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Purdue University Globle
NU551 Advanced Physiology and Pathophysiology Across the Lifespan
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Cells are the basic structural and functional units of the human body, and their ability to survive depends on coordinated genetic, metabolic, and adaptive processes. DNA stores genetic information, RNA helps convert that information into proteins, mitochondria produce cellular energy, and cellular adaptations allow tissues to respond to changing demands. When these processes are disrupted, cellular injury, genetic disorders, chronic inflammation, neurodegenerative disease, and cancer can occur. Understanding these fundamental concepts is essential for studying pathophysiology and explaining how diseases develop at the cellular level.
Messenger RNA (mRNA) is essential for converting genetic information stored in DNA into functional proteins. It acts as an intermediary between DNA in the nucleus and ribosomes, where proteins are produced.
Protein synthesis occurs through two major processes: transcription and translation. During transcription, a specific section of DNA is used as a template to create an mRNA molecule. The mRNA then leaves the nucleus and enters the cytoplasm. Ribosomes read the mRNA sequence during translation and use that information to arrange amino acids into a specific protein.
Proteins produced through this process perform many essential functions, including regulating metabolism, supporting cellular growth, repairing damaged tissues, coordinating immune responses, and maintaining normal cellular structure.
RNA exists in several forms, but three types are particularly important for protein synthesis: messenger RNA, ribosomal RNA, and transfer RNA.
mRNA carries genetic instructions from DNA to ribosomes. Its nucleotide sequence provides the information needed to determine the amino acid sequence of a protein.
rRNA is a major structural and functional component of ribosomes. It helps create the cellular machinery responsible for translating mRNA into proteins.
tRNA transports specific amino acids to the ribosome during translation. Each tRNA recognizes particular codons on the mRNA and delivers the corresponding amino acid, allowing the protein chain to be assembled in the correct order.
Together, mRNA, rRNA, and tRNA allow cells to accurately translate genetic information into proteins.
Deoxyribonucleic acid (DNA) is the primary molecule responsible for storing hereditary information. It contains the instructions needed for cellular development, reproduction, metabolism, and normal tissue maintenance.
DNA is composed of repeating units called nucleotides. Each nucleotide contains a phosphate group, a deoxyribose sugar, and one nitrogenous base. The four bases found in DNA are adenine, thymine, cytosine, and guanine.
The bases pair in a specific pattern: adenine pairs with thymine, while cytosine pairs with guanine. These complementary base pairs form the characteristic double-helix structure of DNA.
DNA must be accurately replicated when cells divide so that new cells receive the genetic information necessary for normal function. Changes in DNA sequences, however, can alter protein production and contribute to disease.
Mitochondria are membrane-bound organelles responsible for producing much of the cell’s usable energy. They generate adenosine triphosphate (ATP) primarily through cellular respiration.
ATP supplies energy for processes such as muscle contraction, active transport, cellular repair, biosynthesis, and other metabolic activities. Mitochondria also participate in calcium regulation and programmed cell death.
Cells with high energy requirements generally contain more mitochondria. Examples include cardiac muscle cells, skeletal muscle cells, and neurons.
Mitochondrial dysfunction can reduce ATP production and increase cellular stress. Because highly active cells depend heavily on mitochondrial energy production, mitochondrial damage can contribute to tissue injury and degenerative disease.
Jaundice occurs when bilirubin accumulates in the blood and tissues, causing a characteristic yellow discoloration of the skin and sclera. Bilirubin is produced during the normal breakdown of heme, particularly from aging red blood cells.
Jaundice can develop when bilirubin production increases, the liver cannot adequately process bilirubin, or bile flow is obstructed. Common causes include hemolysis, liver disease, and biliary obstruction.
At the cellular level, pigments may accumulate within tissues and cells. The resulting yellow coloration becomes clinically visible when bilirubin levels rise sufficiently.
Cells are constantly exposed to changes in workload, nutrition, oxygen availability, hormones, and environmental conditions. When cells experience stress that is not severe enough to cause immediate injury, they may adapt by changing their size, number, phenotype, or metabolic activity.
The four major forms of cellular adaptation are:
Hypertrophy: Increase in cell size
Hyperplasia: Increase in cell number
Atrophy: Decrease in cell size
Metaplasia: Reversible change from one mature cell type to another
These responses can help cells maintain function under stressful conditions. However, persistent or excessive stimulation can eventually contribute to cellular injury and disease.
Muscle atrophy occurs when muscle fibers become smaller, resulting in decreased muscle mass and strength. It commonly develops when muscles are not used regularly or when the body cannot maintain normal protein synthesis.
Prolonged bed rest, immobilization, aging, malnutrition, nerve damage, and chronic disease can all contribute to muscle atrophy.
Reduced mechanical stimulation decreases anabolic signaling and protein synthesis while increasing pathways responsible for protein breakdown. Hormonal and metabolic changes can further accelerate muscle loss.
The result is a reduction in muscle fiber size, metabolic activity, and functional capacity.
Hypertrophy occurs when existing cells increase in size. Unlike hyperplasia, hypertrophy does not primarily involve an increase in the number of cells.
In skeletal muscle, repeated resistance exercise stimulates the production of structural and contractile proteins, including actin and myosin. These changes increase the size and strength of individual muscle fibers.
Hypertrophy can also occur as a pathological adaptation. For example, chronic hypertension increases the workload placed on the left ventricle. In response, cardiac muscle cells may enlarge, producing left ventricular hypertrophy.
Although hypertrophy can initially help tissues cope with increased demands, persistent pathological hypertrophy may eventually impair organ function.
Hyperplasia is an increase in the number of cells within a tissue. It occurs when cells retain the ability to divide and proliferate.
Physiological hyperplasia can occur in response to hormonal stimulation or increased functional demand. Examples include endometrial proliferation during the menstrual cycle and regeneration of liver tissue after certain types of injury.
Hyperplasia is different from cancer because normal hyperplasia remains regulated by physiological signals. When cell proliferation becomes uncontrolled or associated with additional genetic abnormalities, the risk of neoplastic disease may increase.
Apoptosis is a controlled form of programmed cell death that removes unnecessary, damaged, or potentially harmful cells. Unlike uncontrolled cell injury and necrosis, apoptosis generally occurs without producing the same degree of surrounding inflammation.
Normal apoptosis is essential for development, tissue remodeling, and maintenance of cellular balance. Problems occur when apoptosis becomes excessive or insufficient.
Excessive apoptosis can contribute to loss of functional cells, particularly in some neurodegenerative disorders. In contrast, inadequate apoptosis may allow abnormal cells to survive when they should normally be eliminated, which can contribute to cancer development.
Parkinson disease is a progressive neurodegenerative disorder characterized by the loss of dopamine-producing neurons, particularly those located in the substantia nigra.
The reduction in dopamine signaling contributes to characteristic motor symptoms such as:
Resting tremor
Bradykinesia
Muscle rigidity
Postural and gait disturbances
Several cellular mechanisms have been associated with neuronal injury in Parkinson disease, including mitochondrial dysfunction, oxidative stress, abnormal protein accumulation, impaired cellular clearance, and neuroinflammation.
Abnormal activation of cell-death pathways may contribute to the progressive loss of vulnerable neurons.
Ionizing radiation can damage cells by directly affecting DNA or indirectly generating reactive oxygen species and other free radicals. DNA is particularly important because severe or improperly repaired DNA damage can cause mutations, cell death, or loss of reproductive capacity.
Radiation-induced cellular injury may involve:
DNA strand breaks
Oxidative stress
Mitochondrial dysfunction
Plasma membrane alterations
Nuclear damage
Impaired cellular repair
The biological effects depend on factors such as radiation dose, exposure duration, radiation type, and the sensitivity of the affected tissue.
Severe exposure can result in apoptosis or necrosis, while DNA damage that remains unrepaired may increase the risk of mutations and cancer.
A genetic mutation is a change in the DNA sequence. Mutations can occur naturally during DNA replication or result from environmental exposures and other biological processes.
Common forms include base substitutions, insertions, deletions, and frameshift mutations. The effect of a mutation depends on where it occurs and how it changes the resulting gene product.
Some mutations have little or no effect on health. Others can alter protein structure or function and contribute to inherited disorders, cancer, or other diseases.
Mutations may be inherited from a parent through germ cells or acquired during a person’s lifetime in somatic cells.
Genetic abnormalities can involve individual DNA sequences, genes, or entire chromosomes. They may occur because of inherited changes, errors during cell division, or environmental factors that damage genetic material.
Important examples include:
DNA sequence mutations
Gene deletions
Gene duplications
Chromosomal translocations
Abnormal chromosome numbers
Chromosomal abnormalities can alter the amount or organization of genetic material and may lead to developmental disorders, congenital abnormalities, reproductive problems, or disease.
The endoplasmic reticulum (ER) is responsible for important cellular functions, including protein folding and processing. When unfolded or misfolded proteins accumulate within the ER, cells experience endoplasmic reticulum stress.
Cells respond through a protective mechanism known as the unfolded protein response. If the stress is prolonged or cannot be resolved, cellular injury and apoptosis may occur.
Research has linked abnormal ER stress responses with inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis. Disrupted ER function in intestinal epithelial cells can affect the intestinal barrier, immune signaling, inflammation, and cell survival.
Persistent cellular stress may therefore contribute to the chronic inflammation and tissue damage associated with IBD.
The relationship between genetics, cellular function, adaptation, and disease can be summarized through several fundamental concepts. DNA stores genetic information, while RNA helps translate that information into proteins. Mitochondria provide ATP for energy-dependent cellular processes, and cells adapt to stress through changes such as hypertrophy, hyperplasia, atrophy, and metaplasia.
Apoptosis maintains cellular balance, but abnormal cell death can contribute to disease. Radiation and other damaging factors can injure DNA and organelles, potentially producing mutations or cell death. Understanding these mechanisms provides a foundation for explaining how cellular dysfunction progresses into tissue and organ disease.
Messenger RNA carries genetic instructions from DNA to ribosomes, where the information is translated into proteins. These proteins perform many essential functions throughout the body.
The three major types of RNA involved in protein synthesis are messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). mRNA carries genetic instructions, rRNA forms part of the ribosome, and tRNA delivers amino acids during translation.
Hypertrophy increases the size of existing cells, whereas hyperplasia increases the number of cells. Skeletal muscle enlargement from resistance training is an example of hypertrophy, while hormone-driven endometrial proliferation is an example of hyperplasia.
Mitochondria produce ATP through cellular respiration. ATP provides energy for many cellular functions, including active transport, metabolism, movement, synthesis, and repair.
Cellular atrophy is a decrease in cell size that commonly occurs when cells experience reduced workload, inadequate nutrition, aging, loss of nerve supply, or chronic disease.
Ionizing radiation can directly damage DNA or indirectly injure cells by generating reactive oxygen species. Severe damage can cause apoptosis or necrosis, while incorrectly repaired DNA damage can produce mutations.
A genetic mutation is a change in a DNA sequence. Mutations can involve substitutions, insertions, deletions, or other alterations and may be harmless, harmful, or occasionally beneficial depending on their location and effect.
Apoptosis is necessary for removing damaged or unnecessary cells. Excessive apoptosis can contribute to cell loss and some degenerative conditions, while insufficient apoptosis can allow abnormal cells to survive and contribute to cancer.
Persistent ER stress can disrupt protein processing, epithelial cell function, immune regulation, and cell survival. These mechanisms may contribute to the chronic intestinal inflammation associated with Crohn’s disease and ulcerative colitis.
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National Human Genome Research Institute. (n.d.). Talking glossary of genetic terms. National Institutes of Health. https://www.genome.gov/genetics-glossary
National Institute of Neurological Disorders and Stroke. (n.d.). Parkinson’s disease. National Institutes of Health. https://www.ninds.nih.gov/health-information/disorders/parkinsons-disease
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