Biochemistry of heredity:
the structure of Deoxyribonucleic Acid (DNA)
In
principle:
Genes are made of nucleic acids
The
identity of the hereditary substance was unknown until 1940;
its
structure
was
unknown until 1953
"Genetics"
was taught for 50 years without this information (see Orientation)
The history of the discovery
of DNA is a fascinating detective story
The Watson-Crick
structure for Deoxyribonucleic
acid (DNA) (1953)
a double-stranded helix
sugar-phosphate backbone outside
nitrogenous bases (A,C,G,
T) inside
bases
held
together
by hydrogen
bonds ( H- or hydrostatic bonds )
Fundamental insight:
bases
on
alternative
strands pair according to specific rules:
A with T
G with C
each
pair
has
similar structure
A second form of nucleic acid
is ribonucleic
acid (RNA)
Building
blocks
of nucleic acids (DNA & RNA)
bases [IG1_02-04]
pyrimidines (single
ring)
cytosine(C)
thymine (T)
[ uracil in
RNA (U) ]
"PYRamids
were CUT from stone"
purines (double
ring)
adenine (A)
guanine (G)
"AGs are PURe"
nucleoside = base
+ sugar
deoxyribose sugar in DNA (- H on 2'-C) [IG1_02-03]
deoxyadenosine (dA) deoxyguanosine (dG)
deoxycytosine (dC)
deoxythymidine (dT)
nucleoide = nucleoide
+ phosphate(s) [PO4] [IG1_02-02]
one phosphate deoxyucleoide monophosphate (dNMP)
three phosphates deoxynucleoide triphosphate (dNTP)
deoxyadenosine-5'-phosphate or deoxyadenylic
acid
deoxyadenylic acid (dAMP)
/ deoxyguanylic
acid (dGMP)
deoxycytidylic acid (dCMP)
/
deoxythymidylic acid (dTMP)
polynucleotide = nucleotide + nucleotide +
nucleotide + etc
[IG1_02-05]
nucleotides
are
linked by 3' 5' phosphodiester bonds
polynucleotides have directionality
hydroxyl (3') & phosphoryl
(5') ends
Structure
of B-DNA (3-D
model from CSHL)
[IG1_02-06]
1)
Two plectonemic (twisted) right-handed polynucleotide helices ()
2) Helices antiparallel strands wrt
5' 3' orientation
3) Strands held together by hydrogen (H-) bonds between bases
4) H-bonds form according to specific base-pairing rules [IG1_02-07]
A pairs with T:
two H-bonds
G pairs
with C: three H-bonds
A+T & G+C pairs have very similar shapes & sizes
5)
Base pairs co-planar:
interval = 0.34 nM [= 3.4 Ǻngstroms]
6) Period of helix is 10 bp (base pairs) = 3.4 nM
7) 3-D structure has major
& minor grooves
8) Order of bases in each strand aperiodic
Other structures for nucleic
acids
[IG1_02-09, -11] [IG1_02-Tab01]
A-DNA : not
groovy, base pairs not co-planar
Z-DNA: left-handed
helix () [IG1_02-12]
Ribonucleic Acid (RNA):
substitute uracil for thymine [ thymine = 5-methyl-uracil ]
ribose sugar for deoxy-ribose
typically single-stranded
or with complex
double-stranded folding:
mRNA (messenger RNA): long, single-stranded
rRNA (ribosomal RNA): medium-sized, complex 'stem
& loop' folding
tRNA (transfer RNA): small, 'cloverleaf' structure
[more on RNA structures later]
Implications of DNA structure for
its genetic function
"The sequence of bases
on a single chain does not appear to be restricted in any
way....
It has not escaped our notice that the specific
pairing we have postulated
immediately suggests a possible copying mechanism
for the genetic material."
(Watson
& Crick 1953. Nature
112:753)
DNA is an aperiodic crystal:
order of bases conveys
information
Antiparallel strands are self-complementary:
DNA is potentially autocatalytic
All text
material © 2016 by Steven M.
Carr