MOLECULAR BASIS OF INHERITANCE
DNA : Structure of Polynucleotide Chain
● DNA − Polymer of deoxyribonucleotides
● Nucleoside = Nitrogenous base + Pentose sugar (linked through
N−
glycosidic bond
)
Example − adenosine, deoxyadenosine, cytidine, etc.
● Nucleotide = Nucleoside + Phosphate group (linked through
phosphodiester bond
)
● Many nucleotides link together through 3′
−5′
phosphodiester bond
to form polynucleotide chain (as in DNA and RNA).
● In course of formation of polynucleotide chain, a phosphate moiety
remains free at 5′ end of ribose sugar (5′ end of polymer chain) and
one OH group remains free at 3′ end of ribose (3′ end of polymer
chain).
Double Helix Model for the Structure of DNA
● Scientists involved
○ Friedrich Meischer
− First identified DNA as an acidic
substance present in nucleus and named it as ‘Nuclein’
○ Wilkins and Franklin
− Produced Xray diffraction data for
DNA structure
○ Watson and Crick
− Proposed double helix structure model for
DNA based on Xray diffraction data
○ Erwin Chargaff
− Proposed that in ds DNA, ratios A:T and C:G
remain same and are equal to one
Features of double helix structure of DNA:
, ● In a DNA, two polynucleotide chains are coiled to form a helix.
Sugarphosphate forms backbone of this helix while bases project in
wards to each other.
● Complementary bases pair with each other through hydrogen bond.
Purines always pair with their corresponding pyrimidines. Adenine
pairs with thymine through two hydrogen bonds while guanine pairs
with cytosine through three hydrogen bonds.
○ The helix is righthanded.
Pitch − 3.4 nm
10 bp in each turn
○ The plane of one base pair stacks over the other in a double
helix. This provides stability to the helix along with hydrogen
bonding.
Packaging of DNA Helix
, Packaging of DNA Helix
● Distance between two consecutive base pairs in a DNA = 0.34 nm =
0.34 × 10
−9
m
● Total number of base pairs in a human DNA = 6.6 × 10
9
bp
● Total length of human DNA = 0.34 × 10
−9
× 6.6 × 10
9
= ~ 2.2 m
● 2.2 m is too large to be accommodated in the nucleus (10
−6
m).
● Organisation of DNA in prokaryotes:
○ They do not have nucleus. DNA is scattered.
○ In certain regions called nucleoids, DNA (negatively charged) is
organised in large loops and is held by some proteins (positively
charged).
● Organisation of DNA in eukaryotes:
○ They have positively charged basic proteins called histones
(positive and basic due to presence of positive and basic amino
acid residues, lysine and arginine).
○ Histone octamer − Unit of eight molecules of histone
○ DNA (negatively charged) winds around histone octamer
(positively charged) to form nucleosome.
○ 1 nucleosome has approx. 200 bp of DNA.
○ Nucleosomes in a chromatin resemble beads present on strings.
○ Beads on string structure in chromatin are further packaged to
form chromatin fibres, which further coil and condense to form
chromosomes during metaphase.
DNA : Structure of Polynucleotide Chain
● DNA − Polymer of deoxyribonucleotides
● Nucleoside = Nitrogenous base + Pentose sugar (linked through
N−
glycosidic bond
)
Example − adenosine, deoxyadenosine, cytidine, etc.
● Nucleotide = Nucleoside + Phosphate group (linked through
phosphodiester bond
)
● Many nucleotides link together through 3′
−5′
phosphodiester bond
to form polynucleotide chain (as in DNA and RNA).
● In course of formation of polynucleotide chain, a phosphate moiety
remains free at 5′ end of ribose sugar (5′ end of polymer chain) and
one OH group remains free at 3′ end of ribose (3′ end of polymer
chain).
Double Helix Model for the Structure of DNA
● Scientists involved
○ Friedrich Meischer
− First identified DNA as an acidic
substance present in nucleus and named it as ‘Nuclein’
○ Wilkins and Franklin
− Produced Xray diffraction data for
DNA structure
○ Watson and Crick
− Proposed double helix structure model for
DNA based on Xray diffraction data
○ Erwin Chargaff
− Proposed that in ds DNA, ratios A:T and C:G
remain same and are equal to one
Features of double helix structure of DNA:
, ● In a DNA, two polynucleotide chains are coiled to form a helix.
Sugarphosphate forms backbone of this helix while bases project in
wards to each other.
● Complementary bases pair with each other through hydrogen bond.
Purines always pair with their corresponding pyrimidines. Adenine
pairs with thymine through two hydrogen bonds while guanine pairs
with cytosine through three hydrogen bonds.
○ The helix is righthanded.
Pitch − 3.4 nm
10 bp in each turn
○ The plane of one base pair stacks over the other in a double
helix. This provides stability to the helix along with hydrogen
bonding.
Packaging of DNA Helix
, Packaging of DNA Helix
● Distance between two consecutive base pairs in a DNA = 0.34 nm =
0.34 × 10
−9
m
● Total number of base pairs in a human DNA = 6.6 × 10
9
bp
● Total length of human DNA = 0.34 × 10
−9
× 6.6 × 10
9
= ~ 2.2 m
● 2.2 m is too large to be accommodated in the nucleus (10
−6
m).
● Organisation of DNA in prokaryotes:
○ They do not have nucleus. DNA is scattered.
○ In certain regions called nucleoids, DNA (negatively charged) is
organised in large loops and is held by some proteins (positively
charged).
● Organisation of DNA in eukaryotes:
○ They have positively charged basic proteins called histones
(positive and basic due to presence of positive and basic amino
acid residues, lysine and arginine).
○ Histone octamer − Unit of eight molecules of histone
○ DNA (negatively charged) winds around histone octamer
(positively charged) to form nucleosome.
○ 1 nucleosome has approx. 200 bp of DNA.
○ Nucleosomes in a chromatin resemble beads present on strings.
○ Beads on string structure in chromatin are further packaged to
form chromatin fibres, which further coil and condense to form
chromosomes during metaphase.
