Immune Response
What is an infectious disease?
A disease caused by an infectious agent or pathogen.
What are the monomers of nucleic acids?
Nucleotides.
What is the name of the enzyme that unwinds the DNA double helix during replication?
Helicase
Mitosis produces two genetically identical daughter cells for growth, repair, and asexual reproduction.
What is evolution?
The change in the genetic composition of a population during successive generations, which may result in the development of new species.
Name the three lines of defence in the vertebrate immune system.
Barrier defences, innate immune response, adaptive immune response.
What is transcription?
The first step of protein synthesis where a gene's DNA sequence is copied into a messenger RNA (mRNA) molecule.
What is the role of DNA Polymerase III?
DNA polymerase III adds new nucleotides in the 5’ to 3’ direction, following the complementary base pairing rule (A-T, C-G).
During which phase of meiosis do sister chromatids separate?
Sister chromatids separate during anaphase II.
What is convergent evolution?
Where species that do not share a recent common ancestor evolve similar traits independently in response to similar selective pressures.
What is the role of natural killer cells?
To destroy body cells infected with viruses as part of the innate immune response.
A DNA coding strand has the code: 5' ACC GAT CAT TTC GAA 3'. What would the mRNA sequence be?
5' ACC GAU CAU UUC GAA 3'
The coding strand of DNA has the same sequence as the mRNA—except that RNA uses U instead of T.
Why is DNA replication described as “semi‑conservative”?
Each new DNA molecule contains: one original (parental) strand, and one newly synthesised strand.
So half of the original DNA is “conserved” in each daughter molecule.
What is crossing over, and during which stage of meiosis does it occur?
Crossing over is the exchange of genetic material between homologous chromosomes. It occurs during prophase I of meiosis.
What is allopatric speciation?
Where two populations of a species are reproductively isolated due to a geographic barrier, resulting in the evolution of two new species.
How is the adaptive immune response activated?
Antigen-presenting cells phagocytose a pathogen and display fragments of the digested antigen on the cell membrane. When a T helper cell meets an APC with an antigen that matches its receptor, that T helper cell becomes activated. Activated T helper cells activate B cells, starting the humoral pathway, and cytotoxic T cells, starting the cell-mediated pathway.
Explain how a single gene can produce multiple different proteins in eukaryotes.
Eukaryotic genes contain exons (coding regions) and introns (non‑coding regions). After transcription, the primary RNA transcript undergoes several processing steps that can change the final mRNA sequence and therefore the protein produced. During mRNA processing, introns are cut out and the exons are spliced together. This creates a mature mRNA that can be translated into a protein. Different combinations of exons produce different mature mRNAs.
Explain why the lagging strand is synthesised discontinuously.
The lagging strand runs in the 3' → 5' direction relative to the replication fork, but DNA polymerase can only build in the 5' → 3' direction. As a result, it is synthesised in short fragments called Okazaki fragments. DNA ligase later joins these fragments together to form a continuous strand.
What is one key difference between metaphase in mitosis and metaphase I in meiosis.
In mitosis, individual chromosomes line up at the metaphase plate. In meiosis I, homologous chromosome pairs line up together.
Describe one key difference between microevolution and macroevolution, and explain how they are connected.
Microevolution focuses on small genetic changes within populations; macroevolution involves large‑scale patterns such as speciation. Macroevolutionary changes arise from the long‑term accumulation of microevolutionary processes.
In patients with Mast Cell Activation Syndrome (MCAS), mast cells release too much histamine. Describe the role of mast cells in the immune response and suggest what symptoms these patients might experience.
Mast cells recognise pathogens or allergens through surface receptors. When activated, they rapidly release stored chemicals including histamine. Histamine increases blood vessel permeability, allowing immune cells to reach the affected area. This contributes to redness, swelling, and warmth. Patients with MCAS may experience symptoms of the inflammatory response without an infection.
Describe the three steps in translation.
Initiation - The small ribosomal subunit binds to the mRNA. A tRNA carrying methionine (the start amino acid) recognises the start codon AUG. The large ribosomal subunit then joins, forming a complete ribosome ready to build a protein.
Elongation- tRNAs bring amino acids to the ribosome, matching their anticodons to codons on the mRNA. The ribosome forms peptide bonds between amino acids, growing the polypeptide chain. The protein grows as amino acids are added in the order specified by the mRNA sequence.
Termination - When the ribosome reaches a stop codon (UAA, UAG, or UGA), no tRNA matches it. Release factors bind instead, causing the ribosome to release the completed polypeptide.
Imagine a mutation occurs that reduces the activity of DNA ligase. Predict how this would affect DNA replication.
DNA ligase normally joins Okazaki fragments by sealing the sugar‑phosphate backbone. If ligase activity is reduced, Okazaki fragments remain unjoined and the lagging strand stays fragmented and incomplete. DNA replication will fail or produces unstable chromosomes.
A diploid cell has 12 chromosomes. How many chromosomes will each daughter cell have after meiosis II?
A diploid cell with 12 chromosomes has a haploid number of 6. After meiosis II, each daughter cell contains 6 chromosomes.
Why is reproductive isolation important for evolution?
Reproductive isolation prevents populations from sharing genes, allowing them to evolve independently. Once gene flow is cut off, even small genetic changes can accumulate over generations, eventually producing distinct traits, adaptations, and ultimately new species.