seaweedcracker1
Badges: 4
Rep:
?
#1
Report Thread starter 4 years ago
#1
What's the difference?

In semi conservative replication - what does DNA polymerase do? As nucleotides join up to their complementary bases naturally due to attractions, why does DNA polymerase need to be there?

I'm so confused because in Polyermase Chain Reaction, DNA polymerase is used to join together nucleotides and form a sugar phosphate backbone. But I thought DNA ligase does that?!
0
reply
RachelLJ
Badges: 3
Rep:
?
#2
Report 4 years ago
#2
(Original post by seaweedcracker1)
What's the difference?

In semi conservative replication - what does DNA polymerase do? As nucleotides join up to their complementary bases naturally due to attractions, why does DNA polymerase need to be there?

I'm so confused because in Polyermase Chain Reaction, DNA polymerase is used to join together nucleotides and form a sugar phosphate backbone. But I thought DNA ligase does that?!
No chance you're doing Higher Biology is there? Just wondering
But I'm pretty sure DNA polymerase unwinds the DNA double helix and adds on nucleotides to the primer(s) on the leading and lagging strands while forming the sugar-phosphate backbone. Oh, and it is taq DNA polymerase that does it in PCR because it has to be heat tolerant
But your DNA ligase description was only the tiniest bit off. DNA ligase joins together the fragments of DNA on the lagging strand and, if it is relevant to you, it also joins together the complementary sticks ends of the DNA transferred into the plasmid in recombinant DNA technology. Hope that helps!


Posted from TSR Mobile
0
reply
GailQ
Badges: 10
Rep:
?
#3
Report 4 years ago
#3
(Original post by RachelLJ)
No chance you're doing Higher Biology is there? Just wondering
But I'm pretty sure DNA polymerase unwinds the DNA double helix and adds on nucleotides to the primer(s) on the leading and lagging strands while forming the sugar-phosphate backbone. Oh, and it is taq DNA polymerase that does it in PCR because it has to be heat tolerant
But your DNA ligase description was only the tiniest bit off. DNA ligase joins together the fragments of DNA on the lagging strand and, if it is relevant to you, it also joins together the complementary sticks ends of the DNA transferred into the plasmid in recombinant DNA technology. Hope that helps!


Posted from TSR Mobile
DNA polymerase doesn't unwind, helicase does, but other than that, yes.

OP, to answer the question of why DNA polymerase needs to be there, I would guess that seeing as it is an enzyme, although the process may happen naturally the enzyme likely catalyses it? DNA polymerase is needed therefore to efficiently bind the free
nucleotides and initiate replication.


Also, I think you may be confusing the sugar phosphate backbone with a sugar-phosphate bond (phosphodiester). The sugar-phosphate bond forms the structure of DNA strands and it is contains sugar-phosphate bonds which join the nucleotides. DNA ligase forms sugar-phosphate bonds between the fragments of DNA on the lagging strand (i.e. sealing the "nicks" or gaps).
DNA polymerase does indeed form a sugar phosphate backbone by joining nucleotides.

Hope that helps
0
reply
seaweedcracker1
Badges: 4
Rep:
?
#4
Report Thread starter 4 years ago
#4
(Original post by GailQ)
DNA polymerase doesn't unwind, helicase does, but other than that, yes.

OP, to answer the question of why DNA polymerase needs to be there, I would guess that seeing as it is an enzyme, although the process may happen naturally the enzyme likely catalyses it? DNA polymerase is needed therefore to efficiently bind the free
nucleotides and initiate replication.


Also, I think you may be confusing the sugar phosphate backbone with a sugar-phosphate bond (phosphodiester). The sugar-phosphate bond forms the structure of DNA strands and it is contains sugar-phosphate bonds which join the nucleotides. DNA ligase forms sugar-phosphate bonds between the fragments of DNA on the lagging strand (i.e. sealing the "nicks" or gaps).
DNA polymerase does indeed form a sugar phosphate backbone by joining nucleotides.

Hope that helps
But to form the sugar phosphate backbone in the first place you need to form phosphodiester bonds? When I said that DNA polymerase forms a sugar phosphate backbone, doesn't that basically mean it joins together phosphates and pentose sugars on nucleotides?

My question is, why isn't DNA ligase present in PCR, when DNA ligase joins the backbone together in the new DNA strand. How does it do it without it?
0
reply
GailQ
Badges: 10
Rep:
?
#5
Report 4 years ago
#5
(Original post by seaweedcracker1)
But to form the sugar phosphate backbone in the first place you need to form phosphodiester bonds? Yes When I said that DNA polymerase forms a sugar phosphate backbone, doesn't that basically mean it joins together phosphates and pentose sugars on nucleotides? Yes

My question is, why isn't DNA ligase present in PCR, when DNA ligase joins the backbone together in the new DNA strand. Only between nicks.
How does it do it without it?
Replication on the lagging strand is discontinuous as DNA polymerase initiates replication at many different points along the lagging strand, provided that there is a primer present. This results in many short fragments.

Once the primer is removed, it is replaced with DNA. This still leaves a gap between the fragments. The role of DNA ligase is only to seal these gaps with a sugar-phosphate bond so the strand becomes a continuous one.

The sugar-phosphate bonds of the fragments themselves are still made by DNA polymerase adding free nucleotides.

Here's an answer to why DNA ligase is not needed (googled, as I did not know this myself):
"PCR is only used on short DNA sequences, and it's done in a controlled environment (a test tube), so it only requires the bare minimum to work. Ligase isn't needed because there should never be any nicks that need to be sealed or cut ends that need to be bound."

I hope this is now clearer
0
reply
seaweedcracker1
Badges: 4
Rep:
?
#6
Report Thread starter 4 years ago
#6
(Original post by GailQ)
Replication on the lagging strand is discontinuous as DNA polymerase initiates replication at many different points along the lagging strand, provided that there is a primer present. This results in many short fragments.

Once the primer is removed, it is replaced with DNA. This still leaves a gap between the fragments. The role of DNA ligase is only to seal these gaps with a sugar-phosphate bond so the strand becomes a continuous one.

The sugar-phosphate bonds of the fragments themselves are still made by DNA polymerase adding free nucleotides.

Here's an answer to why DNA ligase is not needed (googled, as I did not know this myself):
"PCR is only used on short DNA sequences, and it's done in a controlled environment (a test tube), so it only requires the bare minimum to work. Ligase isn't needed because there should never be any nicks that need to be sealed or cut ends that need to be bound."

I hope this is now clearer
I know that sugar phosphate bonds already exist within free nucleotides as nucleotides have a phoshate, sugar and base.
Does DNA polymerase or DNA ligase create the sugar phosphate bonds between the nucleotides to create a continuous strand?

This is what I've learnt in class:
I've learnt that DNA ligase's role is to join sugar phosphate backbones together in making recombinant DNA and is it not involved in DNA replication.
I've also learnt that DNA polymerase's role in DNA replication and PCR is to form the sugar phosphate backbone, joining the phosphate and sugar together by a sugar phosphate bond (phosphodiester)

Sorry, I'm still confused!
0
reply
maya_vv
Badges: 9
Rep:
?
#7
Report 1 year ago
#7
I'm still confused about this three years later
0
reply
macpatgh-Sheldon
Badges: 16
Rep:
?
#8
Report 1 year ago
#8
My advice to you, OP and others (incl maya_vv) is to try to understand the meanings of the technical terms used (NOT JUST with DNA replication, but with ALL topics in biology as I have reiterated in numerous posts on biology revision. This way, not only is the meaning/function easier to remember, but you can work out the meanings of new words yourself (you don't need to become a Google doctor!).

DNA "ligase" originates from the word ligate, which means to tie up i.e. when there are a number of (short) strands of DNA, the ends of these (the correct ends, of course, i.e. 5' to 3') are tied up or ligated by DNA ligase. [think of ligament as in anterior cruciate ligament in knee OR ligand in chemistry)

DNA polymerase catalyses the production of the polymer of nucleotides i.e. DNA i.e. it joins up two nucleotides to make a dinucleotide, then a 3rd one to make a trinucleotide, etc. etc. etc.

Ok now what about DNA helicase - let us take it step by step, yeah? What is a Helix? Yes well done, it means a spiral - since DNA structure consists of a double helix (originally described by Watson & Crick, for which they won the Nobel Prize) or double spiral, you can work out that the helicase makes or opens up the double helix (since all enzymes can catalyze reactions in either direction under the right conditions).

M (specialist biology tutor)
1
reply
maya_vv
Badges: 9
Rep:
?
#9
Report 1 year ago
#9
Many thanks, regardless of the low-key sarcasm it has really helped!
1
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • The University of Law
    Open Day – BPC, GDL, LPC and MA Law - Bristol campus Postgraduate
    Sat, 18 Jan '20
  • Norwich University of the Arts
    Postgraduate Open Day Postgraduate
    Thu, 23 Jan '20
  • SOAS University of London
    Development Studies, Interdisciplinary Studies, Anthropology and Sociology, Languages, Cultures and Linguistics, Arts, Economics, Law, History, Religions and Philosophies, Politics and International Studies, Finance and Management, East Asian Languages & Cultures Postgraduate
    Sat, 25 Jan '20

How many universities have you heard back from?

0 (66)
15.46%
1 (54)
12.65%
2 (55)
12.88%
3 (88)
20.61%
4 (87)
20.37%
5 (77)
18.03%

Watched Threads

View All