Understanding DNA: History and Structure

• Chromatin is wound tightly to form a visible chromosome.
• Unwinding chromatin reveals DNA, which stands for Deoxyribonucleic Acid.
• Chromatin consists of DNA, RNA, and proteins.

History of DNA Discovery

• DNA's existence was discovered in 1869 by Friedrich Miescher.
• In 1944, Oswald Avery determined DNA's importance as a "transformative factor" responsible for passing on information.
• It wasn’t until 1953 that James Watson and Francis Crick discovered DNA's double helix structure.
• Watson and Crick built upon the work of scientists such as Rosalind Franklin.
• Watson and Crick received the Nobel Prize for their discovery.
• Rosalind Franklin was excluded because the Nobel committee does not award prizes posthumously.
• DNA serves as the blueprint for designing an organism, making everyone's DNA unique, except in the case of identical twins.

DNA Structure

• DNA is a large polymer molecule.
• It falls under the category of nucleic acids.
• Polymers consist of smaller units called monomers.
• Nucleic acids are made up of nucleotide monomers linked together.
• Nucleotides have three parts: a phosphate group, a deoxyribose (5-sided sugar), and a nitrogenous base.

Nitrogenous Bases

• The phosphate and deoxyribose components remain constant in a nucleotide, while the nitrogenous bases vary.
• The nitrogenous bases can be adenine, guanine, cytosine, or thymine.
• Adenine and guanine are purines, while cytosine and thymine are pyrimidines.
• Erwin Chargaff's research contributed to Watson and Crick's work.
• Chargaff discovered that adenine always binds with thymine (A-T), and guanine always binds with cytosine (G-C).
• This is known as Chargaff's Rule.
• Hydrogen bonds link the bases.
• A sequence of bases creates genes, which determine the traits of living things.

DNA Replication

• DNA copying occurs during interphase before cell division.
• DNA unzipping facilitated by the enzyme helicase separates the hydrogen bonds between base pairs.
• Unzipped strands act as templates.
• DNA polymerase pairs free-floating nucleotides with the unzipped templates.
• One strand is the leading strand and the other is the lagging strand.
• The terms correspond to the direction of their deoxyribose molecules.
• The leading strand builds towards the replication fork.
• The lagging strand builds away from the replication fork in segments called Okazaki fragments.
• The original DNA molecule continues to separate, and new strands are simultaneously assembled.
• When the molecule is completely separated, two identical DNA molecules are created.