DNA Structure and replication

Stucture of DNA
Polymers- chains of nucleotide subunits linked together
Two types of nucleotide subunit
1.      Purines- Adenine and Guanine which have Two rings
2.      Pyrimidines, Cytosine, Thymine and Uracil which are Single ring

Structure of DNA contains:
Nitrogenous base- Adonine, Cytosine, Guanine, Thymine
1 pentose sugar (5 carbon)
1 phosphate group

Nucleotides joined together via phosphodiester bonds
Forms the sugar phosphate backbone
Bond is formed between the three prime and five prime end
Hydrogen group attached- deoxyribose

·        Two polynucleotide chains which are anti-paralell

·        DNA has a 5" and 3" end

·        Sugar phosphate backbone on the outside of the structure

·        The bases point inwards

·        Hydrogen bonds are what connect the two base pairs

·        Adenine pairs to thymine with two hydrogen bonds

·        Guanine pairs to cytosine with three hydrogen bonds

·        Adenine pairs with thymine and guanine pairs with cytosine

Protein synthesis via translation
Contains:
Nitrogenous base:
Adenine
Cytosine
Guanine
Uracil

A pentose
A Phosphate group
Hydroxyl group attached- ribose

Chargaff's rule
In DNA the amount of one base pair units is always the same as the other base pair units. For example: there is always the same amount of adenine units as there is thymine units and also the same amount of cytosine as there is guanine

Higher order structure of chromatin
Chromatin is DNA and protein
Tightly packaged into the nucleus
The protein helps to compact the DNA into the nucleus
 Highly organised
Multiple levels
Achieve compaction of DNA into the nucleus
Linked to the regulation of transcription
Chromatin is what forms a chromosome
Chromosomes contain two types of protein:
Histones
Small basic proteins with net positive charge so they can bind to DNA
Non-histones
Often negatively charged

DNA replication
Occurs in the S phase of the cycle
Semi conservative
Three stages
Initiation
 Origins of replication- Where DNA synthesis is initiated
Multiple sites exist
When DNA replication begins, these sites are called replication forks
These origins are targeted by initiator proteins

Enzymes involved:
DNA helicase
Unwinds the double helix to expose each of the two strands
Strands are used as a template for replication
The enzyme hydrolises the ATP holding the bonds together which breaks the bond pulling the two strands apart from eachother
DNA primase
Synthesises short RNA primer
This starts off DNA polymerase
DNA polymerase
Creating and expanding new strands of DNA
Binds nucleotides to create new strand of DNA
Can only add nucleotides to the 5'-3' end

Elongation
DNA polymerase is attached to the template strands
 Lagging strand is antiparalell to leading strand
DNA polymerase cannot add bases to the 5' end
As the helix unwinds, RNA primers are added to the newly exposed bases on the lagging strand
DNA synthesis occurs in fragments
These fragments are called okazaki fragments
Termination
Expanding DNA continues until there is no DNA template strand left or two replication forks meet
RNAase H removes RNA primer at the beginning of each okazaki fragment
DNA ligase joins the fragments together to create one complete strand