Did you know our DNA is 99.9% same?
Let’s dig into the nucleus and get entangled with DNA. DNA stands for deoxyribonucleic acid, it is a long polymer of deoxyribonucleotides. DNA is mostly localised in the nucleus and it carries the information for protein formation. Do you know that DNA has a double helix structure? Let’s dig in deeper!
What is a nucleotide?
A nucleotide is a biomolecule that comprises of:
- Phosphoric acid i.e. PO4–
- Pentose sugar (called deoxyribose sugar in the case of DNA), a sugar that has 5 carbon atoms.
- Nitrogenous base– one nucleotide has one nitrogenous base (nitrogen-containing molecules with chemical properties of a base).
Key features of DNA double helix Structure:
Nitrogenous base and pentose sugar are linked by N-glycosidic bonds. A nucleotide without a phosphate group is called a nucleoside. When a phosphate group is linked to the 5’ end of the nucleoside having hydroxyl group by a phospho-ester linkage it forms a nucleotide. Another key point is that two nucleotides are linked together by 3’-5’ phosphodiester linkage to produce a dinucleotide. Subsequently, many nucleotides join together to form a polynucleotide chain. Basically, the phosphodiester linkages especially impart an inherent polarity to the DNA chain.
A polynucleotide chain has two free ends:
- 5’ end- which has a free phosphate group moiety.
- 3’ end – which has a free hydroxyl (OH) group moiety.
The pentose sugar and the phosphate group form the backbone of the DNA while the nitrogenous bases project outwards. N-glycosidic linkage is what joins the nitrogenous base to the pentose sugar in the polynucleotide chain of the DNA molecules.
Types of Nitrogenous bases:
N-glycosidic linkage is what joins the nitrogenous base to the pentose sugar in the polynucleotide chain of the DNA molecules. Nitrogenous bases are of two types, purines and pyrimidines. It is equally important to understand that the purines include adenine and guanine, and the pyrimidines include cytosine and thymine. Basically, the purines characteristically have a double-ringed structure while the pyrimidines have a single-ring structure.
Base Pairing in DNA:
The major hallmark of the Watson and Crick model of the double helix structure of DNA is base pairing between the nitrogenous bases. This was based on the observations that were made by Erwin Chargaff. He observed that for double-stranded DNA the ratios between Adenine and Thymine and Guanine and Cytosine were constant and equal to one. Finally, this led to the discovery that adenine base pairs with thymine by two hydrogen bonds and guanine base pairs with cytosine by three hydrogen bonds.
DNA double helix structure by Watson and Crick:
Watson and Crick proposed the double helix model of the DNA based on the findings of the X-Ray Diffraction experiments of Maurice Wilkins and Rosalind Franklin. The data from this experiment along with the base-pairing discovery from Erwin Chargaff’s observations helped produce this model. B-DNA is used to indicate the canonical right-handed DNA helix (the most common form of DNA).
Following are some of the features of the double helix model (B-DNA):-
- DNA exists as a right-hand coiled double helix structure of polynucleotide chains in which the backbone is particularly made of sugar-phosphate. The nitrogenous bases project inside.
- The two polynucleotide chains run antiparallel to each other named as 3’-5’ chain and 5’-3’ chain. Basically, these two polynucleotide chains are twisted around each other.
- Due to constant base-pairing between adenine and thymine and guanine and cytosine, a uniform width is obtained between the two strands which are 2nm.
- The Pitch of the DNA helix is 3.4nm, there are about 10 base pairs in each turn, therefore the distance between two base pairs is 0.34nm.
- Basically, a purine always pairs with a pyrimidine.
- Hydrogen bonds between the base pairs and the plane of one base pair stacked over the other are what confers stability to the DNA double helix structure.
Noble Prize on DNA double helix structure:
In 1962 Watson, Crick and Wilkins received the Nobel prize for discovering the double helix structure of DNA with complementary base pairing. Wilkin’s contribution by his X-Ray Diffraction experiments was recognised however Rosalind Franklin’s contribution was not. However, some speculations suggest Wilkins, Watson and Crick stole the data that was experimentally discovered by Rosalind Franklin.
Genetic Implications of DNA Structure:
To summarize, due to the discovery of base pairing between two polynucleotide strands of the double helix structure of DNA, if the sequence of bases on one strand is known the sequence of the other strand, as well as daughter strands that will be generated by this DNA through replication particularly, can be predicted easily. Hence this structure has huge genetic implications and opens a new field of genetics.
“In my view, all that is necessary for faith is the belief that by doing our best we shall succeed in our aims: the improvement of mankind.”
– Rosalind Franklin
Don’t let Franklin’s efforts go to waste, keep reading at MBD.
For more on DNA: Adenine & Guanine structure – My Biology Dictionary