Genetics and Evolution: Molecular Basis of Inheritance

  • Models of replication

Introduction to Molecular Basis of Inheritance

  • DNA structure and composition
  • Central dogma of molecular biology
  • DNA replication
  • Transcription and translation

DNA Structure and Composition

  • Deoxyribonucleic acid (DNA)
  • Double helix structure
  • Nucleotides: sugar, phosphate, nitrogenous base
  • Base pairing: A-T and C-G
  • Complementary strands

Central Dogma of Molecular Biology

  • DNA contains genes
  • Genes encode instructions for protein synthesis
  • Information flow: DNA -> RNA -> Protein
  • Transcription: synthesis of RNA from DNA template
  • Translation: synthesis of protein from RNA template

DNA Replication

  • Semiconservative replication
  • Steps involved in DNA replication:
    • Helicase unwinds the DNA double helix
    • DNA polymerase adds complementary nucleotides
    • Leading and lagging strands
    • DNA ligase joins Okazaki fragments

Transcription

  • Process of converting DNA into RNA
  • RNA polymerase binds to promoter region
  • Initiation, elongation, and termination
  • mRNA, tRNA, and rRNA

Translation

  • Process of protein synthesis
  • Ribosomes and amino acids
  • mRNA codons and tRNA anticodons
  • Initiation, elongation, and termination
  • Genetic code and codon table

Models of Replication

  • Meselson and Stahl experiment
  • Three proposed models of DNA replication:
    • Semiconservative replication
    • Conservative replication
    • Dispersive replication
  • Experimental evidence supporting semiconservative replication

Meselson and Stahl Experiment

  • Aim: to determine the mode of DNA replication
  • Use of isotopes of nitrogen (14N and 15N)
  • Centrifugation and density gradient
  • Results supported the semiconservative replication model

Three Models of DNA Replication

  1. Semiconservative Replication:
    • Each daughter DNA molecule consists of one parental strand and one newly synthesized strand
  1. Conservative Replication:
    • Parental DNA remains intact, and a completely new double-stranded DNA is formed
  1. Dispersive Replication:
    • Both parental and newly synthesized DNA strands are interspersed

DNA Replication: Semiconservative Replication

  • Proposed by Watson and Crick in 1953
  • Each strand of the parental DNA molecule serves as a template for the synthesis of a new complementary strand
  • DNA replication is initiated at specific sites called origins of replication
  • Replication proceeds in both directions from the origin, forming replication forks

Steps in DNA Replication

  1. Initiation:
    • DNA unwinds at the origin of replication
    • Helicase enzyme breaks hydrogen bonds between the DNA strands
  1. Elongation:
    • DNA polymerase adds complementary nucleotides to the growing daughter strands
    • Leading strand is synthesized continuously in the 5’ to 3’ direction
    • Lagging strand is synthesized discontinuously in the form of Okazaki fragments
  1. Termination:
    • DNA replication terminates when the entire DNA molecule is replicated

Leading and Lagging Strands

  • Leading strand:
    • Synthesized continuously in the direction of DNA unwinding
    • Requires only one RNA primer for DNA polymerase to bind and initiate synthesis
  • Lagging strand:
    • Synthesized discontinuously in the opposite direction of DNA unwinding
    • Requires multiple RNA primers for the synthesis of Okazaki fragments

Okazaki Fragments

  • Discontinuous synthesis of the lagging strand results in the formation of Okazaki fragments
  • Okazaki fragments are short DNA segments ranging from 100 to 200 nucleotides
  • RNA primers are added by primase to initiate the synthesis of each Okazaki fragment
  • DNA polymerase extends each fragment from the 5’ to 3’ direction

DNA Ligase

  • DNA ligase is an enzyme that joins the Okazaki fragments together
  • Ligase forms phosphodiester bonds between adjacent nucleotides, sealing the backbone of the daughter DNA strand
  • Ligase plays a crucial role in the overall completion of DNA replication

Errors in DNA Replication

  • DNA replication is highly accurate, but errors can still occur
  • DNA polymerase has a proofreading function that detects and corrects incorrect base pairing
  • Mismatch repair system further corrects errors missed by DNA polymerase
  • Errors in DNA replication can lead to mutations, which can have detrimental effects on an organism

Significance of DNA Replication

  • DNA replication is essential for:
    • Growth and development of organisms
    • Cellular repair and maintenance
    • Transmission of genetic information from one generation to the next
    • Evolution of species through genetic variation and inheritance

Examples of DNA Replication

  • DNA replication occurs during the S phase of the cell cycle
  • Examples of DNA replication in living organisms include:
    • Formation of identical cells during mitotic cell division
    • Reproduction in sexually reproducing organisms
    • Genetic recombination in bacteria and viruses

Equation for DNA Replication

  • DNA replication is summarized by the equation:
    • DNA (parental) -> DNA (daughter strands)
  • The complementary base pairing ensures the accurate synthesis of daughter strands
  • The overall process of DNA replication is highly regulated and tightly controlled

Recap: Molecular Basis of Inheritance - Models of Replication

  • DNA replication is the process of copying a DNA molecule to produce two identical daughter molecules
  • The semiconservative replication model proposed by Watson and Crick is the most accurate model of DNA replication
  • Steps in DNA replication include initiation, elongation, and termination
  • Leading and lagging strands are synthesized in different directions
  • Okazaki fragments are formed during the synthesis of the lagging strand I apologize, but I am unable to provide you with the slides you requested.