Molecular Basis of Inheritance

5.1 The DNA

Structure of Polynucleotide Chain

• DNA is a long polymer of deoxyribonucleotides.
• Basic components:
  • Nitrogenous base
  • Pentose sugar (ribose for RNA, deoxyribose for DNA)
  • Phosphate group
• Types of nitrogenous bases:
  • Purines: Adenine (A), Guanine (G)
  • Pyrimidines: Cytosine (C), Thymine (T for DNA), Uracil (U for RNA)
• DNA structure supports base pairing; A pairs with T, and G pairs with C, forming a double helix.
• DNA is packaged with proteins (histones) forming nucleosomes in eukaryotes.

5.2 The Search for Genetic Material

Transforming Principle

• Griffith's experiments showed that a 'transforming principle' could transfer genetic information.

Hershey-Chase Experiment

• Demonstrated that DNA, not protein, is the genetic material.
• Used radioactive labels to distinguish DNA from proteins in bacteriophages.

5.3 RNA World

• RNA was likely the first genetic material due to its ability to catalyze reactions and carry genetic information.
• DNA evolved from RNA, offering greater stability.

5.4 Replication

• DNA replication is semi-conservative.
• Each strand of DNA serves as a template for a new strand.
• Meselson-Stahl experiment confirmed semi-conservative replication in E. coli.

5.5 Transcription

• Process of copying genetic information from DNA to RNA.
• Involves promoter, structural gene, terminator (transcription unit).
• mRNA is synthesized complementary to the DNA template strand.

5.6 Genetic Code

• Genetic code is a triplet, each codon codes for an amino acid.
• Some codons are stop signals, and AUG is a start codon.
• Code is universal, degenerate, and read without punctuation.

5.7 Translation

• Process of synthesizing proteins from mRNA.
• Occurs in ribosomes; tRNA brings amino acids to the ribosome.
• mRNA codons are translated into a sequence of amino acids.

5.8 Regulation of Gene Expression

• Gene expression is regulated at transcriptional, processing, and translational levels.
• Lac operon is a model for gene regulation in prokaryotes.
• Operon system allows coordinated regulation of gene clusters.

5.9 Human Genome Project

• Aimed to sequence the entire human genome.
• Identified about 20,000-25,000 genes and determined 3 billion base pairs sequence.
• Led to advancements in bioinformatics.

5.10 DNA Fingerprinting

• Based on DNA polymorphisms, particularly in repetitive DNA sequences.
• Used in forensic science and paternity testing.
• Involves techniques such as PCR and Southern blotting.

Summary

• DNA and RNA are polymers of nucleotides; DNA is more stable and primarily stores genetic information.
• Genetic code is universal and consists of codons that dictate protein synthesis.
• Regulation of gene expression is critical for cellular function and response to environmental changes.
• The Human Genome Project provided extensive data on human DNA, fostering new research methods.
• DNA fingerprinting exploits polymorphisms for identification in forensic and genetic studies.