Genetics and Evolution - Molecular Basis of Inheritance

  • Introduction to DNA replication
  • Importance of DNA replication
  • DNA polymerases and other enzymes
  • Replication in prokaryotes and eukaryotes
  • Semi-conservative replication

Basics of DNA Replication

  • DNA: Deoxyribonucleic Acid
  • Composed of nucleotides
  • Double-stranded helical structure
  • Complementary base pairing
    • Adenine (A) with Thymine (T)
    • Cytosine (C) with Guanine (G)

Importance of DNA Replication

  • Essential for growth and development
  • Ensures continuity of genetic information
  • Allows inheritance of traits and characteristics
  • Enables cell division and tissue repair
  • Replication errors can lead to diseases like cancer

Enzymes Involved in DNA Replication

  • DNA Polymerase: synthesizes new DNA strands
  • Helicase: unwinds the DNA double helix
  • Primase: synthesizes RNA primers
  • Ligase: joins Okazaki fragments
  • Topoisomerase: relieves DNA torsional stress

Replication in Prokaryotes

  • Prokaryotic DNA is circular
  • Replication starts at a specific origin site
  • Bidirectional replication
  • Replication forks moving in opposite directions
  • Leading and lagging strands formed

Replication in Eukaryotes

  • Eukaryotic DNA is linear
  • Multiple origins of replication
  • Replication forks progress in both directions
  • Telomeres at ends of chromosomes
  • Problematic replication of telomeres

Semi-Conservative Replication

  • Proposed by Watson and Crick
  • Each new DNA molecule has one original and one new strand
  • Demonstrated by Meselson and Stahl experiment
  • Conservative and dispersive replication disproved

Steps of DNA Replication

  1. Initiation:
    • Replication origin recognized by initiator proteins
    • Helicase unwinds the double helix
    • Single-stranded DNA binding proteins stabilize the strands
  1. Elongation:
    • DNA polymerase adds new nucleotides according to base pairing rules
    • Leading and lagging strands synthesized simultaneously

Steps of DNA Replication (continued)

  1. Priming:
    • Primase synthesizes RNA primers on the lagging strand
    • Provides a starting point for DNA polymerase
    • Okazaki fragments formed on the lagging strand
  1. Joining:
    • DNA ligase joins the Okazaki fragments together on the lagging strand
    • Completes the synthesis of both strands

DNA Replication Summary

  • DNA replication ensures genetic continuity
  • Involves DNA polymerase, helicase, primase, ligase, and topoisomerase
  • Follows a semi-conservative mechanism
  • Prokaryotic and eukaryotic replication differs
  • Steps include initiation, elongation, priming, and joining

Why DNA Replication is Essential

  • DNA replication is essential for the transmission of genetic information from one generation to the next
  • It plays a crucial role in cell division, growth, and development
  • DNA replication ensures genetic continuity and stability
  • Errors during replication can lead to mutations and genetic disorders
  • Examples of genetic disorders caused by replication errors:
    • Down syndrome
    • Cystic fibrosis
    • Huntington’s disease
    • Sickle cell anemia

DNA Replication Errors

  • Errors during DNA replication can occur due to various reasons:
    • Spontaneous DNA damage
    • Exposure to mutagenic agents (chemicals, radiation, etc.)
    • Errors by DNA polymerase
    • Replication fork stalling or collapse
  • DNA repair mechanisms exist to correct replication errors and maintain genetic integrity

DNA Replication in Prokaryotes

  • Prokaryotes have a circular DNA molecule
  • DNA replication starts at a specific origin site
  • Enzymes involved:
    • Helicase unwinds DNA at the origin
    • Single-stranded DNA binding proteins stabilize the unwound strands
    • DNA polymerase synthesizes new DNA strands in both directions
    • DNA ligase joins the Okazaki fragments on the lagging strand

DNA Replication in Eukaryotes

  • Eukaryotes have linear DNA molecules
  • Multiple origins of replication
  • Enzymes involved:
    • Helicase unwinds DNA at each origin
    • DNA polymerase synthesizes new strands in both directions
    • DNA repair proteins check for errors and fix them
    • Telomerase extends the telomeres to prevent shortening with each replication cycle

Leading and Lagging Strands

  • During DNA replication, one strand is synthesized continuously, called the leading strand
  • The other strand is synthesized in short fragments, called Okazaki fragments, on the lagging strand
  • The discontinuous synthesis on the lagging strand is due to its anti-parallel orientation to the leading strand
  • DNA ligase joins the Okazaki fragments on the lagging strand

Telomeres and Telomerase

  • Telomeres are protective caps at the ends of eukaryotic chromosomes
  • They prevent degradation and fusion of chromosomal ends
  • Telomeres shorten with each replication cycle due to incomplete replication at the ends
  • Telomerase is an enzyme that can extend the telomeres, maintaining chromosomal stability
  • Telomerase activity is regulated in normal cells to prevent unlimited cell division (cancer)

Mutations and DNA Replication

  • Mutations are changes in the DNA sequence
  • Some mutations can be beneficial, neutral, or harmful
  • Replication errors are a common cause of mutations
  • Types of mutations:
    • Point mutations: single-base changes (substitutions, insertions, deletions)
    • Frameshift mutations: insertion or deletion of bases, altering the reading frame

DNA Replication and Cancer

  • DNA replication errors can contribute to the development of cancer
  • DNA damage response mechanisms can detect and repair replication errors
  • If these mechanisms fail, mutations can accumulate in critical genes (oncogenes or tumor suppressor genes)
  • The accumulation of mutations can lead to uncontrolled cell division and the formation of tumors

Replication and Transcription

  • DNA replication and transcription are two separate processes
  • DNA replication occurs in the nucleus during the S-phase of the cell cycle
  • Transcription occurs in the nucleus or cytoplasm
  • DNA is replicated to ensure the transmission of genetic information, while transcription produces mRNA for protein synthesis

DNA Replication and Evolution

  • DNA replication is a key process in evolution
  • Accurate replication ensures the transmission of genetic information across generations
  • Errors during replication can introduce genetic variations
  • Genetic variations contribute to the diversity of species and the process of natural selection
  • Evolutionary processes rely on the fidelity and variability of DNA replication.

Differences between Prokaryotic and Eukaryotic DNA Replication

  • Prokaryotic DNA Replication:
    • Circular DNA molecule
    • Single replication origin
    • Bidirectional replication
    • Simultaneous synthesis of leading and lagging strands
  • Eukaryotic DNA Replication:
    • Linear DNA molecules
    • Multiple replication origins
    • Replication forks progress in both directions
    • Telomeres and telomerase for end replication

Steps of DNA Replication - Initiation

  • Replication initiation occurs at specific origins
  • Initiator proteins recognize and bind to the origin
  • Helicase unwinds the DNA double helix at the origin
  • Single-stranded DNA binding proteins stabilize the unwound strands
  • Replication forks are formed

Steps of DNA Replication - Elongation

  • DNA polymerase synthesizes new DNA strands
  • DNA polymerase can only add nucleotides in the 5’ to 3’ direction
  • Leading strand synthesized continuously in the same direction as the replication fork
  • Lagging strand synthesized discontinuously in short fragments (Okazaki fragments)

Steps of DNA Replication - Priming

  • Primase synthesizes RNA primers on the lagging strand
  • RNA primers provide a starting point for DNA synthesis
  • Primers are required because DNA polymerase can only add nucleotides to existing strands
  • RNA primers are later removed and replaced with DNA by DNA polymerase

Steps of DNA Replication - Joining

  • DNA polymerase synthesizes DNA by adding nucleotides to the RNA primers
  • On the leading strand, DNA synthesis is continuous
  • On the lagging strand, DNA synthesis is discontinuous, forming Okazaki fragments
  • DNA ligase joins the Okazaki fragments together by sealing the nicks

Semi-Conservative DNA Replication

  • Watson and Crick proposed that DNA replication is semi-conservative
  • Each newly synthesized DNA molecule consists of one original strand and one new strand
  • This was confirmed by the Meselson-Stahl experiment using labeled isotopes of nitrogen
  • Other replication models like conservative and dispersive were disproved

DNA Replication and Protein Synthesis

  • DNA replication and protein synthesis are interconnected processes
  • DNA replication provides the genetic information for protein synthesis
  • During transcription, DNA is transcribed into mRNA
  • The mRNA then moves to the cytoplasm for translation into proteins
  • Accurate DNA replication is crucial for maintaining the integrity of the genetic information

Replication Errors and Genetic Disorders

  • Replication errors can lead to mutations and genetic disorders
  • Examples of genetic disorders caused by replication errors:
    • Down syndrome: caused by an extra copy of chromosome 21
    • Cystic fibrosis: caused by mutations in the CFTR gene
    • Huntington’s disease: caused by CAG repeat expansion in the HTT gene
    • Sickle cell anemia: caused by a point mutation in the beta-globin gene

Proofreading and DNA Repair

  • DNA polymerase has proofreading activity to correct errors during replication
  • Mismatch repair enzymes recognize and remove mismatched bases after replication
  • Nucleotide excision repair removes bulky DNA lesions caused by UV radiation
  • Base excision repair corrects chemically altered bases
  • DNA repair mechanisms help maintain the integrity of the genetic information

Conclusion

  • DNA replication is essential for the transmission of genetic information from one generation to the next
  • It ensures the continuity and stability of genetic material
  • DNA replication occurs in a semi-conservative manner
  • Prokaryotic and eukaryotic DNA replication differ in their organization and mechanism
  • Replication errors can lead to mutations and genetic disorders, highlighting the importance of DNA repair mechanisms