MDCAT Biology Syllabus

State the complementary base-pairing rules in DNA and the number of hydrogen bonds in each pair.

Adenine pairs with Thymine (2 hydrogen bonds); Guanine pairs with Cytosine (3 hydrogen bonds). In RNA, adenine pairs with uracil.

According to Chargaff's rule, what quantitative relationships hold between DNA bases?

$\%A = \%T$ and $\%G = \%C$; therefore $A + G = T + C$ (total purines equal total pyrimidines).

What is an enzyme and what is its chemical nature?

An enzyme is a biological catalyst that speeds up a reaction by lowering its activation energy without being consumed. Most enzymes are globular proteins (some are RNA molecules called ribozymes).

List four key characteristics of enzymes.

They are highly specific for their substrate; they are efficient (increase reaction rate greatly); they are not consumed and can be reused; their activity is affected by temperature and pH; and they lower activation energy.

Distinguish a cofactor, a coenzyme, and an apoenzyme/holoenzyme.

A cofactor is a non-protein helper (often a metal ion); a coenzyme is an organic cofactor (e.g., NAD, FAD, derived from vitamins). Apoenzyme is the protein part alone (inactive); apoenzyme + cofactor = holoenzyme (active).

Compare the lock-and-key and induced-fit models of enzyme action.

Lock-and-key: the substrate fits precisely into a rigid, complementary active site. Induced-fit (Koshland): the active site is flexible and changes shape to mould around the substrate for a better fit upon binding.

How does an enzyme lower activation energy in terms of the enzyme-substrate complex?

The substrate binds the active site forming an enzyme-substrate (ES) complex, which stabilizes the transition state, orients reactants, and strains bonds—reducing the activation energy so products form faster; then products are released and enzyme is regenerated.

How do temperature and pH affect enzyme activity?

Activity rises with temperature up to an optimum (~$37^{\circ}C$ in humans), then falls sharply as the enzyme denatures. Each enzyme has an optimum pH (e.g., pepsin ~$2$, trypsin ~$8$); deviations denature it and reduce activity.

How does substrate concentration affect enzyme reaction rate, and what is $V_{max}$?

Rate increases with substrate concentration until all active sites are saturated, then it plateaus. $V_{max}$ is the maximum reaction velocity reached at enzyme saturation.

Differentiate competitive and non-competitive enzyme inhibition.

A competitive inhibitor resembles the substrate and binds the active site, blocking substrate binding (overcome by raising substrate concentration). A non-competitive inhibitor binds a site other than the active site (allosteric), changing enzyme shape; it is not overcome by more substrate.

Name the phases of the cell cycle in order.

Interphase ($G_1$, S, $G_2$) followed by the mitotic (M) phase, which includes mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis. Non-dividing cells may enter $G_0$.

What happens during the S phase and each gap phase of interphase?

$G_1$: cell growth and organelle synthesis; S phase: DNA replication (chromosomes duplicated); $G_2$: further growth and synthesis of proteins needed for mitosis.

Briefly state the key event of each stage of mitosis.

Prophase: chromosomes condense, spindle forms, nuclear envelope breaks down. Metaphase: chromosomes align at the equator (metaphase plate). Anaphase: sister chromatids separate to opposite poles. Telophase: nuclear envelopes reform, chromosomes decondense.

What molecules regulate the cell cycle and how?

Cyclins and cyclin-dependent kinases (CDKs). Cyclins bind and activate CDKs, which phosphorylate target proteins to drive the cell from one phase to the next; cyclin levels rise and fall cyclically.

Name the main cell cycle checkpoints and what each monitors.

$G_1$ (restriction) checkpoint: cell size, nutrients, DNA damage—decides whether to divide. $G_2$/M checkpoint: DNA replication completeness and damage. M (spindle) checkpoint: proper attachment of chromosomes to the spindle before anaphase.

What is apoptosis, and how does it differ from necrosis?

Apoptosis is programmed, controlled cell death that is genetically regulated and does not trigger inflammation (cell shrinks, forms apoptotic bodies). Necrosis is unplanned death from injury, causing cell swelling, lysis, and inflammation.

What are the morphological features of a cell undergoing apoptosis?

Cell shrinkage, chromatin condensation, DNA fragmentation, membrane blebbing, and breakup into membrane-bound apoptotic bodies that are engulfed by phagocytes without releasing contents.

Which enzymes execute apoptosis and what is the role of mitochondria?

Caspases (cysteine-aspartic proteases) carry out apoptosis. Mitochondria release cytochrome c (intrinsic pathway), which activates caspases; the extrinsic pathway is triggered by death-receptor signals at the cell surface.

Name the formed elements of blood and their primary functions.

Erythrocytes (RBCs) carry $\ce{O2}$ (and some $\ce{CO2}$) via hemoglobin; leukocytes (WBCs) provide immune defense; thrombocytes (platelets) initiate blood clotting.

Differentiate innate (non-specific) and adaptive (specific) immunity.

Innate immunity is present from birth, fast, non-specific (skin, phagocytes, inflammation) with no memory. Adaptive immunity is acquired, slower initially, specific to antigens (lymphocytes), and has immunological memory.

Contrast the roles of B-lymphocytes and T-lymphocytes.

B-cells mediate humoral immunity by producing antibodies against antigens. T-cells mediate cell-mediated immunity: helper T-cells (CD4) coordinate responses; cytotoxic T-cells (CD8) kill infected cells.

What is a neuron, and what are its three main structural parts?

A neuron is the structural and functional unit of the nervous system that transmits impulses. Parts: dendrites (receive signals), cell body/soma (contains nucleus), and axon (conducts impulse away, often myelinated).

What is the resting membrane potential of a neuron and how is it maintained?

About $-70\ \text{mV}$ (inside negative). It is maintained by the $\ce{Na+}/\ce{K+}$ pump, which pumps 3 $\ce{Na+}$ out and 2 $\ce{K+}$ in per ATP, plus selective membrane permeability to $\ce{K+}$.

Divide the human nervous system into its main structural and functional parts.

Structurally: Central NS (brain + spinal cord) and Peripheral NS (nerves). Functionally the PNS divides into somatic (voluntary) and autonomic (involuntary) systems; the autonomic further splits into sympathetic and parasympathetic divisions.

Trace the path of food through the human alimentary canal in order.

Mouth $\to$ pharynx $\to$ esophagus $\to$ stomach $\to$ small intestine (duodenum, jejunum, ileum) $\to$ large intestine (colon) $\to$ rectum $\to$ anus.

Match these digestive enzymes to the nutrients they break down: amylase, pepsin, lipase.

Amylase (salivary/pancreatic) digests starch into maltose; pepsin (stomach) digests proteins into peptides; lipase (pancreatic) digests fats (triglycerides) into fatty acids and glycerol.

Name the parts of a nephron in order of filtrate flow.

Bowman's capsule (glomerulus) $\to$ proximal convoluted tubule $\to$ loop of Henle (descending and ascending limbs) $\to$ distal convoluted tubule $\to$ collecting duct.

What are the three basic processes of urine formation in the nephron?

Glomerular filtration (blood filtered into Bowman's capsule), tubular reabsorption (useful substances like glucose, water, ions returned to blood), and tubular secretion (wastes like $\ce{H+}$, $\ce{K+}$ added to filtrate).

Compare the three types of muscle tissue.

Skeletal muscle: striated, voluntary, multinucleate—attached to bones. Cardiac muscle: striated, involuntary, branched with intercalated discs—in the heart. Smooth muscle: non-striated, involuntary, spindle-shaped—in walls of internal organs.

According to the sliding filament theory, how does a muscle contract?

Actin (thin) filaments slide over myosin (thick) filaments, pulled by myosin cross-bridges; the sarcomere shortens (H-zone and I-band narrow) while filament lengths stay constant. It requires $\ce{Ca^{2+}}$ and ATP.