Ch11DNAreplicationV2+Rev_fdfe33afa98bed784117bb54e890532b

DNA Replication and Structure

Overview of DNA

• ,DNA (Deoxyribonucleic Acid) is the molecule that carries genetic information.

• DNA is structured as a double helix composed of two strands.

Nucleic Acid Structure

• Nucleic Acids consist of Nucleotides:

  • Components of Nucleotides: 5 Carbon Sugar + Phosphate + Nitrogenous Base.

  • Types of Sugars:

    • Ribose in RNA

    • Deoxyribose in DNA

Nucleotide Structure

• Pentose Sugars:

  • Ribose (RNA)

  • Deoxyribose (DNA)

• Nitrogenous Bases:

  • Purines: Adenine (A), Guanine (G)

  • Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

Structure of DNA

• DNA structure includes:

  • Antiparallel strands

  • Phosphodiester bonds connect nucleotides within a strand.

  • Hydrogen bonds form between bases of opposite strands:

    • Adenine (A) pairs with Thymine (T)

    • Guanine (G) pairs with Cytosine (C)

DNA Replication

• Purpose: To create an identical DNA molecule for cell division.

• Type: Semiconservative (each new double helix contains one original and one new strand).

• Bidirectional Process: Replication occurs in two directions from the origin of replication (ORI).

Enzymes Involved in DNA Replication

• Helicase: Unwinds the double-stranded DNA.

• Topoisomerase: Alleviates supercoiling by creating nicks in the DNA.

• Single-Stranded Binding Proteins (SSB): Prevent re-annealing of unwound DNA.

• Primase: Synthesizes short RNA primers to begin DNA synthesis.

• DNA Polymerase III: Main enzyme synthesizing new DNA strands.

• DNA Polymerase I: Removes RNA primers and fills gaps with DNA.

• DNA Ligase: Joins DNA fragments by creating phosphodiester bonds.

Steps in DNA Replication:

1. Unwinding DNA:

   • Helicase unwinds DNA at the ORI.

2. Single-Stranded Binding:

   • SSB proteins stabilize unwound strands.

3. Primer Synthesis:

   • Primase synthesizes short RNA primers.

4. DNA Synthesis:

   • DNA Polymerase III adds dNTPs in the 5' to 3' direction.

5. Joining DNA Fragments:

   • DNA Ligase connects Okazaki fragments on the lagging strand.

Leading and Lagging Strands

• Leading Strand: Synthesized continuously in the direction of the replication fork.

• Lagging Strand: Synthesized in short segments (Okazaki fragments) away from the fork.

Chromosome Structure

• Chromatin: The less condensed form of DNA intertwined with proteins.

• Chromosome: Highly condensed DNA, visible during cell division.

• Nucleosome Structure: DNA wrapped around histone proteins, creating a 'beads-on-a-string' appearance.

• Levels of Chromatin Structure:

  • Level 1: DNA double helix

  • Level 2: Nucleosome (10 nm fiber)

  • Level 3: 30 nm fiber

  • Level 4: Radial loop domains leading to the final chromosome structure.

Summary of Differences Between DNA and RNA

• DNA: Double-stranded, contains deoxyribose, uses Thymine (T).

• RNA: Single-stranded, contains ribose, uses Uracil (U).

Review: Steps of DNA Replication

1. Unwinding: Helicase and Topoisomerase

2. SSB Proteins: Stabilizing unwound DNA

3. Primer Synthesis: Primase

4. DNA Synthesis: Via DNA Polymerase III

5. Completing the DNA: Through DNA Ligase

• Result: Two identical DNA molecules, each consisting of one old strand and one new strand.