Molecular Basis of Inheritance — Practice Questions with Answers

A single DNA nucleotide consists of three components: a deoxyribose sugar molecule, a phosphate group, and one of four possible nitrogenous bases (adenine, thymine, guanine, or cytosine).

These nucleotides are linked together, with the sugar-phosphate components forming the structural "backbone" of each DNA strand, while the nitrogenous bases project inward, forming the specific base-pairing interactions responsible for the double helix's characteristic structure.

Complementary base pairing refers to the specific, predictable pairing pattern between nitrogenous bases on opposite DNA strands — adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C), via hydrogen bonding.

Chargaff's rule states that, within any given DNA sample, the amount of adenine is always approximately equal to the amount of thymine, and the amount of guanine is always approximately equal to the amount of cytosine — a direct consequence of this specific, complementary base-pairing pattern.

Sugar component: DNA contains deoxyribose sugar; RNA contains ribose sugar (which has an additional hydroxyl group compared to deoxyribose).

Nitrogenous bases: DNA contains thymine as one of its four bases; RNA contains uracil instead of thymine. Additionally, DNA is typically double-stranded, while RNA is typically single-stranded.

DNA helicase is the specific enzyme responsible for unwinding and separating the two complementary strands of the DNA double helix at the start of replication, by breaking the hydrogen bonds holding the paired bases together.

This unwinding creates a "replication fork," exposing the single-stranded DNA template necessary for DNA polymerase to subsequently synthesise new, complementary daughter strands.

Transcription is the process by which genetic information encoded in a specific segment of DNA (a gene) is copied into a complementary messenger RNA (mRNA) molecule.

RNA polymerase is the enzyme responsible for this process — it binds to a specific starting region (promoter) on the DNA, unwinds the relevant DNA segment, and synthesises a complementary mRNA strand using one of the DNA strands as a template, ultimately producing an mRNA molecule carrying the same genetic information (in RNA form) as the original DNA gene sequence.

The genetic code refers to the specific set of rules by which the sequence of nucleotide bases (in triplet codons) in mRNA is translated into the corresponding sequence of amino acids during protein synthesis.

The genetic code is described as "degenerate" (or redundant) because, since there are 64 possible codons but only 20 standard amino acids, most amino acids are actually specified by more than one different codon — this redundancy means multiple different codons can code for the exact same amino acid, providing some degree of built-in tolerance/buffering against the potentially harmful effects of certain mutations.

The start codon (typically AUG, which also codes for the amino acid methionine) signals the specific point at which the ribosome should begin translation, establishing the correct reading frame for the entire subsequent translation process.

The stop codons (there are three: UAA, UAG, and UGA) do not code for any amino acid — instead, they signal the ribosome to terminate translation and release the completed polypeptide chain, marking the defined end point of the protein-coding sequence.

Exons are the coding segments of a eukaryotic gene, which contain the genetic information ultimately retained in the mature mRNA and translated into protein.

Introns are non-coding segments interspersed between the exons within the gene — introns are initially transcribed into the primary RNA transcript, but are subsequently removed during RNA processing (splicing), with only the remaining exons being joined together to form the final, mature mRNA molecule used for translation.

A mutation is any change occurring in the DNA nucleotide sequence.

A point mutation involves a change affecting just a single nucleotide base (such as substitution of one base for another), typically affecting only a single corresponding codon/amino acid. A frameshift mutation involves the insertion or deletion of one or a small number of bases (not in multiples of three) — since the genetic code is read in continuous, non-overlapping triplet codons, such an insertion/deletion shifts the entire subsequent "reading frame," typically altering the entire downstream amino acid sequence and often significantly disrupting normal protein function.

Gene regulation refers to the various cellular mechanisms that control whether, when, and to what extent a particular gene is actively expressed (transcribed and translated) within a given cell.

Not all genes are expressed at all times because different genes are needed only under specific circumstances or in specific cell types — for example, genes required for a particular metabolic pathway are typically only actively expressed when that specific metabolic function is actually needed (such as lactose metabolism genes only being expressed when lactose is present); efficient, appropriately regulated gene expression allows cells to conserve resources and appropriately, dynamically adapt their protein production according to their current specific functional needs and environmental circumstances.

The Watson-Crick model describes DNA as a double helix, consisting of two long polynucleotide strands wound around each other in a helical, spiral structure.

Antiparallel orientation: The two strands run in opposite directions relative to each other (antiparallel) — one strand runs in the 5' to 3' direction, while the complementary strand runs in the 3' to 5' direction; this specific antiparallel arrangement is essential for proper complementary base pairing and, later, for the mechanics of DNA replication.

Complementary base pairing: The nitrogenous bases project inward from each sugar-phosphate backbone, forming specific hydrogen-bonded pairs with the bases on the opposite strand — adenine always pairs with thymine (via two hydrogen bonds), and guanine always pairs with cytosine (via three hydrogen bonds).

Significance: This complementary base pairing means that, given the sequence of one DNA strand, the sequence of the complementary strand can always be precisely predicted — this property is fundamentally important for both accurate DNA replication (allowing each strand to serve as a template for producing a new, complementary partner strand) and for maintaining the overall structural stability of the double helix, with the specific base-pairing rules ensuring the two strands fit together correctly, maintaining a fairly uniform helix width throughout the molecule.

Unwinding: DNA helicase unwinds and separates the two parental DNA strands at the replication fork, breaking the hydrogen bonds between complementary base pairs.

Primer synthesis: Since DNA polymerase cannot initiate synthesis of an entirely new strand from scratch, the enzyme primase first synthesises a short RNA primer, providing the necessary starting point (a free 3' end) for DNA polymerase to begin its work.

DNA polymerase synthesis: DNA polymerase then extends this primer, synthesising a new, complementary DNA strand — critically, DNA polymerase can only synthesise new DNA in the 5' to 3' direction.

Leading versus lagging strand: Since the two parental strands are antiparallel, but DNA polymerase can only synthesise in one direction (5' to 3'), one new strand (the leading strand) can be synthesised continuously, directly following the replication fork as it progressively opens. The other new strand (the lagging strand) must instead be synthesised discontinuously, in a series of short fragments (called Okazaki fragments), each requiring its own separate RNA primer, synthesised in the direction away from the replication fork.

Role of ligase: Finally, the enzyme DNA ligase joins together the multiple separate Okazaki fragments on the lagging strand (after their RNA primers have been removed and replaced with DNA), creating one continuous, complete new DNA strand — this combination of continuous leading strand synthesis and discontinuous, fragment-based lagging strand synthesis together allows both new DNA strands to be accurately, efficiently synthesised despite DNA polymerase's inherent directional (5' to 3' only) synthesis constraint.

Initiation: The small ribosomal subunit binds to the mRNA molecule, positioning itself at the start codon (AUG) — an initiator tRNA (carrying the amino acid methionine) then binds to this start codon via complementary anticodon-codon base pairing, following which the large ribosomal subunit joins, forming the complete, functional ribosome-mRNA-tRNA complex.

Elongation: The ribosome progressively moves along the mRNA, one codon at a time — for each new codon encountered, a specific tRNA molecule (carrying the appropriate amino acid, as dictated by complementary anticodon-codon pairing) enters the ribosome, and a peptide bond is formed between the new amino acid and the growing polypeptide chain, catalysed by the ribosome's inherent enzymatic activity — this process repeats continuously, progressively extending the growing polypeptide chain, one amino acid at a time, as the ribosome moves codon-by-codon along the mRNA.

Termination: When the ribosome encounters one of the three specific stop codons (which do not correspond to any tRNA/amino acid), specific release factor proteins bind instead, triggering release of the completed polypeptide chain from the ribosome, and subsequent disassembly of the ribosomal complex — this three-stage process (initiation, elongation, termination), precisely coordinated through the specific interactions between mRNA codons, tRNA anticodons, and the ribosome's catalytic machinery, ensures accurate, sequential translation of the genetic code into the correctly ordered amino acid sequence of the resulting protein.

The lac operon is a cluster of genes in E. coli bacteria, all required for lactose metabolism, positioned together under the shared control of a single regulatory system, allowing them to be coordinately switched on or off together as needed.

Structure: The lac operon includes structural genes (coding for the enzymes required for lactose metabolism), a promoter region (where RNA polymerase binds), and an operator region (where a specific regulatory repressor protein can bind) — a separate regulatory gene, located elsewhere, continuously produces the lac repressor protein.

In the absence of lactose: The lac repressor protein binds directly to the operator region, physically blocking RNA polymerase from proceeding past this point — this prevents transcription of the structural genes, meaning the lactose-metabolising enzymes are not produced when lactose is absent (and hence not needed).

In the presence of lactose: A specific lactose-derived molecule (allolactose) binds directly to the repressor protein, causing a conformational change that causes the repressor to release from the operator region — with the operator now unblocked, RNA polymerase can freely proceed, transcribing the structural genes, leading to production of the enzymes required for lactose metabolism, precisely when lactose is actually present and needs to be metabolised.

This elegant, responsive regulatory system allows the bacterium to efficiently, dynamically produce lactose-metabolising enzymes only when lactose is actually available in the environment, avoiding the metabolic waste of continuously producing these specific enzymes when they are not currently needed.

In eukaryotes, the initial RNA transcript produced by RNA polymerase (the primary transcript, or pre-mRNA) undergoes several important processing/modification steps before becoming the mature, functional mRNA molecule.

5' capping: A modified guanine nucleotide "cap" structure is added to the 5' end of the pre-mRNA — this cap helps protect the mRNA from premature degradation, and plays an important role in later recognition by the ribosome during translation initiation.

3' polyadenylation: A long chain of adenine nucleotides (a "poly-A tail") is added to the 3' end of the pre-mRNA — this poly-A tail similarly helps protect the mRNA from degradation, and can influence the mRNA's stability and export from the nucleus.

Splicing: The non-coding intron sequences, interspersed between the coding exon sequences within the primary transcript, are precisely removed, and the remaining exon sequences are joined together, forming the continuous, mature coding sequence — this splicing process is carried out by a specialised molecular complex called the spliceosome.

Overall significance: Together, these processing steps convert the initial, unprocessed primary RNA transcript into the mature, stable, properly formatted mRNA molecule required for efficient nuclear export and subsequent, accurate translation at cytoplasmic ribosomes — additionally, alternative splicing (where different combinations of exons can be selectively included in the final mRNA) allows a single gene to potentially produce multiple different protein variants, significantly expanding the coding/functional diversity achievable from a comparatively limited number of actual genes.

Griffith's experiment (1928): Frederick Griffith worked with two strains of Streptococcus pneumoniae bacteria — a virulent (disease-causing) smooth (S) strain, and a non-virulent rough (R) strain. When Griffith injected mice with heat-killed S strain bacteria (rendered harmless) mixed with live, harmless R strain bacteria, the mice unexpectedly died, and living S strain bacteria were subsequently recovered from them — Griffith concluded that some "transforming principle" from the dead S strain bacteria had been transferred to the living R strain bacteria, converting them into the virulent form, though Griffith's experiment itself did not identify the specific chemical nature of this transforming substance.

Hershey-Chase experiment (1952): Alfred Hershey and Martha Chase used bacteriophages (viruses that infect bacteria) to definitively resolve the question of whether DNA or protein was the actual genetic material. They separately radioactively labelled the phage's protein coat (using radioactive sulfur, since protein contains sulfur but DNA does not) and the phage's DNA (using radioactive phosphorus, since DNA contains phosphorus but protein essentially does not). After allowing labelled phages to infect bacteria, they found that primarily the radioactive phosphorus (DNA) had entered the bacterial cells, while the radioactive sulfur (protein) largely remained outside, in the phage's now-empty protein coats — since the infected bacteria went on to produce new, complete phage progeny despite only the DNA (not the protein) having actually entered the cell, this provided direct, compelling experimental evidence that DNA, not protein, is the genetic material responsible for carrying and transmitting hereditary information.

The central dogma of molecular biology describes the generally observed, fundamental directional flow of genetic information within living cells, first articulated by Francis Crick.

DNA → RNA (Transcription): Genetic information encoded within DNA is first transcribed into a complementary messenger RNA (mRNA) molecule.

RNA → Protein (Translation): This mRNA is then translated at ribosomes, with its coded genetic information used to direct synthesis of a specific corresponding protein, determining the protein's precise amino acid sequence.

DNA → DNA (Replication): Additionally, DNA can be accurately replicated, producing new DNA copies, allowing genetic information to be faithfully passed on to subsequent generations of cells (or organisms).

Exceptions and refinements: While this DNA→RNA→protein flow represents the predominant, general pattern, certain notable exceptions exist — for example, retroviruses (like HIV) can perform reverse transcription, synthesising DNA from an RNA template (via the enzyme reverse transcriptase), representing a flow of genetic information in the reverse direction (RNA→DNA) compared to the standard central dogma pattern — such exceptions have led to some refinement/expansion of the originally proposed central dogma concept, while still preserving its core, generally applicable framework describing the fundamental flow of genetic information within most living systems.

PCR (Polymerase Chain Reaction) is a widely used laboratory technique that allows rapid, exponential amplification of a specific, targeted segment of DNA, generating millions of copies from even a very small initial DNA sample.

Step 1 (Denaturation): The DNA sample is heated to a high temperature, causing the two DNA strands to separate (denature) into single strands.

Step 2 (Annealing): The sample is then cooled, allowing short, specifically designed DNA primers (complementary to the specific target sequence boundaries) to bind (anneal) to their corresponding complementary sequences on the now-single-stranded template DNA.

Step 3 (Extension): A heat-stable DNA polymerase enzyme (commonly Taq polymerase, originally isolated from a heat-tolerant bacterium) then extends each primer, synthesising a new, complementary DNA strand, effectively duplicating the targeted DNA segment.

Significance: Since this three-step cycle (denaturation, annealing, extension) can be repeated many times in rapid succession (typically 25-35 cycles), and since each cycle approximately doubles the amount of the specific target DNA sequence present, PCR can generate an enormous number of copies of a specific DNA segment from an extremely small starting sample — this powerful amplification capability has made PCR an indispensable tool across numerous applications, including forensic DNA analysis, medical diagnostic testing (including infectious disease detection), genetic research, and paternity testing, among many other applications.

The Human Genome Project was a major, international collaborative scientific effort aimed at determining the complete nucleotide sequence of the entire human genome, and identifying/mapping the location of all human genes.

Key findings: The project determined the sequence of the approximately 3 billion base pairs comprising the human genome, and revealed that the human genome contains a considerably smaller number of protein-coding genes than had originally been anticipated by many scientists prior to the project's completion — the project also revealed that a substantial proportion of the human genome does not directly code for protein, consisting instead of various regulatory sequences and other non-coding DNA regions, whose specific functions in many cases remain subjects of ongoing research.

Broader implications: The complete human genome sequence has provided an invaluable foundational resource for numerous subsequent areas of biological and medical research — it has substantially advanced our understanding of human genetic variation and its relationship to disease susceptibility, has enabled more precise, targeted identification of specific disease-causing genetic mutations, has facilitated development of new genetic diagnostic tests, and has opened important new avenues for personalised medicine approaches (tailoring medical treatment based on an individual's specific genetic profile) — the project has fundamentally transformed the landscape of modern genetics and molecular medicine, providing a comprehensive genetic reference framework that continues to underpin extensive ongoing biomedical research efforts.

A point mutation involves a change affecting a single nucleotide base within the DNA sequence — the specific functional consequence of such a mutation depends on precisely how it affects the corresponding codon and the resulting amino acid sequence.

Silent mutation: Due to the degenerate (redundant) nature of the genetic code, a base change can sometimes result in a new codon that still codes for the exact same amino acid as the original, unmutated codon — in this case, the resulting protein's amino acid sequence remains completely unchanged, and the mutation is termed "silent," typically having no functional effect on the protein.

Missense mutation: A base change can result in a new codon that codes for a different amino acid than the original codon — this substitutes one specific amino acid for a different one at that particular position within the protein. Depending on the specific properties of the original versus the substituted amino acid, and the specific role of that position within the overall protein structure, a missense mutation may have minimal functional effect, or may substantially impair the protein's normal structure/function (as occurs, for example, in sickle cell anaemia).

Nonsense mutation: A base change can result in a new codon that becomes a premature stop codon (rather than continuing to code for the correct, intended amino acid) — this causes translation to terminate prematurely, well before the ribosome reaches the normal, intended stop codon, resulting in a truncated (incompletely synthesised), typically non-functional protein — nonsense mutations are frequently associated with significant loss of normal protein function, since a substantial portion of the intended protein sequence is simply never synthesised at all.