Molecular Basis of Inheritance

The One Thing

DNA stores hereditary information, replicates through complementary base pairing, and directs protein production through the pathway DNA → RNA → protein. Genome organisation, gene regulation, and mutation determine how inherited information is expressed and how genetic variation arises.

Reactions, Processes and Experiments

Key Terms

• Gene: A functional unit of heredity, usually a specific DNA sequence that carries information for a protein or functional RNA.
• Genome: The complete genetic material present in an organism or cell.
• Nucleotide: The basic unit of DNA or RNA, made of a nitrogenous base, a pentose sugar, and a phosphate group.
• DNA: Deoxyribonucleic acid, the main hereditary material in most organisms.
• RNA: Ribonucleic acid, generally involved in gene expression and serving as genetic material in some viruses.
• DNA double helix: A twisted ladder-like structure consisting of two antiparallel, complementary polynucleotide strands.
• Complementary base pairing: A pairs with T in DNA through two hydrogen bonds, while G pairs with C through three hydrogen bonds; in RNA, U replaces T.
• Antiparallel strands: The two DNA strands run in opposite directions: one from 5′ to 3′ and the other from 3′ to 5′.
• Semiconservative replication: DNA replication in which each daughter DNA molecule contains one parental strand and one newly synthesized strand.
• Origin of replication: The specific DNA region where replication begins.
• Replication fork: The Y-shaped region where the two DNA strands separate and new strands are formed.
• DNA polymerase: An enzyme that adds nucleotides to a growing DNA strand in the 5′ to 3′ direction.
• Leading strand: The new DNA strand synthesized continuously toward the replication fork.
• Lagging strand: The new DNA strand synthesized discontinuously away from the replication fork as short Okazaki fragments.
• Okazaki fragments: Short DNA segments formed on the lagging strand and later joined together.
• Helicase: An enzyme that unwinds the DNA double helix by separating its two strands.
• Primase: An enzyme that produces a short RNA primer needed to begin DNA synthesis.
• DNA ligase: An enzyme that joins Okazaki fragments by forming phosphodiester bonds.
• Transcription: The process of copying genetic information from DNA into RNA.
• RNA polymerase: The enzyme that synthesizes RNA using one DNA strand as a template.
• Promoter: A DNA sequence where RNA polymerase binds to begin transcription.
• Terminator: A DNA sequence that signals the end of transcription.
• Template strand: The DNA strand read by RNA polymerase to make a complementary RNA molecule.
• Coding strand: The DNA strand whose sequence resembles the RNA sequence, except that DNA has thymine while RNA has uracil.
• Messenger RNA (mRNA): RNA that carries genetic information from DNA to ribosomes for protein synthesis.
• Transfer RNA (tRNA): RNA that carries specific amino acids to the ribosome and recognizes mRNA codons through its anticodon.
• Ribosomal RNA (rRNA): RNA that forms part of ribosomes and helps catalyse peptide-bond formation.
• RNA processing: In eukaryotes, the modification of primary RNA by 5′ capping, 3′ poly-A tail addition, and removal of introns.
• Intron: A non-coding region removed from eukaryotic primary RNA during processing.
• Exon: A region retained in mature RNA after introns are removed.
• Translation: The process by which ribosomes use the codon sequence of mRNA to assemble a polypeptide.
• Codon: A sequence of three mRNA nucleotides that specifies an amino acid or a stop signal.
• Anticodon: A three-base sequence on tRNA that pairs with a complementary mRNA codon.
• Start codon: AUG, the usual codon that initiates translation and codes for methionine.
• Stop codons: UAA, UAG, and UGA; these do not code for amino acids and terminate translation.
• Genetic code: The rule by which nucleotide codons specify amino acids in proteins.
• Central dogma: The general flow of genetic information from DNA to RNA to protein.
• Mutation: A heritable change in the nucleotide sequence of genetic material.
• Point mutation: A change involving a single nucleotide or base pair, such as the substitution responsible for sickle-cell anaemia.
• Regulatory gene: A gene that controls the expression of other genes, often by producing a regulatory protein.
• Lac operon: An inducible gene-regulation system in bacteria that controls genes needed for lactose utilisation.
• Inducer: A molecule that activates gene expression by inactivating or removing the effect of a repressor.

Easily Confused

• Nucleotide and nucleoside: A nucleotide contains a base, pentose sugar, and phosphate; a nucleoside contains only a base and sugar.
• DNA and RNA: DNA generally contains deoxyribose and thymine and is double-stranded; RNA generally contains ribose and uracil and is single-stranded.
• Template and coding strands: The template strand is read by RNA polymerase; the coding strand resembles the RNA sequence except that it contains T instead of U.
• Leading and lagging strands: The leading strand is synthesized continuously toward the replication fork; the lagging strand is synthesized discontinuously as Okazaki fragments.
• Promoter and terminator: The promoter is where transcription begins; the terminator signals its end.
• Intron and exon: Introns are removed during RNA processing; exons remain in mature RNA.
• Codon and anticodon: A codon is found on mRNA; an anticodon is the complementary sequence on tRNA.
• Start and stop codons: AUG initiates translation and codes for methionine; UAA, UAG, and UGA terminate translation and do not code for amino acids.
• Silent, missense, and nonsense mutations: Silent mutations do not change the encoded amino acid, missense mutations change an amino acid, and nonsense mutations create a premature stop codon.
• DNA replication and transcription: Replication copies DNA to produce DNA; transcription copies information from DNA into RNA.
• Transcription and translation: Transcription produces RNA from DNA; translation produces a polypeptide using mRNA.
• Phosphodiester and hydrogen bonds: Phosphodiester bonds link nucleotides within a DNA strand; hydrogen bonds link complementary bases between strands.
• Purines and pyrimidines: Adenine and guanine are purines; cytosine, thymine, and uracil are pyrimidines.
• Structural genes and regulatory gene: lacZ, lacY, and lacA encode products involved in lactose utilisation; the regulatory gene controls their expression.

What Gets Asked

• DNA structure and base pairing: Questions may ask students to identify the bonds, strand orientation, complementary bases, or DNA dimensions. The mark-losing errors are confusing phosphodiester bonds with hydrogen bonds, or omitting that A–T has two hydrogen bonds and G–C has three.
• Chargaff’s rule calculations: If double-stranded DNA contains 30% adenine, the correct composition is 30% thymine, 20% guanine, and 20% cytosine. The common error is failing to apply A = T and G = C.
• DNA replication: Questions may require comparison of leading- and lagging-strand synthesis or identification of enzyme functions. Marks are lost by stating that DNA polymerase begins synthesis without an RNA primer, or by reversing the 5′ to 3′ direction.
• Replication experiments: The Meselson and Stahl experiment tests semiconservative replication using 15N and 14N, whereas the Hershey and Chase experiment supports DNA as genetic material using 32P and 35S. The specific labels and purposes must not be interchanged.
• Transcription, RNA processing, and translation: Questions may require the roles of the template strand, promoter, RNA polymerase, introns, exons, codons, and anticodons. A frequent error is treating the coding strand as the strand read by RNA polymerase or using thymine rather than uracil in RNA.
• Gene regulation and mutation: Questions may ask how the lac operon responds to lactose and limited glucose, or how silent, missense, and nonsense mutations affect proteins. Marks are lost by confusing the inducer with the regulatory gene or by treating all mutations as producing the same protein effect.