Week 2.2: DNA structure and replication Flashcards
What are the three parts of a DNA nucleotide?
- Sugar
- Phosphate
- A base
–> Sugar and phosphate form the back-bone and then bases have a H bond with bases on other side of strand to create a binary pattern.
What are the three steps of DNA replication?
The three steps of DNA replication are…
1) Unzipping and unwinding
2) Priming
3) DNA polymerase III adding nucleotides
Goes from one piece of chromosome into a chromosome with two identical sister chromatids.
What are some of the key players in DNA replication?
In phase 1): unwinding and unzipping
- Topoisomerase
- DNA helicase
- Single-stranded binding protein
In phase 2): priming.
- RNA primer
- Primase
In phase 3): Addition of nucleotides
- DNA pol III: adds nucleotides to the 3’ end of primer
- DNA pol I: replaces RNA primer with DNA
- Enzymes: connect DNA fragments into one long sequence.
What’s a primer?
A primer is a short nucleic acid sequence, usually short segments of RNA, that provides a starting point for DNA synthesis. A primer must be synthesized by an enzyme called primase; a type of RNA polymerase, before DNA replication can occur (wikipedia).
How big is a DNA molecule?
A DNA molecule is technically the whole chain length of linked nucleotides. Technically as many nucleotides as are covalently bonded.
Role of topoisomerase?
Topoisomerase relieves strain by breaking, swivelling, and rejoining parental DNA ahead of fork. (Necessary because unwinding by DNA helicase creates tension).
Role of DNA helicase?
DNA helicase unwinds + separates strands (creating tension)
Role of single stranded binding protein?
Single stranded binding protein stabilises unwound strands by binding to them and preventing them from reforming into a double helix.
Role of primase?
Primase synthesises RNA primer
DNA pol III vs DNA pol I?
- DNA pol III: adds nucleotides to the 3’ end of primer
- DNA pol I: replaces RNA primer with DNA
Role of ligase?
Ligase connects okazaki DNA fragments.
What is semiconservative replication?
Semiconservative replication means that each new strand from DNA replication contains one of the old strands and one of the new strands.
Where in the strand does DNA replication occur?
DNA replication occurs in multiple replication bubbles (origins of replication) opened up through the whole chain, each with replication forks that move outwards. Over time these bubbles widen until they join up to make two new helices.
What sort of bonds join nucleotides in DNA?
DNA nucleotides are joined by phosphodiester bonds. These are bonds between the incoming phosphate and the OH group of the previous atom, forming O-P
Where does energy come from to power the addition of nucleotides?
Before a nucleotide is added it has three phosphate groups: its a triphosphase e.g. ATP CTP GTP TTP and then two of these groups are broken off and then seperated into ions when a nucleotide is added, this releases energy.
