DNA: The Molecule of Life Quiz

DNA: The Molecule of Life

• DNA is the most complex and important molecule in existence.
• Its structure wasn't understood until about 60 years ago.
• DNA is incredibly diverse; if all the DNA from a single cell were stretched out, it would be longer than the individual.
• The human body contains trillions of cells.
• If all the DNA from all the human body cells were connected, it would reach the sun 600 times.
• DNA stores genetic instructions, programming cellular activities.
• It's a 6-billion-letter code assembling the components that make individuals unique.
• This code holds the same principles for all living things.
• Each cell contains 46 chromosomes, each holding a large DNA molecule.
• Chromosomes are packaged with proteins in the cell nucleus.
• DNA is an acid found in the nucleus.
• RNA is another nucleic acid, discussed later.
• Nucleic acids are the fourth major class of biological molecules after carbohydrates, lipids, and proteins.

DNA Structure

• DNA is a polymer, consisting of repeating nucleotides.
• Nucleotides have three components: a 5-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base.
• Four nitrogenous bases define the genetic code: Thymine (T), Adenine (A), Guanine (G), and Cytosine (C).
• DNA exists as a double-stranded molecule, resembling a twisted ladder.
• Sugar and phosphate groups form the ladder's sides.
• These sides run in opposite directions (5' and 3' ends).
• Bases connect the two strands via weak hydrogen bonds.
• Adenine pairs with Thymine (A-T), and Guanine pairs with Cytosine (G-C).
• (G-C) base pairs form three hydrogen bonds, stronger than the two hydrogen bonds of (A-T) pairs.
• The base sequence determines individual traits.
• For example, the sequence TTCAGTCG on chromosome 1 (247 million base pairs) differs from AGGTCCATG.
• Chromosome 1, if printed, would be a 200,000-page book.

DNA Replication

• Cells constantly divide, requiring complete DNA copies.
• This process, replication, produces 10,000 copies of the DNA "book" within hours.
• Every cell in an organism has identical DNA copies from the original copy.
• During a lifetime, DNA replicates itself trillions of times, using each original DNA strand as a template to build a new complementary strand.
• Helicases unwind the DNA double helix, breaking hydrogen bonds.
• Replication begins at the replication fork, with leading and lagging strands.
• Separated strands serve as templates for new strands.
• DNA polymerase adds nucleotides to the leading strand continuously, complementing the sequence.
• A primer is needed for DNA polymerase to initiate DNA synthesis.
• RNA primase adds a short RNA primer for DNA polymerase to attach and begin building the new strand.
• DNA polymerase adds nucleotides only to the 3' end of the strand.
• Lagging strand replication is more complex, involving short Okazaki fragments.
• RNA primase creates multiple RNA primers for the lagging strand.
• DNA polymerase fills in the gaps corresponding to each primer.
• DNA ligase joins Okazaki fragments into a complete lagging strand.
• Replication errors are rare (approximately 1 in 10 billion nucleotides).
• DNA polymerase can often correct errors by removing mismatched nucleotide pairs.

DNA Discovery

• Friedrich Miescher discovered DNA in 1869 by studying white blood cells from bandages.
• He purified a substance, called nuclein, later renamed DNA.
• Rosalind Franklin, using X-ray diffraction, likely first confirmed DNA's helical structure.
• She also identified the sugar-phosphate backbone on the outside of the DNA helix.
• Franklin's contributions were not fully recognized, partly due to her willingness to share knowledge, providing insights to Watson and Crick, and her untimely death.
• Her X-ray images were used by Watson and Crick without her knowledge.
• Watson and Crick were awarded a Nobel Prize in 1962, a prize not awarded posthumously, thus excluding Franklin.