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DNA Double Helix
The structure of DNA is referred to as a double helix as it resembles a twisted staircase. A molecule of the DNA comprises two strands wound around each other as though a twisted ladder where each strand comprises a backbone comprising alternating groups of phosphate and sugar groups. The nitrogenous bases are centrally placed holding together these two strands.
One of the 4 bases is attached to each of the sugar – (A) adenine (G) guanine (C) cytosine or (T) thymine. Both strands are united with the help of bonds between the bases – adenine forms a base pairing with thymine whereas cytosine pairs with guanine. This pairing is referred to as the base complementary rule since the DNA strands are complementary to one another.
Thus, when the sequence of a strand is TTGGCCAA, then the complementary strand should have AACCGGTT as its sequence.
As a result of this base pairing, DNA strands are antiparallel, complementary to one another and running in the opposite directions. That is to say, the 5′ carbon end of one of the strands faces the 3′ carbon end of its complementing strand. This orientation that runs antiparallel is crucial for replication of DNA and several nucleic acid interactions.
Each DNA strand in the double helix is linear and long, comprising smaller units known as nucleotides which in turn form a chain. The chemical bonds referred to as the sugar-phosphate backbones are formed by the phosphate and sugar molecules.
Also see: DNA Ligase
Who proposed this structure?
James Watson and Francis Crick pioneered in 1953 to put forward the molecular structure of DNA which they termed the “double helix” in a journal. In 1962, Watson and Crick along with Maurice Wilkins (their colleague) were even awarded the Nobel Prize for this breakthrough in the history of medicine.
Each strand of the DNA copies during replication of the DNA leading to the formation of a daughter DNA double helix. It contains one parental DNA strand and one newly synthesized DNA strand. There is a possibility at this juncture for any change in the sequence of the nitrogen base to take place, in short mutation to occur.
In most cases, the DNA proactively takes charge when mutation occurs and is thereby able to fix itself and gain back its original base to the sequence. On the other hand, when repair is not seemingly a possibility, new proteins take birth.
To summarize, the unique structure of DNA allows molecules to copy itself while the cell division process is on. Whenever a cell is ready for division, the DNA helix breaks into two and identifies itself as two individual strands. The single strands hence formed act as templates to build two new double-stranded DNA molecules, each of which is a replica of the original DNA molecule. While this process is on, A (adenine) bases are added where there is a T (thymine) and similar is the case with C and G as aforementioned. Consequently, these bases now have partners once again.
During the formation of proteins, this double helix unwinds enabling a single strand of the DNA to act as a template which is in turn transcribed into the mRNA. The mRNA is a molecule conveying crucial instructions to the protein generating factory of the cell.
That was a brief on the double-helical structure of DNA. Explore other interesting concepts for your NEET preparation, at BYJU’S.
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Frequently Asked Questions
What is Watson and Crick model of DNA?
Watson and Crick proposed the molecular structure of DNA which they termed the “double helix”. The specific pairing of nucleotides is a key feature of the Watson and Crick model of DNA. Here, each DNA strand is linear and long, comprising smaller units known as nucleotides which in turn form a chain.
What is Chargaff’s rule?
According to Chargaff’s rule, the amount of A is equal to T, whereas the amount of C is equal to G. The DNA of any cell from any organism should have a 1:1 ratio of purine (A and G) and pyrimidine (C and T) bases.