DNA REPLICATION Flashcards
What needs ti happen before cell division
DNA within the cell must be replicated to ensure all the daughter cells have the necessary genetic information to produce required proteins
Describe semi conservative replication
The result of DNA replication is that each of the new DNA molecules contain half of the original DNA, onto whuch the other half has eben built
Original strand acts as a template
Free nucleotides join to form a new strand
Which end bears a phosphate
5’
Which end bears a hydroxyl group
3’
Binding of DNA polymerase
DNA polymerase is complementary to the 3’ end of the template strand
Step 1 of replication
Hydrogen bonds between bases are broken by the enzyme helicase
This seperates (unzips) the strands
Hydrogen bonds are weak and easily broken
Step 2 of DNA replication
Each polynucleotide strand acts as a template
Free nucleotides (within the nucleus) are attracted to their complementary bases
Hydrogen bonds form between via complementarybase pairing (no enzyme needed)
Step 3
Once in place the nucleotides are activated using energy from ATP
nucleotides are then joined together by DNA polymerase (condensation) to form Phosphodiester bonds and create the nee sugar phosphate backbone
Semi conservative: new DNA molecules contain one old strand and one new strand
Meselsohn and Stahl
1958
They used the knowledge that all the organic bases in DNA contain nitrogen
Bacteria will incorporate nitrogen from their growing medium into any new DNA they make
They grew E coli with different isotopes of nitrogen
Nitrogen has two forms
Light 14N
Heavy 15N
What happened at the start of M&S experiment
The 15N strand is heavy so makes a band low dowm the tube when spun in a centrifuge
High density sinks further down the tube
What happened after one generation of m&s experiment
There was only one band
14/15 hybrid
What happened in the second generation of the m&s experiment
After another generation there were two bands
A light chain and a 14/15 level
