DNA- the genetic material - structure Flashcards
Structure Chardaff's rule DNA discovery BDNA and DNA melting
When was DNA discovered
1953 - James Watson, Francis Crick and Rosalind Franklin and Maurice Wilkins
What does DNA contain
Deoxyribose sugar
Phosphate
Organic bases
Organic bases family - Pyrimidines
single ringed nitrogenous
Thymine and Cytosine
Organic bases family - Purines
double ringed nitrogenous
Adenine and Guanine
DNA deoxyribose
Sugar attachments and alcohol
in Carbon 1, 3 and 5
carbon 1 - Base attaches to -OH
carbon 3 - (3’) secondary alcohol binds to phosphate
carbon 5 - (5’) primary alcohol binds to another phosphate
nulceoside
sugar + base
names of nucleoside
Adenosine
Thymidine
Guanosine
Cytidine
Nucleoside + phosphate
Deoxynucleoside triphosphate
Bonds between sugar and base
Glycosidic bond
N-linked by condensation reaction
Bonds between phosphate and sugar (3’ and 5’ -OH)
Phosphodiester backbone
By condensation reaction
Chardaff’s rule
No. A= No. T and No. C= No. G
Purines = Pyrimidines
Method of Chardaff’s rule
Break Phosphodiester bond and purify DNA using acid
Separated by paper chromatography
use UV spectrophotometry to identify individual bases and its proportions
Rosalind Franklin and Maurice Wilkins
X-ray diffraction patterns on DNA fibres
makes fundamental repeating distances in DNA
Method for X-ray crystallography
Purify DNA and change humidity = different forms of DNA
fixed piece of DNA on support, sealed in front of film and behind the camera
bombarded DNA with x-ray = x-ray diffraction - (e-) which scatter
Increasing humidity to 92%
forming BDNA
James Watson and Francis Crick
1953
discovered the DNA model with the double helix
properties of bases
hydrophobic and stack on top in the core
distance of bases
optimal separation for VdW interaction
strands direction
antiparallel one from 5’ to 3’ and other 3’ to 5’
strands of BDNA and its direction
double stranded and goes in right handed direction
Distance between base pairs in BDNA
0.34nm
distance per turn of BDNA
3.4nm per turn
10 bp per turn
Meaning of bp
base pairing
Right handed helix - rotation direction
DNA curl in clockwise direction
diameter of BDNA
around 2nm
why is slightly off set - not symmetrical
has major and minor groove
groove
gaps between the sides (backbone)
A-DNA direction
right handed
Cause of A-DNA
Decrease in humidity therefore was dehydrated
structures of A-DNA
wider, bases tilted and off set from axis
Z-DNA direction
Left handed - anitclockwise
Cause of Z-DNA
GC containing sequence at high salt concentration
Max DNA absorbance
260nm - light wave
Comparison of single and double stranded DNA in absorbancy
Single stranded DNA has higher absorbance
Tm (degrees celcius)
mid point of transition
Affect of GC rich DNA
melt at higher temperature due to 3 hydrogen bonds
Affect of longer DNA fragments
require higher temperature to melt
Affect of higher salt concentration
requires higher temperature as positive ions neutralise negative phosphate backbone
hypochromic effect
Limited absorbance until double strand separate
Hyperchromisity occur
at 260nm form single stranded
Proteins in chromatin
Histones , 5 proteins
name of 5 histones
H1, H2A, H2B, H3, H4
Histonecore
2 copies of each H2A, H2B, H3, H4 forming octametric - disk like structure
Nucleosome
DNA wraps around histones 1.6 times
condensed nucleosomes
arranged in ‘bead on string’, condensed to form higher-order structures and compact DNA
Role of H1 histones
linker histones
changes conformation of DNA as it leaves nucleosome - compact DNA - create kink
What H1 histones do
bind to nucleosome, contacts protein and DNA, able to change conformation of DNA as it leaves
histones property
Rich in basic therefore positively charged
number of base pairs per cell
6,000,000,000 around 2m
number of cells in body
10 to power of 14
DNA per cell
0.6pg
