Lecture Notes on Chromosomes and DNA Replication

Overview

• Presenter: Iman
• Topic: Continuation of Chapter 16
• Main Objectives:
  1. DNA as the genetic material (covered previously)
  2. Chromosome structure and DNA replication

Chromosome Structure

• Chromosome Composition: DNA molecule packed with proteins
• DNA Compaction:
  • DNA is compacted into chromosomes during cell division preparation.
  • 46 chromosomes exist in a less compact form called chromatin when not dividing.

DNA in Organelles

• Mitochondria and Chloroplasts:
  • Contain circular DNA
  • Provide energy and genes for cell processes

DNA Compaction Process

1. Nucleosomes Formation:
   • DNA wraps around proteins called histones to form nucleosomes.
2. Chromatin Formation:
   • Nucleosomes coil to form chromatin, the structure in non-dividing cells.
3. Chromosome Formation:
   • Chromatin further condenses to form chromosomes during cell division.

Proteins Involved in DNA Compaction

• Histone Proteins: Bind closely to DNA due to opposite charges.
• Non-Histone Proteins: Involved in spatial organization and gene regulation.

Types of Chromatin

• Heterochromatin: Dense, transcriptionally silent, appears dark under a microscope.
• Euchromatin: Less dense, transcriptionally active, appears light under a microscope.

Key Chromosome Structures

• Telomeres:
  • End of chromosomes
  • High GC content to prevent DNA unraveling
• Centromeres:
  • Middle of chromosomes
  • Hold sister chromatids together during anaphase

DNA Replication

• Definition: Process of creating two identical DNA molecules from one original molecule.
• Nature: Semi-conservative and all-or-none process.

Steps of DNA Replication

1. Origin of Replication:
   • Initiator proteins bind and unwind DNA.
2. DNA Helicase:
   • Unwinds DNA to form replication fork.
3. Topoisomerase (DNA Gyrase):
   • Relieves strain from unwinding.
4. Single-Strand Binding Proteins:
   • Stabilize unwound DNA strands.
5. DNA Primase:
   • Lays down RNA primers for DNA polymerase function.
6. DNA Polymerase III:
   • Synthesizes new DNA strands (5' to 3' direction).
7. Lagging Strand Synthesis:
   • DNA synthesized in Okazaki fragments with multiple RNA primers.
8. DNA Polymerase I:
   • Replaces RNA primers with DNA.
9. DNA Ligase:
   • Links Okazaki fragments together.

Differences in Replication

• Prokaryotes: Circular chromosomes with one origin of replication.
• Eukaryotes: Linear chromosomes with multiple origins to speed up replication.

Conclusion

• The chapter concludes with a detailed explanation of the DNA replication process.