'
5' ---- A ---- T ---- G ---- C ---- 3'
Final Replicated DNA
Strand:
5' ---- A ---- T ---- G ---- C ---- 3'
|
3' ---- 5'
5' ---- A ---- T ---- G ---- C ---- 3'