The three-dimensional structure of DNA proposed by Watson and Crick, as well as recent advances in this field, are summarised below: DNA is made up of two helical chains that are coiled around the same axis to form a right-handed double helix. The two chains in the helix are antiparallel, meaning that the 5′ end of one polynucleotide chain and the 3′ end of the other polynucleotide chain are on the same side and close together. Each turn is separated by 3.6 nm (formerly 3.4 nm). Each turn contains 10.5 nucleotides (formerly 10 nucleotides). Major and minor grooves are formed between the two strands due to their spatial relationship. Some proteins interact in these grooves. The outside of the double helix has hydrophilic backbones made up of alternating deoxyribose and negatively charged phosphate groups. The hydrophobic pyrimidine and purine bases are located inside the double helix, which helps to stabilize the DNA double helix. An inter-chain hydrogen bond formed between a purine and pyrimidine base also helps to stabilise the double helix. A particular purine base forms hydrogen bonds with only one pyrimidine base, i.e Adenine (A) pairs with Thymine (T) and Guanine (G) with Cytosine (C). Adenine and Thymine (A = T) form two hydrogen bond pairs, while Guanine and Cytosine (G ≡ C) form three hydrogen bond pairs. Complementary base pairs are A = T and G ≡ C. The two chains of the DNA double helix are complementary to each other due to the presence of complementary base pairing. One strand of the double helix is known as the sense strand, which codes for RNA/proteins, and the other is known as the antisense strand.

1. Watson &
Crick
Model
By Sajib.
ZOOLOGY EXPLORER

2. Zoology Explorer
Watson & Crick Model
In the year 1953, Watson and Crick proposed the DNA as a supermodel.
X-ray crystallography was used by Maurice Wilkins and Rosalind Franklin to
discover the picture of DNA. The X-ray diffraction image of DNA & concepts of
chargaff's base pairing aided Watson and Crick in their research into the structure
and components of DNA. Watson and Crick proposed a model for DNA known as
Watson and Crick’s model of double-helical DNA as a result of this.
In 1962, they were awarded the Nobel Prize for this outstanding
achievement.

3. Zoology Explorer
10Å 10Å
Nucleotides aren't arranged in a straight line;
instead, they form an angle between
themselves. Angle between two nucleotide is
36°.
A complete turn makes 360°. As a result, a
complete turn contains 10 nucleotides.
360°/10= 36°.
The distance between two nucleotide is 3.4Å.
So a complete turn makes 34Å.
The length between two DNA strand is 20Å.

4. Zoology Explorer

5. Zoology Explorer
SUMMARY
DNA is made up of two helical chains that are coiled around the same axis to form a right-handed
double helix.
The two chains in the helix are antiparallel, meaning that the 5′ end of one polynucleotide chain and
the 3′ end of the other polynucleotide chain are on the same side and close together.
Each turn is separated by 3.6 nm (formerly 3.4 nm).
Each turn contains 10.5 nucleotides (formerly 10 nucleotides).
Major and minor grooves are formed between the two strands due to their spatial relationship.
Some proteins interact in these grooves.
The outside of the double helix has hydrophilic backbones made up of alternating deoxyribose and
negatively charged phosphate groups.
The hydrophobic pyrimidine and purine bases are located inside the double helix, which helps to
stabilize the DNA double helix.

6. An inter-chain hydrogen bond formed between a purine and pyrimidine base also helps to stabilise
the double helix.
A particular purine base forms hydrogen bonds with only one pyrimidine base, i.e Adenine (A) pairs
with Thymine (T) and Guanine (G) with Cytosine (C).
Adenine and Thymine (A = T) form two hydrogen bond pairs, while Guanine and Cytosine (G ≡ C)
form three hydrogen bond pairs.
Complementary base pairs are A = T and G ≡C.
The two chains of the DNA double helix are complementary to each other due to the presence of
complementary base pairing.
One strand of the double helix is known as the sense strand, which codes for RNA/proteins, and the
other is known as the antisense strand.
Zoology Explorer
SUMMARY

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