Recording-2025-02-10T18:38:56.474Z

Composition of DNA and RNA

• DNA and RNA are nucleic acids composed of nucleotides.

• Nitrogen Bases: In DNA, the bases are adenine (A), thymine (T), cytosine (C), and guanine (G). In RNA, uracil (U) replaces thymine (T).

• Sugars: DNA contains deoxyribose, while RNA contains ribose.

Structure of Nucleotides

• Nucleotides consist of three components: a phosphate group, a sugar molecule, and a nitrogen base.

• Carbon Atoms: Nucleotide structure is defined by the arrangement of carbon atoms in the sugar ring:

  • Nitrogen base connects to the 1st carbon of the sugar.

  • Phosphate group connects to the 5th carbon of the sugar.

• Directionality is determined by the orientation of the sugar-phosphate backbone:

  • 5' to 3' end: The phosphate at the 5th carbon and the sugar at the 3rd carbon.

  • 3' to 5' end: The reverse orientation of the phosphates and sugars on the complementary strand.

Nitrogen Bases Classification

• Pyrimidines (single ring): Cytosine (C), thymine (T), uracil (U in RNA).

• Purines (double ring): Adenine (A), guanine (G).

• Pairing Rules:

  • Adenine (A) pairs with thymine (T) in DNA (2 hydrogen bonds).

  • Cytosine (C) pairs with guanine (G) (3 hydrogen bonds).

Bonding in DNA

• Phosphodiester Bonds: Covalent bonds connecting the sugar and phosphate groups, forming the backbone of DNA.

  • This occurs through the linkage of the phosphate on the 5th carbon of one nucleotide to the 3rd carbon of the sugar of another nucleotide.

DNA Structure and Functions

• DNA is double-stranded, forming a double helix shape with complementary strands running in opposite directions (antiparallel).

• Each strand has a specific orientation:

  • Leading strand #1 runs continuously towards the replication fork (3' to 5').

  • Lagging strand runs discontinuously away from the fork, creating Okazaki fragments.

DNA Replication Process

• Enzymes involved:

  • Helicase: Unwinds and separates the double-stranded DNA.

  • Primase: Synthesizes RNA primers needed for DNA polymerization.

  • DNA Polymerase III: Adds nucleotides to the growing daughter strand in the 5' to 3' direction.

  • DNA Polymerase I: Replaces RNA primers with DNA nucleotides.

  • Ligase: Joins Okazaki fragments together on the lagging strand.

Replication Mechanics

• Origin of Replication: Sites of DNA unwinding for synthesis, creating replication bubbles that expand as replication progresses.

• Leading Strand Synthesis: Continuous addition of nucleotides, utilizing one primer.

• Lagging Strand Synthesis: Involves multiple primers and is formed in segments (Okazaki fragments) due to 5' to 3' synthesis direction.

Proofreading and Mutation Correction

• DNA polymerases can proofread each nucleotide as it is added, making corrections to maintain fidelity of DNA replication.

• The accuracy of DNA copying is high, with a low mutation rate even during replication, ensuring genetic stability.

Comparison of Prokaryotic and Eukaryotic DNA

• Prokaryotic DNA has a circular chromosome and often contains plasmids, offering genetic flexibility through horizontal gene transfer.

• Eukaryotic DNA is linear and organized within a nucleus, requiring more complex mechanisms for packing and binding to histone proteins to fit within the cell nucleus.