Biology for Medical Exams

Viruses can convert proto-oncogenes into oncogenes through several mechanisms: by introducing viral oncogenes (v-onc) that are homologous to cellular proto-oncogenes, by inserting near proto-oncogenes and causing their overexpression, or by carrying mutated versions. While radiation can also cause this conversion, viruses are particularly efficient at it.

The cell membrane is selectively permeable because it allows only certain types of molecules to pass through, based on factors like size, polarity, and charge. Small nonpolar molecules diffuse easily, while ions and large polar molecules require transport proteins. It does not allow all particles to pass, nor is it limited to only nutrients.

The alpha-helix is a type of secondary protein structure. Secondary structure refers to local folding patterns stabilized by hydrogen bonds between the backbone amide and carbonyl groups. The alpha-helix and beta-sheet are the two main types of secondary structure. Primary structure is the amino acid sequence, tertiary is overall 3D folding, and quaternary involves multiple polypeptide chains.

Radioactive nitrogen-15 would appear in haemoglobin because haemoglobin is a protein containing nitrogen in its amino acids. Proteins incorporate nitrogen during synthesis. Phosphoenolpyruvate (PEP) contains carbon, hydrogen, oxygen, and phosphorus but no nitrogen. Fatty acids contain only carbon, hydrogen, and oxygen. Glycogen is a polysaccharide of glucose, containing only carbon, hydrogen, and oxygen.

The Golgi body (Golgi apparatus) produces vesicles that transport proteins and lipids to other destinations. It modifies, sorts, and packages materials into secretory vesicles. The nucleus doesn't produce vesicles. Mitochondria produce ATP through cellular respiration. Ribosomes synthesize proteins but don't produce vesicles.

According to Chargaff's rules, in double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). If adenine is 33%, then thymine is also 33%. This accounts for 66% of the nucleotides. The remaining 34% must be guanine + cytosine in equal proportions, so guanine is 17% and cytosine is 17%.

RNA contains the sugar ribose, while DNA contains deoxyribose. This is a fundamental biochemical difference - the 'deoxy' in DNA means it lacks one oxygen atom that ribose has. RNA is found in both nucleus and cytoplasm, not only nucleus. DNA uses thymine and RNA uses uracil, not the reverse. Both RNA and DNA can form helical structures (RNA can be single-stranded but also forms helices like in tRNA).

Dehydration synthesis (also called condensation) joins amino acids by removing a water molecule between them, forming peptide bonds that create polypeptide chains and enzymes. Oxidation involves electron loss, denaturation unfolds proteins, and hydrolysis breaks them down.

High hydrogen ion concentration defines acidic solutions (low pH). Neutral solutions have equal H+ and OH- concentrations, while basic solutions have low H+ and high OH- concentration. Isotonic refers to solute concentration equality across a membrane.

The Na+/K+ ATPase pump actively transports 3 Na+ out and 2 K+ in against their concentration gradients, using ATP to establish the resting membrane potential (~-70mV). Depolarization and repolarization are passive ion flows down gradients once the resting potential is established.

After descending through the esophagus, food enters the stomach. The duodenum is the first part of the small intestine (stomach comes before it). The liver and pancreas are accessory organs that secrete into the duodenum but food doesn't enter them directly.

Rough ER has ribosomes attached to its surface, giving it its 'rough' appearance. These ribosomes synthesize proteins (including enzymes), which the rough ER then transports to the Golgi apparatus. Smooth ER detoxifies drugs, not rough ER.

The liver is the body's primary detoxification organ, filtering toxins from blood, metabolizing drugs and alcohol, and converting harmful substances into less toxic forms for excretion. When this system fails, toxins accumulate in circulation.

Pepsin is produced in the stomach as pepsinogen (inactive precursor) and activated by stomach acid to digest proteins. Trypsin comes from the pancreas, amylase from salivary glands and pancreas (not liver), and maltase is an intestinal brush-border enzyme.

Human chorionic gonadotropin (HCG) is produced by the developing placenta shortly after implantation and is the hormone detected by pregnancy tests. Estrogen, progesterone, and LH are present in menstrual cycles but are not specific pregnancy markers in urine.

The hypothalamus contains osmoreceptors that detect blood osmolarity (salt concentration). When blood is too concentrated, the hypothalamus triggers ADH release and thirst. The medulla regulates vital autonomic functions, glomerulus filters blood, and DCT reabsorbs water.

Antidiuretic hormone (ADH, vasopressin) is produced in the hypothalamus but stored in and released from the posterior pituitary into circulation. The anterior pituitary releases different hormones (FSH, LH, ACTH, etc.), and the adrenal cortex releases cortisol and aldosterone.

A blood cell traveling from renal vein → inferior vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein passes TWO valves: tricuspid (right atrioventricular) and pulmonary semilunar. Mitral and aortic valves are on the left side.

The medulla oblongata contains respiratory centers that control breathing rate and depth. During exercise, increased metabolic activity produces more CO2, which the medulla detects and responds to by increasing breathing rate to maintain gas exchange. The hypothalamus regulates homeostasis but not breathing, while the corpus callosum connects brain hemispheres and distal convoluted tubule is a kidney structure.

Louis Pasteur is credited with establishing the germ theory of disease in the 1860s, demonstrating that microorganisms cause disease and fermentation. His experiments disproved spontaneous generation and laid foundations for modern microbiology and sterilization practices. Darwin proposed evolution by natural selection, Hooke discovered cells, and Edwards is not associated with germ theory.

Chordae tendineae are fibrous cords connecting atrioventricular valve cusps to papillary muscles in the ventricles. They prevent valve inversion (prolapse) when ventricles contract, ensuring unidirectional blood flow. Without them, high ventricular pressure would force valves back into atria, causing regurgitation. They do not open valves or support the septum.

Lysosomes are membrane-bound organelles containing digestive enzymes (hydrolases) that break down worn-out cell components, engulfed pathogens, and debris. This recycling process is called autophagy. Ribosomes synthesize proteins, centrioles aid in cell division, and cytosol is the fluid component of cytoplasm - none have digestive functions.

Sickle cell disease (sickle cell anemia) causes red blood cells to assume a sickle shape due to abnormal hemoglobin S. These misshapen, rigid cells have reduced oxygen-carrying capacity and block blood vessels, causing tissue hypoxia. Hemophilia impairs clotting, thrombocytopenia reduces platelet count, and 'blood transfusion' is a procedure, not a disease.

Restriction enzymes like EcoRI are called 'molecular scissors' because they cut DNA at specific recognition sequences (EcoRI cuts at GAATTC). This ability to precisely cleave DNA makes them essential tools in genetic engineering for creating recombinant DNA. Glucose oxidase, isocitrate dehydrogenase, and homoserine dehydrogenase are metabolic enzymes, not nucleases.