This part of the notes delves deeper into the structural facets of Deoxyribonucleic Acid (DNA), an integral molecule that encodes genetic information. These notes will traverse the journey of the discovery of DNA's structure, elucidate the characteristic double helix and antiparallel strands, base pairing and the importance of DNA structure in information storage.

Discovery of DNA structure

The discovery of the structure of DNA was a pivotal moment in biological science, propelling our understanding of genetic information into new dimensions. This achievement was a combined endeavour of several scientists, but four critical figures, James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin, stand out. In 1953, their collaborative efforts resulted in the disclosure of the DNA double helix model.

• Rosalind Franklin's X-ray diffraction: Franklin's work was essential to our understanding of the DNA structure. She used X-ray diffraction techniques to capture images of DNA, providing a visual representation of the molecule for the first time. Her photograph 51, a diffraction image of the DNA molecule, played an indispensable role in unveiling its helical structure.
• Watson and Crick's double helix model: Watson and Crick, on the other hand, conceptualised the physical model of DNA. They built upon Franklin's X-ray data and other information to propose the double helix model of DNA. The recognition of the antiparallel nature of the two DNA strands and the specific base pairing was a significant contribution from their end.

The Double Helix and Antiparallel Strands

The discovery that DNA takes the form of a double helix, like a twisted ladder, marked a milestone in the field of genetics. This structure comprises two long polymers of simple units, or nucleotides, with backbones made of sugars (deoxyribose) and phosphate groups. These chains of nucleotides are linked together by phosphodiester bonds, forming a sugar-phosphate backbone.

• Antiparallel nature: A salient feature of the double helix is that these two strands run in opposite directions to each other and are therefore referred to as antiparallel. The sugar-phosphate backbone of one strand descends in the 3' to 5' direction (downwards), while the other ascends in the 5' to 3' direction (upwards). This antiparallel nature is fundamental in DNA replication, as it guides the semi-conservative replication process, ensuring the accurate transfer of genetic information.

Base Pairing

At the heart of the DNA structure is the concept of base pairing. Each nucleotide base on one strand interacts via hydrogen bonds with a base on the opposite strand. This interaction forms the rungs of the DNA 'ladder'.

A-T and C-G pairing: The DNA molecule hosts four types of bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Crucially, Adenine always pairs with Thymine (forming A-T pairs), and Guanine pairs with Cytosine (forming G-C pairs), which is known as Chargaff's rule. The A-T pair forms two hydrogen bonds, while the G-C pair forms three. This specific base pairing is the key to DNA's ability to replicate itself accurately.

Significance of DNA Structure in Information Storage

The DNA molecule's capacity to store genetic information hinges on its unique structure. The double helix model, with its base pairing and antiparallel strands, serves several crucial functions in information storage and replication.

• Blueprint of life: The sequence of the nucleotide bases in a DNA molecule forms the blueprint of life. This sequence determines the instructions for constructing and organising an organism – it's the set of rules for creating proteins, the workhorses of cells. Understanding the protein structure is pivotal in grasping how genetic information translates into functional molecules.
• Genetic transmission: This information is then passed on from generation to generation, ensuring the transmission of inherited traits. The base pairing and double-stranded nature of DNA enable it to replicate accurately during cell division, and thus, genetic information is faithfully passed on to the next generation. Techniques like the Polymerase Chain Reaction (PCR) have been instrumental in studying and amplifying specific DNA sequences.
• DNA repair: The complementary nature of the DNA structure also enables repair. If one strand gets damaged, the sequence of the other strand serves as a template for repair, restoring the original sequence. This mechanism is crucial for maintaining the integrity of genetic information, as demonstrated in DNA profiling techniques which rely on the analysis of DNA sequences for identification purposes.
• Broad Implications: Understanding the structure of DNA has far-reaching implications, from genetics to bioinformatics, to biotechnology. In genetic engineering, DNA sequences are manipulated for various applications, such as creating genetically modified organisms or developing gene therapies for diseases. The structure of DNA is, therefore, the key to manipulating life at its most fundamental level.