Molecular Basis of Inheritance

The One Thing

DNA stores hereditary information in the sequence of its nucleotides. Complementary base pairing permits accurate replication, while transcription and translation convert genetic information into functional RNA and proteins whose production is regulated according to cellular conditions.

Reactions, Processes and Experiments

Key Terms

• DNA: Deoxyribonucleic acid, the primary genetic material in most organisms; it stores hereditary information.
• Nucleotide: The basic unit of DNA or RNA, consisting of a sugar, a phosphate group, and a nitrogenous base.
• Nitrogenous bases: DNA contains adenine, thymine, guanine, and cytosine; RNA contains adenine, uracil, guanine, and cytosine.
• Double helix: The twisted-ladder structure of DNA made of two antiparallel polynucleotide strands.
• Complementary base pairing: A pairs with T in DNA and with U in RNA, while G pairs with C; hydrogen bonds hold paired bases together.
• Gene: A functional segment of DNA that contains information for producing a protein or a functional RNA molecule.
• Genome: The complete genetic material present in an organism or cell.
• Chromatin: The DNA–protein complex found in the nucleus, mainly composed of DNA associated with histone proteins.
• Nucleosome: The basic unit of chromatin in which DNA is wrapped around a core of histone proteins.
• Semiconservative replication: DNA replication in which each daughter DNA molecule contains one parental strand and one newly synthesised strand.
• Origin of replication: The specific DNA region where replication begins.
• Replication fork: The Y-shaped region where the DNA strands separate and new strands are synthesised.
• Helicase: An enzyme that unwinds the DNA double helix by breaking hydrogen bonds between bases.
• Primase: An enzyme that synthesises a short RNA primer needed to begin DNA synthesis.
• DNA polymerase: An enzyme that adds nucleotides to a growing DNA strand in the 5′ to 3′ direction and may proofread the new strand.
• Leading strand: The new DNA strand synthesised continuously towards the replication fork.
• Lagging strand: The new DNA strand synthesised discontinuously away from the replication fork as short fragments.
• Okazaki fragments: Short DNA segments formed on the lagging strand during replication.
• DNA ligase: An enzyme that joins Okazaki fragments by forming phosphodiester bonds.
• Transcription: The synthesis of RNA using one DNA strand as a template.
• RNA polymerase: The enzyme that builds an RNA strand by adding complementary ribonucleotides to a DNA template.
• Promoter: A DNA sequence where RNA polymerase and associated factors bind to begin transcription.
• Template strand: The DNA strand read by RNA polymerase to produce a complementary RNA molecule.
• Coding strand: The DNA strand whose sequence resembles the RNA transcript, except that DNA has thymine instead of uracil.
• Messenger RNA: RNA that carries genetic information from DNA to ribosomes for protein synthesis.
• Transfer RNA: RNA that carries specific amino acids to the ribosome and recognises mRNA codons through its anticodon.
• Ribosomal RNA: RNA that forms part of ribosomes and helps catalyse peptide-bond formation.
• RNA processing: Modification of a primary RNA transcript by adding a 5′ cap, a poly-A tail, and removing introns through splicing.
• Intron: A non-coding region removed from a primary RNA transcript during RNA splicing.
• Exon: A region retained in mature RNA after introns are removed; it usually contributes to the coding sequence.
• 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: Usually AUG; it begins translation and codes for methionine.
• Stop codons: UAA, UAG, and UGA; they signal termination of translation and do not code for amino acids.
• Genetic code: The relationship between mRNA codons and amino acids; it is generally triplet, degenerate, nearly universal, and unambiguous.
• Central dogma: The general flow of genetic information from DNA to RNA to protein, with DNA also able to make DNA during replication.
• Operon: A group of related genes controlled together by a common promoter and regulatory region, especially in prokaryotes.
• Lac operon: An inducible bacterial gene-regulation system controlling enzymes used to metabolise lactose.
• Regulatory gene: A gene that produces a regulator, such as a repressor, which controls the expression of other genes.
• Repressor: A regulatory protein that reduces transcription by binding to a regulatory DNA sequence.
• Inducer: A molecule that initiates gene expression, often by inactivating a repressor.
• Mutation: A heritable change in the nucleotide sequence of DNA.
• Point mutation: A change involving a single nucleotide pair, such as a substitution.
• Frameshift mutation: A mutation caused by insertion or deletion of bases not in multiples of three, altering the reading frame.
• DNA repair: Cellular mechanisms that detect and correct damage or copying errors in DNA.

Easily Confused

• Template strand and coding strand: RNA polymerase reads the template strand; the coding strand resembles the RNA sequence, except that thymine replaces uracil in DNA.
• Leading strand and lagging strand: The leading strand is synthesised continuously towards the replication fork, whereas the lagging strand is synthesised discontinuously as Okazaki fragments.
• DNA replication and transcription: Replication produces DNA from DNA, whereas transcription produces RNA from a DNA template.
• Transcription and translation: Transcription converts DNA information into RNA; translation uses mRNA information to assemble a polypeptide.
• Codon and anticodon: A codon is a three-base sequence on mRNA; an anticodon is the complementary three-base sequence on tRNA.
• Intron and exon: Introns are removed during RNA splicing; exons are retained in mature RNA.
• Point mutation and frameshift mutation: A point mutation affects a single nucleotide pair, whereas a frameshift results from an insertion or deletion not in multiples of three and alters subsequent codons.
• Inducer and repressor: An inducer promotes gene expression by inactivating a repressor; a repressor reduces transcription by binding to regulatory DNA.
• Degenerate and unambiguous genetic code: Degeneracy means that several codons may specify the same amino acid; unambiguity means that each codon normally has only one meaning.
• Purines and pyrimidines: Purines have two rings and include adenine and guanine; pyrimidines have one ring and include cytosine, thymine, and uracil.

What Gets Asked

• DNA structure and base ratios: Questions may require identification of nucleotide components, purines and pyrimidines, antiparallel strands, hydrogen-bond numbers, or Chargaff’s relationships. Marks are lost by confusing adenine–thymine pairing with guanine–cytosine pairing or by omitting that G–C has three hydrogen bonds.
• Replication diagrams and explanations: Questions may ask students to label the origin of replication, replication fork, helicase, primase, DNA polymerase, DNA ligase, leading strand, lagging strand, and Okazaki fragments. Marks are lost by stating that DNA polymerase synthesises 3′ to 5′ or by describing both strands as continuous.
• The Meselson–Stahl experiment: Questions may ask how nitrogen isotopes provided evidence for semiconservative replication. Marks are lost by failing to state that each daughter DNA molecule contains one parental strand and one newly synthesised strand.
• Information-flow equations: Questions may require the relationships DNA replication: DNA → DNA, transcription: DNA → RNA, and translation: RNA → protein. Marks are lost by reversing transcription and translation or by treating replication as DNA → RNA.
• RNA processing and translation: Questions may ask for the roles of the 5′ cap, poly-A tail, introns, exons, codons, anticodons, start codon, stop codons, and polyribosomes. Marks are lost by saying that stop codons code for amino acids or that introns remain in mature mRNA.
• Gene regulation and mutation: Questions may ask how the lac operon responds to lactose and scarce glucose, or how mutations affect proteins. Marks are lost by stating that the inducer activates the repressor, or by failing to distinguish a frameshift from a single-nucleotide point mutation.