Slide 1

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

Slide 2

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)

Slide 3

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

Slide 4

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

Slide 5

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

Slide 6

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

Slide 7

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

Slide 8

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

2. Elongation:

   • DNA polymerase adds new nucleotides according to base pairing rules
   • Leading and lagging strands synthesized simultaneously

Slide 9

Steps of DNA Replication (continued)

3. Priming:

   • Primase synthesizes RNA primers on the lagging strand
   • Provides a starting point for DNA polymerase
   • Okazaki fragments formed on the lagging strand

4. Joining:

   • DNA ligase joins the Okazaki fragments together on the lagging strand
   • Completes the synthesis of both strands

Slide 10

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

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Slide 11

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

Slide 12

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

Slide 13

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

Slide 14

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

Slide 15

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

Slide 16

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)

Slide 17

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

Slide 18

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

Slide 19

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

Slide 20

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.

Slide 21

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

Slide 22

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

Slide 23

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)

Slide 24

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

Slide 25

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

Slide 26

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

Slide 27

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

Slide 28

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

Slide 29

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

Slide 30

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