Molecular Basis of Inheritance Study Notes

CHAPTER 5: MOLECULAR BASIS OF INHERITANCE

5.1 Overview of DNA

  • Inheritance patterns and the genetic basis of inheritance were not fully understood at the time of Mendel.
  • The realization that DNA (deoxyribonucleic acid) is the genetic material emerged over time.
  • DNA is a polymer of nucleotides, with RNA (ribonucleic acid) also playing roles as messenger and catalyst in certain cases.
  • Key topics of discussion include:
    • Structure of DNA
    • DNA replication
    • Transcription
    • Genetic code
    • Translation
    • Regulation of gene expression
    • Human genome project
    • DNA fingerprinting

5.1.1 Structure of DNA

  • DNA is a long polymer consisting of deoxyribonucleotides.
  • Characteristics:
    • Length defined by the number of nucleotides or base pairs.
    • Examples of organisms and their DNA lengths:
    • Bacteriophage φ ×174: 5386 nucleotides
    • Bacteriophage lambda: 48502 base pairs
    • Escherichia coli: 4.6imes1064.6 imes 10^6 bp
    • Human haploid content: 3.3imes1093.3 imes 10^9 bp
  • Components of a nucleotide:
    • Nitrogenous base: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Uracil, Thymine).
    • Pentose sugar: Ribose in RNA and Deoxyribose in DNA.
    • Phosphate group.
  • Nucleotides linked via:
    • N-glycosidic linkage forms nucleosides.
    • Phosphoester linkage forms nucleotides.
    • Phosphodiester linkage forms dinucleotides and polynucleotides.

5.1.2 DNA Structure Characteristics

  • Antiparallel Strands: One DNA strand runs 5' to 3'; the other runs 3' to 5'.
  • Base Pairing: A pairs with T (2 hydrogen bonds), and G pairs with C (3 hydrogen bonds).
  • Right-Handed Helix: Pitch of the helix is 3.4 nm with $ ext{~10 bp per helical turn}$. Distance between base pairs is approximately 0.34 nm.
  • Stability of helical structure comes from base stacking and hydrogen bonds.

5.2 The Search for Genetic Material

  • Initial hypotheses doubted DNA as the genetic material due to historical context.
  • Griffith's Experiment (1928): Showed transformation in Streptococcus pneumoniae by mixing heat-killed S strain and live R strain, leading to recovery of living S strain.
  • Avery et al. (1933 - 1944): Identified DNA as the transforming principle by showing that only DNA from S bacteria transformed R bacteria.
  • Hershey-Chase Experiment (1952): Confirmed DNA as the genetic material using bacteriophages marked with radioactive phosphorus and sulfur; only the radioactive DNA entered the bacteria.

5.2.1 Properties of Genetic Material

  • Essential properties:
    • Must replicate (self-replicating).
    • Stable chemically and structurally.
    • Allow for mutation and evolution.
    • Express phenotypic traits (as per Mendelian inheritance).
  • Stability: DNA is more stable than RNA due to the lack of a 2’ -OH group in its nucleotides.
  • RNA, while less stable, is dynamic, performs catalytic roles, and can evolve.

5.3 RNA World

  • RNA is thought to be the original genetic material from which DNA evolved due to its stability and dual functionality as both genetic material and a catalyst.

5.4 Replication

  • Semiconservative Replication: Proposed by Watson and Crick; each new DNA molecule consists of one parental and one new strand.
  • Meselson-Stahl Experiment (1958): Confirmed semiconservative replication using heavy nitrogen to distinguish DNA strands.
  • Key Enzymes:
    • DNA-dependent DNA polymerase (catalyzes polymerization)
    • DNA ligase (joins fragments)
  • Replication Process:
    • Initiation at origin of replication.
    • Continuous replication on one strand and discontinuous (Okazaki fragments) on the other due to the antiparallel nature.

5.5 Transcription

  • Transcription involves copying a segment of DNA into RNA using a template strand.
  • Transcription Unit: Composed of a promoter, structural gene, and terminator.
  • Strands Defined:
    • Template Strand: 3' to 5' polarity, used for transcription.
    • Coding Strand: 5' to 3' polarity with a sequence similar to RNA.
  • RNA Types: mRNA, tRNA, rRNA; all required for protein synthesis.
  • Eukaryotic Complexity: Introns and exons exist; splicing and processing need to occur for functional mRNA.

5.6 Genetic Code

  • The genetic code consists of triplets (codons) that specify amino acids. Key features:
    • Comprised of 64 codons (61 for amino acids, 3 stop codons).
    • Degeneracy: More than one codon can specify a single amino acid.
    • Universality: Most codons code for the same amino acids across species.
  • Mutations:
    • Point mutations can lead to diseases like sickle cell anemia.
    • Insertions/deletions can cause frameshift mutations impacting protein synthesis.

5.6.1 tRNA – The Adapter Molecule

  • tRNA binds to specific amino acids and pairs with mRNA codons via anticodons, thus serving as an adapter during protein synthesis.

5.7 Translation

  • Translation synthesizes polypeptides from amino acids. Key aspects:
    • Ribosomes play a central role in the synthesis process, catalyzing peptide bonds.
    • Initiation involves specific recognition of start codon by initiator tRNA.

5.8 Regulation of Gene Expression

  • Gene expression can be regulated at multiple levels:
    • Transcriptional, processing, transport, translational.
  • Lac Operon: A model of regulation involving genes that metabolize lactose, demonstrating negative and positive regulatory mechanisms.

5.9 Human Genome Project

  • A large initiative aimed at sequencing the entire human genome, uncovering the genetic makeup of humans, with significant implications for health, understanding genetic disorders, and medical research.

5.10 DNA Fingerprinting

  • Involves analyzing specific regions of DNA (repetitive DNA) to identify individual genetic differences. Useful in forensic science and paternity testing.

SUMMARY

  • DNA and RNA serve as genetic materials, with various functions in heredity, stability, mutations, and gene regulation.