Genetics and Molecular Biology

The One Thing

Genetics and molecular biology explain how hereditary information is stored in DNA, accurately copied, expressed through RNA and protein, and altered by mutation. The central information flow is generally DNA → RNA → protein, linking genotype to phenotype.

Reactions, Processes and Experiments

Key Terms

• Genetics: The branch of biology that studies heredity and variation among organisms.
• Gene: A functional segment of DNA that carries information for a product, usually a protein or functional RNA.
• Genome: The complete genetic material present in an organism or cell.
• DNA: Deoxyribonucleic acid, the main hereditary material in most organisms.
• Nucleotide: The basic unit of DNA or RNA, made 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.
• Complementary base pairing: In DNA, adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds.
• Chromosome: A condensed structure made mainly of DNA and proteins that carries genes.
• Replication: The process by which DNA makes an identical copy of itself before cell division.
• Semiconservative replication: A mode of DNA replication in which each daughter DNA molecule contains one original strand and one newly synthesised strand.
• Origin of replication: A specific DNA region where replication begins.
• Helicase: An enzyme that unwinds the DNA double helix by breaking hydrogen bonds between bases.
• DNA polymerase: An enzyme that adds complementary nucleotides to form a new DNA strand.
• Leading strand: The new DNA strand synthesised continuously in the same general direction as the replication fork.
• Lagging strand: The new DNA strand synthesised discontinuously in short segments called Okazaki fragments.
• DNA ligase: An enzyme that joins Okazaki fragments by sealing breaks in the sugar-phosphate backbone.
• Transcription: The synthesis of RNA using one DNA strand as a template.
• RNA polymerase: The enzyme that joins RNA nucleotides during transcription.
• Messenger RNA: RNA that carries genetic instructions from DNA to ribosomes for protein synthesis.
• Transfer RNA: RNA that carries specific amino acids to the ribosome and recognises codons through its anticodon.
• Ribosomal RNA: RNA that forms part of ribosomes and helps in protein synthesis.
• Translation: The process in which ribosomes use the information in mRNA to assemble amino acids into 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: The mRNA codon AUG, which usually begins translation and codes for methionine.
• Stop codon: A codon that ends translation; the main stop codons are UAA, UAG, and UGA.
• Central dogma: The general flow of genetic information from DNA to RNA to protein.
• Mutation: A sudden, stable change in the DNA sequence or chromosome structure.
• Point mutation: A change involving a single nucleotide pair, such as substitution.
• Insertion: The addition of one or more nucleotides to a DNA sequence.
• Deletion: The removal of one or more nucleotides from a DNA sequence.
• Frameshift mutation: A mutation caused by insertion or deletion of bases in numbers not divisible by three, altering the reading frame of codons.
• Mutagen: A physical or chemical agent that increases the rate of mutation, such as certain radiation or chemicals.
• Genotype: The genetic constitution of an organism.
• Phenotype: The observable characteristics of an organism produced by its genotype and environmental influences.

Easily Confused

• DNA and RNA: DNA contains deoxyribose and thymine and is usually double-stranded; RNA contains ribose and uracil and is generally single-stranded.
• Leading and lagging strands: The leading strand is synthesised continuously, whereas the lagging strand is synthesised discontinuously as Okazaki fragments.
• Replication and transcription: Replication produces a DNA copy; transcription produces a complementary RNA strand from DNA.
• Transcription and translation: Transcription is DNA → RNA; translation is the assembly of a polypeptide using mRNA.
• Codon and anticodon: A codon is a three-base sequence on mRNA; an anticodon is the complementary three-base sequence on tRNA.
• Insertion/deletion and frameshift mutation: Insertion and deletion describe the DNA change; a frameshift occurs when the number of inserted or deleted bases is not divisible by three.
• Genotype and phenotype: Genotype is an organism’s genetic constitution; phenotype is its observable characteristics produced by genotype and environmental influences.
• Body-cell and germ-cell mutations: Body-cell mutations are generally not inherited by offspring, whereas germ-cell mutations may be inherited.
• Exons and introns: Exons generally remain in mature mRNA; introns are removed during RNA processing.
• Silent, missense, and nonsense substitutions: A silent substitution leaves the amino acid unchanged, a missense substitution changes the amino acid, and a nonsense substitution creates a stop signal.

What Gets Asked

• Explain semiconservative DNA replication, including the roles of the origin of replication, helicase, DNA polymerase, and DNA ligase. Marks are lost by failing to distinguish continuous leading-strand synthesis from discontinuous lagging-strand synthesis involving Okazaki fragments.
• Compare DNA and RNA structures and bases. The specific error is stating that RNA contains thymine rather than uracil, or that DNA contains ribose rather than deoxyribose.
• Describe transcription and translation using the sequence DNA → RNA → protein. Marks are lost by confusing transcription with translation or by omitting the roles of mRNA, tRNA, rRNA, codons, and anticodons.
• Interpret codons and translation signals, including AUG as the usual start codon and UAA, UAG, and UGA as stop codons. A common error is treating a stop codon as coding for an amino acid.
• Classify mutations as substitution, insertion, deletion, frameshift, silent, missense, or nonsense. The key distinction is that insertion or deletion causes a frameshift only when the number of bases involved is not divisible by three.
• Apply quantitative and base-pairing relationships, including A = T and G = C, the approximate DNA helix width of 2 nm, approximately 10 base pairs per turn, and DNA length = number of base pairs × distance between adjacent base pairs, with adjacent base pairs approximately 0.34 nm apart.