Molecular Biology of the Gene: Structure, Function, and Expression

Molecular Biology of the Gene

Nucleic Acid Structure

Nucleic acids, including DNA and RNA, are essential macromolecules that store and transmit genetic information in all living organisms.

• DNA (Deoxyribonucleic Acid): Contains the sugar deoxyribose; typically double-stranded and forms a double helix.

• RNA (Ribonucleic Acid): Contains the sugar ribose; usually single-stranded but can form complex secondary structures.

• Sugar-Phosphate Backbone: The repeating chain of sugar and phosphate groups that forms the structural framework of nucleic acids.

• Nucleotide: The monomer of nucleic acids, consisting of a sugar, a phosphate group, and a nitrogenous base.

• Nitrogenous Bases: The information-carrying part of nucleic acids. DNA bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). RNA bases: Adenine (A), Uracil (U), Cytosine (C), Guanine (G).

• Purines: Double-ringed bases (Adenine and Guanine).

• Pyrimidines: Single-ringed bases (Cytosine, Thymine, and Uracil).

• Base Pairs: Specific pairing between bases: A-T (or A-U in RNA), C-G, stabilized by hydrogen bonds.

• 5’ End and 3’ End: Refer to the carbon positions in the sugar; the 5’ end has a phosphate group, the 3’ end has a hydroxyl group (-OH).

• Antiparallel: The two strands of DNA run in opposite directions (5’ to 3’ and 3’ to 5’).

Example: In DNA, the sequence 5’-ATCG-3’ pairs with 3’-TAGC-5’.

Discovery of DNA Structure

The structure of DNA was elucidated through a series of key experiments and discoveries.

• Hershey-Chase Experiment: Used bacteriophages labeled with radioactive sulfur (for proteins) and phosphorus (for DNA) to show that DNA is the genetic material.

• Chargaff’s Experiment and Rules: Demonstrated that the amount of adenine equals thymine, and cytosine equals guanine in DNA, suggesting base pairing.

• Rosalind Franklin: Used X-ray crystallography to reveal the helical structure and consistent diameter of DNA.

• Watson and Crick: Built the first accurate model of DNA as a double helix based on available data.

Example: The Hershey-Chase experiment used radioactive sulfur to label proteins and radioactive phosphorus to label DNA, proving DNA is the hereditary material.

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division, ensuring genetic continuity.

• Models of Replication:

  • Conservative Model: Parental DNA remains intact; new molecule is entirely new DNA.

  • Semiconservative Model: Each new DNA molecule consists of one parental and one new strand (supported by Meselson-Stahl experiment).

  • Dispersive Model: Parental and new DNA are interspersed in both strands.

• Meselson-Stahl Experiment: Used isotopes of nitrogen to demonstrate semiconservative replication.

• Origin of Replication: Specific sequence where DNA replication begins.

• Replication Bubble and Fork: The area where the DNA double helix is unwound for replication.

• Key Enzymes and Proteins:

  • Helicase: Unwinds the DNA double helix.

  • Single-Stranded Binding Proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves tension ahead of the replication fork.

  • Primase: Synthesizes RNA primers to initiate DNA synthesis.

  • DNA Polymerase III: Main enzyme that adds nucleotides to the growing DNA strand.

  • DNA Polymerase I: Removes RNA primers and replaces them with DNA.

  • DNA Ligase: Joins Okazaki fragments on the lagging strand.

• Leading Strand: Synthesized continuously in the 5’ to 3’ direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

Example: The semiconservative model means each daughter DNA molecule has one old and one new strand.

Transcription and RNA Processing

Transcription is the synthesis of RNA from a DNA template, followed by processing to produce mature mRNA.

• RNA Polymerase: Enzyme that synthesizes RNA from the DNA template.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Terminator Sequence: Signals the end of transcription.

• mRNA (Messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis.

• RNA Processing (in eukaryotes):

  • 5’ Cap: Modified guanine nucleotide added to the 5’ end for stability and ribosome binding.

  • Poly-A Tail: String of adenine nucleotides added to the 3’ end for stability and export from the nucleus.

  • Introns: Non-coding sequences removed from pre-mRNA.

  • Exons: Coding sequences that remain in mature mRNA.

  • Spliceosome: Complex that removes introns and joins exons.

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

Example: Mature mRNA contains only exons, a 5’ cap, and a poly-A tail.

Translation and the Genetic Code

Translation is the process by which ribosomes synthesize proteins using the information in mRNA.

• Codon: A sequence of three mRNA nucleotides that codes for a specific amino acid.

• tRNA (Transfer RNA): Brings amino acids to the ribosome; contains an anticodon complementary to the mRNA codon.

• Anticodon: Three-nucleotide sequence on tRNA that pairs with the mRNA codon.

• Wobble Pairing: Flexibility in base pairing at the third codon position, allowing some tRNAs to pair with multiple codons.

• Genetic Code: The set of rules by which codons specify amino acids; it is degenerate/redundant (multiple codons for one amino acid) and universal (shared by most organisms).

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

• Ribosome: Composed of large and small subunits; has A (aminoacyl), P (peptidyl), and E (exit) sites for tRNA binding.

• Start Codon: AUG (codes for methionine); signals the start of translation.

• Stop Codons: UAA, UAG, UGA; signal the end of translation.

Example: The codon UUU codes for phenylalanine; the anticodon is AAA.

Mutations

Mutations are changes in the DNA sequence that can affect gene function and phenotype.

• Nucleotide-Pair Substitution (Point Mutation): Replacement of one base pair with another.

  • Silent Mutation: No change in amino acid sequence.

  • Missense Mutation: Changes one amino acid to another.

  • Nonsense Mutation: Changes a codon to a stop codon, truncating the protein.

• Nucleotide-Pair Insertion or Deletion: Addition or loss of base pairs.

  • Frameshift Mutation: Alters the reading frame, usually resulting in a nonfunctional protein.

Example: A frameshift mutation early in a gene is likely to severely disrupt protein function.

Comparison Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strandedness	Double-stranded	Single-stranded
Function	Genetic information storage	Information transfer, catalysis

Key Equations and Concepts

• Base Pairing:

  • A pairs with T (or U in RNA): $A = T$ (in DNA), $A = U$ (in RNA)

  • C pairs with G: $C = G$

• Directionality: DNA and RNA are synthesized in the 5’ to 3’ direction.

Practice Questions (Selected)

1. In the Hershey-Chase experiment, sulfur was used to label proteins and phosphorus to label DNA because only proteins contain sulfur and only DNA contains phosphorus.

2. Chargaff’s rules (A = T, C = G) helped reveal the base-pairing nature of DNA.

3. Rosalind Franklin’s X-ray crystallography showed DNA is a double helix with a consistent diameter.

4. Hydrogen bonds hold DNA base pairs together.

5. Purines (A, G) pair with pyrimidines (C, T/U) to maintain uniform DNA structure.

6. The 5’ and 3’ ends refer to the numbering of carbons in the sugar ring of nucleotides.

7. The Meselson-Stahl experiment demonstrated semiconservative DNA replication using isotopic labeling.

8. DNA and RNA differ in sugar, bases, and structure (see table above).

9. Complementary bases: A-T (or A-U), C-G.

10. Pre-mRNA contains introns; mature mRNA has exons only, plus a 5’ cap and poly-A tail.

11. The genetic code is degenerate (redundant): multiple codons can specify the same amino acid.

12. Most disruptive mutations: nonsense or frameshift in early exons; least disruptive: silent mutations or those in introns.

Additional info: The above content integrates definitions, examples, and context for all major terms and concepts listed in the study guide, providing a comprehensive overview suitable for exam preparation in a college-level molecular biology course.