Molecular Basis of Inheritance

Overview

• DNA is identified as the genetic material of most organisms.
• RNA acts mainly as a messenger, adapter, structural molecule, and sometimes as a genetic material.
• The chapter covers DNA structure, replication, transcription, genetic code, translation, and gene expression regulation.

5.1 The DNA

• Long polymer of deoxyribonucleotides.
• Length defined by nucleotides/base pairs (bp).
• Examples: Bacteriophage λ: 48502 bp, E. coli: 4.6 x 10^6 bp, Human DNA: 3.3 x 10^9 bp.

Structure of Polynucleotide Chain

• Nucleotide: nitrogenous base, pentose sugar, phosphate group.
• Two types of nitrogenous bases:
  • Purines: Adenine (A) & Guanine (G)
  • Pyrimidines: Cytosine (C), Uracil (U), Thymine (T)
• DNA has Thymine, RNA has Uracil.
• DNA discovered by Friedrich Meischer as Nuclein.
• Double Helix model by Watson & Crick based on Chargaff's rule and X-ray diffraction data.

Packaging of DNA Helix

• DNA packages into nucleosomes in eukaryotes with histones.
• Prokaryotic DNA organized in nucleoids.
• Chromatin: Euchromatin (active), Heterochromatin (inactive).

5.2 The Search for Genetic Material

• Early 20th-century studies focused on chromosomes.
• Griffith's experiment with Streptococcus pneumoniae showed transformation.
• Avery, MacLeod, and McCarty concluded that DNA is the transforming principle.
• Hershey-Chase experiment confirmed DNA as genetic material.

5.3 RNA World

• RNA possibly the first genetic material.
• Catalytic properties but less stable than DNA.
• DNA evolved from RNA with greater stability.

5.4 Replication

• Semi-conservative replication model by Watson & Crick.
• Experimentally proven by Meselson and Stahl.
• Involves DNA-dependent DNA polymerase, replication fork, and origin of replication.

5.5 Transcription

• Process of creating RNA from DNA.
• Only a segment of DNA is transcribed.
• Transcription unit: Promoter, Structural gene, Terminator.
• In eukaryotes, includes splicing, capping, and tailing.

5.6 Genetic Code

• Triplet codon system with 64 codons.
• Genetic code is redundant but universal.
• AUG as start codon; UAA, UAG, UGA as stop codons.

5.7 Translation

• Process of assembling proteins from amino acids according to mRNA.
• Involves ribosomes, tRNA, and mRNA.
• Peptide bonds form between amino acids.

5.8 Regulation of Gene Expression

• Regulated at transcriptional, processing, and translational levels.
• Lac operon as a model of transcriptional regulation in prokaryotes.

5.9 Human Genome Project

• Aimed to sequence the entire human genome.
• Completed in 2003, revealed information about genes and polymorphisms.

5.10 DNA Fingerprinting

• Technique to identify differences in DNA sequences among individuals.
• Based on polymorphisms, useful for forensic science and paternity testing.

Summary

• DNA is the genetic material with RNA as a messenger.
• DNA replication is semi-conservative.
• Transcription converts DNA to RNA; translation uses RNA to synthesize proteins.
• Human Genome Project expanded understanding of human genetics.
• DNA fingerprinting utilizes polymorphisms for identification.