DNA Structure and Replication: Crash Course Biology #10

Key Concepts:

• DNA (Deoxyribonucleic acid): Stores genetic instructions.
• RNA (Ribonucleic acid): Crucial in protein production and DNA replication.
• Nucleotides: The building blocks of DNA and RNA, consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base.
• Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G) in DNA; Adenine (A), Uracil (U), Cytosine (C), Guanine (G) in RNA.
• Base Pairs: A-T and G-C in DNA; A-U and G-C in RNA.
• Double Helix: The intertwined, double spiral staircase structure of DNA.
• Sugar-Phosphate Backbone: The main support structure of DNA.
• 5' and 3' Ends: Designations based on the carbon atoms of the deoxyribose sugar, indicating the direction of the DNA strand.
• Replication: The process of copying DNA.
• Helicase: Enzyme that unwinds the DNA double helix.
• Replication Fork: The point where the DNA double helix splits during replication.
• Leading Strand: The DNA strand that is synthesized continuously during replication.
• Lagging Strand: The DNA strand that is synthesized in segments (Okazaki fragments) during replication.
• DNA Polymerase: Enzyme that adds matching nucleotides to the main stem during replication.
• RNA Primase: Enzyme that creates RNA primers to initiate DNA synthesis.
• Okazaki Fragments: Short DNA segments synthesized on the lagging strand during replication.
• DNA Ligase: Enzyme that joins Okazaki fragments together.

1. DNA: The Molecule of Life

• DNA stores genetic instructions, programming cell activities. It's a 6-billion letter code.
• Human somatic cells contain 46 chromosomes, each with a DNA molecule.
• DNA is a nucleic acid, like RNA. Nucleic acids are polymers made of repeating nucleotide units.
• If untangled, the DNA from one cell would be taller than the presenter. The DNA in all cells would stretch to the sun 600 times.

2. Nucleotide Structure

• Nucleotides consist of:
  • A five-carbon sugar molecule (deoxyribose in DNA).
  • A phosphate group.
  • One of four nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
• The genetic code resides in the sequence of these nitrogenous bases.

3. DNA's Double Helix Structure

• DNA exists as a pair of polynucleotide molecules forming a double helix.
• The sugar and phosphate groups form the sugar-phosphate backbone.
• The two strands run in opposite directions, designated as 5' to 3' and 3' to 5'. The 5' end has a phosphate group connected to the sugar molecule's 5th carbon, while the 3' end has a free end at the sugar's 3rd carbon.
• Nitrogenous bases link the two strands via hydrogen bonds: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).
• G-C pairing has three hydrogen bonds, making it slightly stronger than the A-T base-pair, which only has two bonds.
• The base sequence (e.g., AGGTCCATG) determines genetic information.
• Human chromosome 1 contains 247 million base pairs.

4. RNA: A Close Cousin

• RNA is similar to DNA but has three key differences:
  1. It is single-stranded.
  2. The sugar is ribose (with one more oxygen atom than deoxyribose).
  3. It contains uracil (U) instead of thymine (T), so A pairs with U.
• RNA is crucial for protein production and DNA replication.

5. The Discovery of DNA: A Collaborative Effort

• DNA was discovered in 1869 by Friedrich Miescher, who isolated "nuclein" from white blood cells.
• Rosalind Franklin used X-ray diffraction to confirm DNA's helical structure and the location of the sugar-phosphate backbone.
• James Watson and Francis Crick are often credited, but they built upon Franklin's work, and she did not receive due credit, partly due to her early death and the rules of the Nobel Prize.

6. DNA Replication: Copying the Code

• Replication is the process of creating a complete copy of DNA.
• The enzyme Helicase unwinds the double helix, creating a replication fork.
• The leading strand is synthesized continuously by DNA polymerase, which adds nucleotides in the 5' to 3' direction. RNA primase creates a primer for DNA polymerase to start.
• The lagging strand is synthesized in segments (Okazaki fragments) because DNA polymerase can only add nucleotides to the 3' end. RNA primase lays down short RNA primers, and DNA polymerase works backward along the strand.
• Another DNA polymerase replaces the RNA primers, and DNA ligase joins the Okazaki fragments.
• DNA replication has a low error rate (one in every 10 billion nucleotides), and DNA polymerases can proofread and correct mismatches.

7. Conclusion

DNA is a complex and vital molecule that stores genetic information and replicates with high fidelity. Its discovery and understanding involved the contributions of multiple scientists, including Friedrich Miescher and Rosalind Franklin, though the latter's contributions were not fully recognized during her lifetime. The process of DNA replication involves several enzymes and occurs with remarkable accuracy.