The Molecular Basis of Inheritance & Gene Expression: Study Notes (Campbell Biology, Ch. 16 & 17)

Chapter 16: The Molecular Basis of Inheritance

DNA as the Genetic Material

Early 20th-century biologists sought to identify the molecules responsible for inheritance. The discovery that genes are located on chromosomes led to the hypothesis that either DNA or protein could be the genetic material. Key experiments with bacteria and viruses established DNA as the hereditary molecule.

• Griffith's Experiment: Demonstrated transformation, where non-pathogenic bacteria became pathogenic by assimilating DNA from heat-killed pathogenic bacteria.

• Avery, McCarty, MacLeod: Identified DNA as the transforming substance, though skepticism remained due to limited knowledge of DNA.

• Hershey-Chase Experiment: Showed that DNA, not protein, is the genetic material in phage T2 by tracking radioactive labels during infection of E. coli.

• Chargaff's Rules: DNA composition varies by species; in any species, the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C).

The Structure of DNA

DNA is a polymer of nucleotides, each consisting of a nitrogenous base (A, T, G, C), a deoxyribose sugar, and a phosphate group. The double helix model, proposed by Watson and Crick, was based on X-ray crystallography data from Rosalind Franklin and Maurice Wilkins.

• Double Helix: Two antiparallel strands form a right-handed helix.

• Base Pairing: A pairs with T, G pairs with C, via hydrogen bonds. Pairing a purine with a pyrimidine maintains uniform helix width.

• Antiparallel Orientation: The two strands run in opposite 5' to 3' directions.

DNA Replication

DNA replication ensures genetic information is accurately transmitted during cell division. The process is semiconservative: each new DNA molecule consists of one parental and one new strand.

• Origins of Replication: Specific sequences where replication begins, forming replication bubbles.

• Replication Fork: Y-shaped region where DNA is unwound and new strands are synthesized.

• Key Enzymes:

  • Helicase: Unwinds the DNA double helix.

  • Single-Strand Binding Proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves strain ahead of the replication fork.

  • Primase: Synthesizes short RNA primers.

  • DNA Polymerase: Adds nucleotides to the 3' end of the primer; requires a template and primer.

  • Ligase: Joins Okazaki fragments on the lagging strand.

• Leading vs. Lagging Strand: The leading strand is synthesized continuously toward the fork; the lagging strand is synthesized discontinuously as Okazaki fragments away from the fork.

Enzyme/Protein	Function
Helicase	Unwinds parental DNA
Single-Strand Binding Protein	Stabilizes single-stranded DNA
Topoisomerase	Relieves overwinding strain
Primase	Synthesizes RNA primer
DNA Polymerase III	Main enzyme for DNA synthesis
DNA Polymerase I	Replaces RNA primer with DNA
Ligase	Joins DNA fragments

Proofreading and Repair

DNA polymerases proofread newly synthesized DNA, correcting errors. Additional repair mechanisms, such as mismatch repair and nucleotide excision repair, maintain genetic fidelity. Mutations that escape repair can be inherited and contribute to genetic variation.

Telomeres and Telomerase

Linear eukaryotic chromosomes have telomeres—repetitive sequences at their ends that protect genes from erosion during replication. The enzyme telomerase extends telomeres in germ cells, stem cells, and some cancer cells.

Chromosome Structure

DNA is packaged with proteins to form chromatin. In eukaryotes, DNA wraps around histone proteins to form nucleosomes, which further fold into higher-order structures. Chromatin structure regulates gene expression and DNA accessibility.

Chapter 17: Gene Expression: From Gene to Protein

Gene Expression Overview

Gene expression is the process by which DNA directs the synthesis of proteins (or functional RNAs). It involves two main stages: transcription (DNA to RNA) and translation (RNA to protein).

• Central Dogma: The flow of genetic information is DNA → RNA → Protein.

• One Gene–One Polypeptide Hypothesis: Each gene codes for a single polypeptide (or functional RNA).

The Genetic Code

The genetic code is a triplet code: three nucleotide bases (codon) specify one amino acid. The code is nearly universal and redundant but not ambiguous.

• Codons: 64 possible codons; 61 code for amino acids, 3 are stop signals.

• Reading Frame: Codons must be read in the correct grouping for proper translation.

Transcription: DNA-Directed RNA Synthesis

Transcription is the synthesis of RNA from a DNA template. It occurs in three stages: initiation, elongation, and termination.

• RNA Polymerase: Enzyme that synthesizes RNA; does not require a primer.

• Promoter: DNA sequence where RNA polymerase binds and initiates transcription; includes the TATA box in eukaryotes.

• Transcription Factors: Proteins that help RNA polymerase bind to the promoter in eukaryotes.

• Termination: In bacteria, transcription ends at a terminator sequence; in eukaryotes, after a polyadenylation signal.

RNA Processing in Eukaryotes

Pre-mRNA undergoes processing before becoming mature mRNA:

• 5' Cap: Modified guanine nucleotide added to the 5' end.

• Poly-A Tail: Series of adenine nucleotides added to the 3' end.

• RNA Splicing: Removal of introns (noncoding regions) and joining of exons (coding regions) by spliceosomes.

• Alternative Splicing: Allows a single gene to code for multiple proteins by varying exon combinations.

Translation: RNA-Directed Polypeptide Synthesis

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. It involves tRNA molecules that bring amino acids to the ribosome, matching codons with anticodons.

• tRNA: Transfers specific amino acids to the growing polypeptide chain; has an anticodon complementary to mRNA codons.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches the correct amino acid to its tRNA.

• Ribosome: Composed of rRNA and proteins; has A, P, and E sites for tRNA binding and polypeptide synthesis.

• Initiation: Small ribosomal subunit binds mRNA and initiator tRNA; large subunit completes the initiation complex.

• Elongation: Amino acids are added one by one to the C-terminus of the chain.

• Termination: Occurs when a stop codon is reached; a release factor releases the polypeptide.

Protein Folding and Targeting

Newly synthesized polypeptides fold into their functional shapes, sometimes with the help of chaperone proteins. Post-translational modifications and signal peptides direct proteins to their correct cellular locations.

Mutations and Their Effects

Mutations are changes in the DNA sequence that can affect protein structure and function.

• Point Mutations: Single nucleotide changes; can be silent, missense, or nonsense mutations.

• Insertions/Deletions: Addition or loss of nucleotides; may cause frameshift mutations, altering the reading frame.

• Mutagens: Physical or chemical agents that cause mutations; many are carcinogenic.

Gene Editing: CRISPR-Cas9

CRISPR-Cas9 is a revolutionary gene-editing tool derived from bacterial immune systems. It uses a guide RNA to direct the Cas9 enzyme to a specific DNA sequence, where it introduces a double-strand break. This can disable a gene or allow for precise correction by providing a repair template.

• Applications: Gene knockout, correction of genetic diseases, research on gene function.

• Concerns: Potential off-target effects and ethical considerations in human applications.

What Is a Gene?

The modern definition of a gene is a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule.