BioMedical Admissions Test (BMAT) Section 2: Scientific Knowledge and Applications — Biology Syllabus

Explain why DNA replication is described as semi-conservative.

Each new DNA molecule consists of one original (parental) strand and one newly synthesised strand. The two strands separate and each acts as a template, so half of each new molecule is conserved from the parent.

Name the two key enzymes in DNA replication and their roles.

DNA helicase unwinds the double helix and breaks the hydrogen bonds between bases. DNA polymerase joins free nucleotides to the template strand by forming phosphodiester bonds, synthesising the new complementary strand.

What are the two main stages of protein synthesis and where do they occur?

Transcription occurs in the nucleus, where mRNA is made from a DNA template. Translation occurs at the ribosomes (cytoplasm/rough ER), where the mRNA codon sequence is decoded to assemble amino acids into a polypeptide.

What is a codon and how many bases code for one amino acid?

A codon is a sequence of three adjacent bases (a triplet) on mRNA that codes for one amino acid. The genetic code is therefore a triplet code; it is also degenerate (most amino acids have more than one codon) and non-overlapping.

Describe the role of tRNA in translation.

Transfer RNA carries a specific amino acid and has an anticodon that is complementary to an mRNA codon. It binds to the ribosome so its anticodon pairs with the codon, delivering the correct amino acid in sequence to form the polypeptide.

Define gene, allele, genotype, and phenotype.

Gene: a length of DNA coding for a polypeptide/characteristic. Allele: a different version of a gene. Genotype: the alleles an organism possesses. Phenotype: the observable characteristics, determined by genotype and environment.

Distinguish between homozygous and heterozygous, and dominant and recessive alleles.

Homozygous: two identical alleles for a gene; heterozygous: two different alleles. A dominant allele is expressed in the phenotype even when only one copy is present; a recessive allele is only expressed when two copies are present (homozygous recessive).

In a monohybrid cross between two heterozygotes (Aa × Aa), what is the expected phenotypic ratio?

3:1 dominant to recessive. The genotypic ratio is 1 AA : 2 Aa : 1 aa, giving 3 showing the dominant phenotype to 1 showing the recessive phenotype.

What is codominance? Give an example.

Codominance is when both alleles in a heterozygote are fully expressed in the phenotype. Example: human ABO blood group, where alleles $I^A$ and $I^B$ are codominant, so genotype $I^A I^B$ gives blood group AB.

How are sex-linked characteristics inherited, and why are males more often affected by X-linked recessive conditions?

Sex-linked genes are carried on the sex chromosomes (usually X). Males (XY) have only one X, so a single recessive allele (e.g. for haemophilia or red-green colour blindness) is always expressed; females (XX) need two copies to be affected and can be carriers.

Distinguish between continuous and discontinuous variation, giving an example of each.

Continuous variation shows a range of values controlled by many genes and influenced by environment (e.g. height, mass). Discontinuous variation shows distinct categories with no intermediates, usually controlled by one or few genes (e.g. blood group, sex).

Outline the process of natural selection.

Variation exists within a population (from mutation). There is competition for limited resources. Individuals with advantageous alleles are better adapted and more likely to survive and reproduce (survival of the fittest). They pass on these favourable alleles, so over generations the allele frequency increases in the population.

What is the difference between a mutation and the role it plays in variation?

A mutation is a random change in the base sequence of DNA (or chromosome structure/number). Mutations are the original source of new alleles and therefore of genetic variation, providing the raw material on which natural selection acts.

Describe the double circulatory system of mammals.

Blood passes through the heart twice for each complete circuit. The pulmonary circuit carries deoxygenated blood from the heart to the lungs and back. The systemic circuit carries oxygenated blood from the heart to the body and back, maintaining high pressure for efficient delivery.

Compare the structure and function of arteries, veins, and capillaries.

Arteries: thick muscular/elastic walls, narrow lumen, carry blood at high pressure away from the heart. Veins: thinner walls, wide lumen, valves to prevent backflow, carry blood at low pressure to the heart. Capillaries: one-cell-thick walls for short diffusion distance, site of exchange between blood and tissues.

Trace the path of blood through the heart starting from the vena cava.

Vena cava → right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → bicuspid (mitral) valve → left ventricle → aorta → body.

Why is the wall of the left ventricle thicker than the right ventricle?

The left ventricle pumps oxygenated blood around the whole body (systemic circulation) at high pressure, so it needs a thicker, more muscular wall. The right ventricle only pumps blood the short distance to the lungs at lower pressure.

Write the balanced equation for aerobic respiration.

$$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}$$ This releases energy (ATP). In words: glucose + oxygen → carbon dioxide + water.

Compare aerobic and anaerobic respiration in terms of oxygen, products, and energy yield.

Aerobic respiration uses oxygen, fully oxidises glucose to $\ce{CO2}$ and water, and releases a large amount of energy (~38 ATP). Anaerobic respiration occurs without oxygen, gives a small energy yield (~2 ATP), and produces lactate in animals or ethanol and $\ce{CO2}$ in yeast/plants.

List the adaptations of the alveoli for efficient gas exchange.

Large surface area (many alveoli), thin walls (one cell thick) giving a short diffusion distance, moist surface for gases to dissolve, and a dense capillary network with continuous blood flow to maintain a steep concentration gradient.

Describe the changes during inhalation (breathing in).

The diaphragm contracts and flattens, and the external intercostal muscles contract, raising the ribs up and out. This increases thoracic volume, decreases pressure below atmospheric, so air is drawn into the lungs.

Describe the structure and function of a motor neurone.

A motor neurone has a cell body with dendrites, a long myelinated axon, and motor end plates. It carries electrical impulses from the central nervous system to effectors (muscles or glands). Myelin (Schwann cells) insulates the axon and speeds up impulse transmission.

Outline the sequence of a reflex arc.

Stimulus → receptor → sensory neurone → relay (intermediate) neurone in the spinal cord/CNS → motor neurone → effector → response. It is rapid and automatic, bypassing conscious brain processing for protection.

How is an impulse transmitted across a synapse?

An impulse arrives at the presynaptic terminal, triggering vesicles to release neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across and binds to receptors on the postsynaptic membrane, generating a new impulse. It is then broken down/reabsorbed.

Compare nervous and hormonal (endocrine) control.

Nervous control uses electrical impulses along neurones, is very fast, short-lived, and targets specific cells. Hormonal control uses chemical hormones transported in the blood, is slower, longer-lasting, and may affect many target organs.

Define homeostasis and give two examples of conditions it regulates.

Homeostasis is the maintenance of a constant internal environment within narrow limits despite external changes. Examples include regulation of blood glucose concentration, body temperature, blood pH, and water/osmotic balance.

Explain how insulin and glucagon control blood glucose concentration (negative feedback).

When blood glucose is high, the pancreas releases insulin, which stimulates cells to take up glucose and the liver to convert glucose to glycogen, lowering it. When glucose is low, glucagon is released, stimulating the liver to break glycogen down to glucose, raising it. This is negative feedback.

What is a food chain, and what do the arrows represent?

A food chain shows the feeding relationships and transfer of energy/biomass between organisms, starting with a producer. The arrows point in the direction of energy flow, i.e. from the organism being eaten to the organism that eats it.

Why is energy lost between trophic levels, and roughly how much is transferred?

Energy is lost as heat from respiration, in movement, and in undigested material (faeces) and excretion (urine). Only about 10% of energy is transferred to the next trophic level, which limits food chains to about 4–5 links.

Distinguish between pathogens and the main types of pathogen.

A pathogen is a microorganism that causes disease. Main types: bacteria (e.g. tuberculosis), viruses (e.g. influenza, HIV), fungi (e.g. athlete's foot), and protists/protozoa (e.g. malaria, caused by Plasmodium).

Distinguish between the body's non-specific defences and the specific immune response.

Non-specific (innate) defences act the same against all pathogens: skin, mucus, stomach acid, and phagocytosis. The specific (adaptive) response is targeted: lymphocytes recognise particular antigens, with B-cells producing specific antibodies and T-cells destroying infected cells, and it creates immunological memory.

Explain how vaccination produces immunity.

A vaccine introduces a dead, weakened, or antigenic part of a pathogen. The antigens stimulate B-lymphocytes to produce specific antibodies and memory cells. On later infection by the real pathogen, memory cells trigger a faster, larger secondary response, destroying it before symptoms develop (active immunity).