Molecular Biology of the Gene: Structure, Replication, Transcription, Translation, and Mutations

Nucleic Acid Structure

DNA and RNA: Chemical Composition and Structure

Nucleic acids are essential biomolecules that store and transmit genetic information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA: Contains deoxyribose sugar; typically double-stranded and forms a double helix.

• RNA: Contains ribose sugar; usually single-stranded but can fold into complex shapes.

• Sugar-phosphate backbone: The structural framework of nucleic acids, consisting of alternating sugar and phosphate groups.

• Nucleotide: The basic unit of nucleic acids, composed of a sugar, a phosphate group, and a nitrogenous base.

• Nitrogenous bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T, in DNA), Uracil (U, in RNA).

• Purines: Double-ring bases (A and G).

• Pyrimidines: Single-ring bases (C, T, and U).

• Base pairs: A-T (or A-U in RNA), C-G; stabilized by hydrogen bonds.

• 5’ end: The end of the nucleic acid strand with a free phosphate group attached to the 5’ carbon of the sugar.

• 3’ end: The end with a free hydroxyl (-OH) group attached to the 3’ carbon.

• Anti-parallel: DNA strands run in opposite directions (5’ to 3’ and 3’ to 5’).

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

Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Strands	Double	Single
Bases	A, T, C, G	A, U, C, G
Function	Genetic storage	Protein synthesis, regulation
Location	Nucleus	Nucleus & cytoplasm

DNA Structure Discovery

Key Experiments and Scientists

The structure of DNA was elucidated through several landmark experiments.

• Hershey-Chase experiment: Used bacteriophages and radiolabeling (sulfur for proteins, phosphorus for DNA) to show DNA is the genetic material.

• Chargaff’s experiment: Demonstrated that the amount of A equals T, and C equals G in DNA (Chargaff’s rules).

• Rosalind Franklin: Used X-ray crystallography to reveal DNA’s double helix and consistent diameter.

• Watson and Crick: Built the first accurate model of DNA’s double helix.

Example: The Hershey-Chase experiment used radioactive sulfur (for proteins) and phosphorus (for DNA) to track which molecule entered bacteria during infection.

DNA Replication

Models and Mechanisms

DNA replication is the process by which DNA is copied before cell division. Three models were proposed: conservative, semiconservative, and dispersive.

• Semiconservative model: Each new DNA molecule contains one old strand and one new strand.

• Meselson-Stahl experiment: Used heavy and light nitrogen isotopes to demonstrate semiconservative replication.

• Origin of replication: Specific sequence where replication begins.

• Replication bubble: Region where DNA is unwound for replication.

• Replication fork: Y-shaped region where new DNA strands are synthesized.

Enzymes and Steps in DNA Replication

• Helicase: Unwinds the DNA helix.

• Single-stranded binding proteins: Stabilize unwound DNA.

• Topoisomerase: Relieves strain ahead of the replication fork.

• Primase: Synthesizes RNA primers.

• DNA polymerase III: 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.

• Lagging strand: Synthesized discontinuously in Okazaki fragments.

Example: The lagging strand is synthesized in short segments called Okazaki fragments, which are later joined by DNA ligase.

Transcription

Process and Key Components

Transcription is the synthesis of RNA from a DNA template. It is the first step in gene expression.

• RNA polymerase: Enzyme that synthesizes RNA.

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

• Terminator sequence: Signals the end of transcription.

• mRNA: Messenger RNA, carries genetic information from DNA to ribosomes.

RNA Processing

• 5’ cap: Added to the 5’ end for stability and ribosome recognition.

• Poly-A tail: Added to the 3’ end for stability.

• Intron: Non-coding sequence removed during splicing.

• Exon: Coding sequence retained in mature mRNA.

• Spliceosome: Complex that removes introns and joins exons.

• Alternate splicing: Allows different proteins to be produced from the same gene.

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

Translation

Genetic Code and Protein Synthesis

Translation is the process by which mRNA is decoded to synthesize proteins.

• Codon: Three-nucleotide sequence in mRNA that specifies an amino acid.

• tRNA: Transfer RNA, brings amino acids to the ribosome.

• Anticodon: Three-nucleotide sequence in tRNA complementary to the mRNA codon.

• Wobble pairing: Flexibility in base pairing at the third codon position.

• Genetic code: Universal and redundant; multiple codons can code for the same amino acid.

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

• Ribosome: Composed of large and small subunits; site of protein synthesis.

• A site: Entry site for tRNA.

• P site: Site where peptide bonds are formed.

• Start codon: AUG; signals the beginning of translation.

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

Example: The codon AUG codes for methionine and serves as the start signal for translation.

How to Use the Genetic Code Table

• Find the first, second, and third bases of the codon in the table to determine the corresponding amino acid.

Mutations

Types and Effects

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

• Nucleotide-pair substitution (Point mutation): Replacement of one nucleotide pair.

• Silent mutation: No effect on protein function.

• Missense mutation: Changes one amino acid; may affect protein function.

• Nonsense mutation: Creates a premature stop codon; often severely disrupts protein function.

• Nucleotide-pair insertion or deletion: Addition or removal of nucleotides.

• Frameshift mutation: Alters the reading frame; usually highly disruptive.

Example: A frameshift mutation early in an exon can render a protein nonfunctional.

Mutation Impact Table

Mutation Type	Effect	Location Impact
Silent	No change in protein	Any location
Missense	Change in one amino acid	Exon; effect depends on position
Nonsense	Premature stop codon	Early exon: severe; late exon: less severe
Frameshift	Disrupts reading frame	Early exon: severe; late exon: less severe
Point mutation in intron	Usually no effect	Intron

Key Experiments and Concepts

Hershey-Chase Experiment

• Proteins: Labeled with radioactive sulfur (S); DNA does not contain sulfur.

• DNA: Labeled with radioactive phosphorus (P); proteins do not contain phosphorus.

• Showed DNA is the genetic material.

Chargaff’s Rules

• A = T; C = G in DNA.

• Helped establish base pairing and double helix structure.

Rosalind Franklin’s X-ray Crystallography

• Revealed double helix structure and consistent diameter.

Meselson-Stahl Experiment

• Used heavy and light nitrogen to show semiconservative replication.

Additional info:

• DNA and RNA are both rich in nitrogen, carbon, and oxygen, so these elements are not useful for distinguishing between them in labeling experiments.

• Complementary base pairing is essential for accurate DNA replication and transcription.

• The genetic code’s redundancy (degeneracy) protects against some mutations.

• Splicing removes introns from pre-mRNA, producing mature mRNA ready for translation.