Molecular Biology And Biochemistry DNA Flashcards
Characteristic components of nucleotides
Nitrogenous base
One or more phosphates
A Pentose sugar
Why is ribose sugar more reactive the deoxyribose
It’s more reactive and unstable because of the OH group on the 2 carbon
Which bases are purines
Adenine and guanine
Which bases are pyrimidines
Thymine, uracil and cytosine
What is a pyrimidine
Single ring nitrogenous base
Structure of dna
Two anti parallel strands
Each strand has a 5’ (phosphate) and 3’ end (hydroxyl)
Held together by hydrogen bonds
Antiparallel nature allows for h bond formation
Bases planar to sugar phosphate backbone
What evidence proved dna helical structure
X ray diffraction
How does the x ray diffraction of dna show a helical structure
X shape which is indicative
Layer line which shows the length of one complete turn
The common features of B-DNA
Right handed double helix
Hydrophobic core
Hydrophilic backbone
Major and minor groves
Factors towards the stability and structure of the double helix
Backbones separated as far apart as possible to avoid electrostatic repulsion
Bases form stablizing h bonds
Bases stack to maximise aromatic ring interactions
How many hydrogen bonds form between the bases
A +t 2
C + g 3 (
Why doesn’t non standard base pairing work
Distorts the geometry of the double helix the bases are no longer 1.1 nm apart
Why are their aromatic stack interactions
Benzene contains an e in all p orbitals which can delocalise and form a ring of electron density around the planar ring
Why is displacement stacking better then eclipse stacking
Maximises attractive interactions between dipoles
Why are there major and minor groves
The glycosidic bonds between bases are the same distance apart so nucleotides must be at an angle
They allow for dna bonding proteins to read dna without unwinding the helix
Why does DNA have thymine not uracil
Cytosine can delaminate to uracil therefore uracil is identified and removed in dna repair processes
Why does RNA have uracil not thymine
Thymine is more energetically costly to produce as a methylated uracil
There’s no bio synthetic pathway for it to be produced
Why is rna less stable then dna
OH group on c2 acts as an internal nucleophile and causes the breakage of the phosphodiester backbone
What end are nucleotides added to in replication
3’
How does chain elongation occurs
The OH group on c3 has a nucleophilic attack on the primary phosphate group of another nucleotide. Hydrolysis of the other two phosphates provide energy for the reation
What are the steps to separate the double helix
Helicase opens the helix by breaking the weak h bonds between bases
Topoisomerase releaves the tension in the helix by breaking the dna and reasealing to the right of the replication fork ahead of the helicase
Single stranded dna binding proteins bind to separate strands to prevent annealing
How does dna get a 3’ OH end to start replication
Short rna primer is made by dna primase
How does the antiparallrl nature of dna cause problems
Lagging strand is synthesised discontinuously
What are the discontinuous stands of dna in the lagging strand called
Okazaki fragments