What is apoptosis, and in what types of organisms does it occur?

Apoptosis is a form of programmed cell death that occurs in multicellular organisms and also in some eukaryotic, single-celled microorganisms like yeast. It is a highly regulated process involving specific biochemical events that lead to characteristic cell changes and eventual death.

What are the key morphological changes a cell undergoes during apoptosis?

During apoptosis, a cell typically undergoes shrinkage and rounding due to cytoskeleton breakdown, the cytoplasm becomes dense with tightly packed organelles, chromatin condenses into patches against the nuclear envelope (pyknosis), and the nuclear envelope fragments, leading to DNA fragmentation (karyorrhexis) and the formation of apoptotic bodies.

How does apoptosis differ from necrosis?

Unlike necrosis, which is a traumatic cell death resulting from acute injury, apoptosis is a controlled, programmed process. Apoptosis results in the formation of membrane-bound apoptotic bodies that are engulfed by phagocytes, preventing damage to surrounding tissues, whereas necrosis involves cell swelling and lysis, releasing cellular contents.

What is the etymological origin of the term 'apoptosis'?

The term 'apoptosis' comes from Ancient Greek, meaning 'falling off.' It was originally used to describe the falling off of petals from flowers or leaves from trees, and later adopted into medical terminology to describe cell death.

Who were the key figures involved in the initial description and naming of apoptosis?

Carl Vogt first described the principle of apoptosis in 1842, and Walther Flemming provided a more precise description in 1885. However, John Kerr, Alastair Currie, and Andrew Wyllie were instrumental in distinguishing apoptosis from traumatic cell death and published a seminal article in 1972 that popularized the term 'apoptosis,' with James Cormack suggesting the name.

What significant discoveries led to apoptosis becoming a major field of research?

The field of apoptosis research gained significant momentum with two key discoveries: the identification of the first components of the cell death control and effector mechanisms, and the linkage of abnormal cell death processes to human diseases, particularly cancer, exemplified by the 1988 discovery that the BCL2 gene inhibits cell death.

Which scientists were awarded the Nobel Prize in Physiology or Medicine in 2002 for their work on apoptosis?

Sydney Brenner, H. Robert Horvitz, and John Sulston were awarded the 2002 Nobel Prize in Physiology or Medicine for their groundbreaking work in identifying genes that control apoptosis, primarily through studies in the nematode C. elegans.

What are the two primary pathways that initiate apoptosis?

Apoptosis can be initiated through two main pathways: the intrinsic pathway (also known as the mitochondrial pathway), triggered by intracellular signals like cell stress, and the extrinsic pathway, activated by extracellular signals binding to cell-surface death receptors.

How does the intrinsic pathway of apoptosis lead to cell death?

The intrinsic pathway is activated by cellular stress, leading to the release of proteins like cytochrome c from mitochondria. These proteins, along with Apaf-1 and ATP, form an apoptosome complex that activates caspase-9, which in turn activates executioner caspases (like caspase-3) to dismantle the cell.

What role do Bax and Bak proteins play in the intrinsic apoptotic pathway?

Bax and Bak are proteins that insert into the outer mitochondrial membrane, forming pores or increasing membrane permeability. This action facilitates the release of cytochrome c and other apoptotic effectors from the mitochondria into the cytosol, initiating the caspase cascade.

What are SMACs, and how do they contribute to apoptosis?

SMACs (second mitochondria-derived activators of caspases) are proteins released from mitochondria during apoptosis. They bind to and inhibit Inhibitor of Apoptosis Proteins (IAPs), thereby removing the brakes on caspases and allowing the cell death process to proceed.

How is the extrinsic pathway of apoptosis initiated?

The extrinsic pathway is triggered by external signals, such as ligands binding to cell-surface death receptors like the Fas receptor or TNF receptors. This binding leads to the formation of a Death-Inducing Signaling Complex (DISC), which recruits and activates initiator caspases.

What is the role of caspases in apoptosis?

Caspases are a family of cysteine proteases that play a central role in apoptosis. Initiator caspases are activated first, which then activate executioner caspases. These executioner caspases dismantle the cell by degrading key cellular proteins.

What are the two main types of caspases involved in apoptosis?

There are two main types of caspases: initiator caspases (such as caspases 2, 8, 9, and 10) which are activated by apoptotic signals, and effector caspases (such as caspases 3, 6, and 7) which are activated by initiator caspases and carry out the cell degradation program.

What is the significance of the Bcl-2 family of proteins in apoptosis?

The Bcl-2 family proteins act as crucial regulators of apoptosis, particularly in the intrinsic pathway. They exist in pro-apoptotic (like Bax, Bak, Bid) and anti-apoptotic (like Bcl-2, Bcl-xL) forms. The balance between these proteins determines whether a cell commits to apoptosis, often by influencing mitochondrial membrane permeability.

Can apoptosis occur without the activation of caspases?

Yes, a caspase-independent apoptotic pathway exists, mediated by factors like Apoptosis-Inducing Factor (AIF). AIF is released from mitochondria and translocates to the nucleus, where it contributes to cell death without involving the caspase cascade.

How does the TNF pathway contribute to apoptosis?

Tumor Necrosis Factor-alpha (TNF-alpha), a cytokine produced by macrophages, is a major extrinsic mediator of apoptosis. When TNF-alpha binds to its receptor TNFR1, it initiates a signaling cascade involving proteins like TRADD and FADD, ultimately leading to caspase activation.

What is the function of the Fas receptor and Fas ligand in apoptosis?

The Fas receptor (also known as CD95 or Apo-1) is a transmembrane protein that binds to its ligand, FasL. This interaction triggers the formation of the DISC, which contains FADD and caspases 8 and 10, initiating the extrinsic apoptotic pathway.

What are the two types of cells described in relation to Fas-mediated apoptosis?

In Fas-mediated apoptosis, cells are classified into Type I and Type II. In Type I cells, processed caspase-8 directly activates other caspases to execute apoptosis. In Type II cells, the Fas-DISC initiates a feedback loop that amplifies the release of pro-apoptotic factors from mitochondria, leading to increased caspase-8 activation.

What morphological changes are observed during apoptotic cell disassembly?

Apoptotic cell disassembly involves membrane blebbing, where the cell membrane forms irregular buds. Some cells may also develop membrane protrusions like apoptopodia. Finally, the cell fragments into vesicles called apoptotic bodies, which are then cleared by phagocytes.

What is efferocytosis, and how is it related to apoptosis?

Efferocytosis is the process by which phagocytic cells, such as macrophages, engulf dying cells. During apoptosis, cells display signals like phosphatidylserine on their surface, marking them for efferocytosis, which ensures their orderly removal without triggering inflammation.

What is the significance of phosphatidylserine exposure during apoptosis?

Phosphatidylserine is normally located on the inner surface of the cell membrane. During apoptosis, it is redistributed to the outer surface by scramblase proteins. This external exposure acts as a 'eat me' signal, marking the cell for recognition and engulfment by phagocytes.

How can gene knockouts be used to study apoptosis pathways?

Gene knockouts, where specific genes involved in apoptosis pathways are inactivated, are used to test the function of individual proteins. For example, knocking out TNF or Apaf-1 has revealed their critical roles in the apoptotic process and organismal development.

What are some methods used to distinguish between apoptotic and necrotic cells?

Methods to distinguish apoptotic from necrotic cells include live cell imaging, time-lapse microscopy, flow cytometry (analyzing cell surface markers like phosphatidylserine exposure or cell permeability), transmission electron microscopy, and biochemical assays for caspase activation or DNA fragmentation.

What is DNA laddering, and how is it used in apoptosis research?

DNA laddering refers to the characteristic fragmentation of DNA into regularly spaced units during apoptosis. This fragmentation can be visualized on an agar gel after electrophoresis, and the presence of this 'ladder' pattern helps differentiate apoptosis from other forms of cell death like necrosis.

How can defects in apoptotic pathways contribute to diseases like cancer?

Defects that inhibit apoptosis can allow damaged or abnormal cells to survive and proliferate, a hallmark of cancer. For instance, overexpression of anti-apoptotic proteins like XIAP or mutations in genes like p53 can disrupt the normal cell death process, promoting tumor development and resistance to therapy.

What is the role of the p53 protein in apoptosis?

The tumor-suppressor protein p53 plays a critical role in apoptosis. When DNA damage is detected, p53 can halt the cell cycle to allow for repair. If the damage is too extensive, p53 can induce apoptosis, thereby preventing the propagation of potentially cancerous cells.

How does the human papillomavirus (HPV) affect apoptosis in HeLa cells?

HPV proteins E6 and E7, expressed in HeLa cells, inhibit apoptosis. E6 deactivates p53, and E7 binds to retinoblastoma proteins, both of which are involved in cell cycle regulation. This inhibition of apoptosis contributes to the immortality and cancerous nature of HeLa cells.

What are some therapeutic strategies aimed at manipulating apoptosis?

Therapeutic strategies can either induce or inhibit apoptosis depending on the disease. To induce apoptosis, treatments might involve increasing death receptor ligands (like TRAIL), antagonizing anti-apoptotic proteins (like Bcl-2), or using Smac mimetics. To inhibit apoptosis, treatments might target specific caspases or block survival signaling pathways.

How is apoptosis involved in neurodegenerative diseases?

Hyperactive apoptosis, or excessive programmed cell death, is implicated in neurodegenerative diseases such as Alzheimer's and Parkinson's. In these conditions, neurons may undergo apoptosis due to various cellular stresses, leading to progressive loss of neural function.

What is the connection between HIV infection and apoptosis?

HIV infection leads to the depletion of CD4+ T-helper lymphocytes, partly through apoptosis. HIV proteins can interfere with anti-apoptotic proteins like Bcl-2, activate pro-apoptotic pathways, and trigger Fas-mediated apoptosis in these crucial immune cells, ultimately compromising the immune system and leading to AIDS.

How can viruses manipulate apoptosis to their advantage?

Viruses can trigger apoptosis in host cells as part of their life cycle or to evade the immune system. Conversely, many viruses also encode proteins that inhibit apoptosis, allowing the infected cell to survive longer, replicate the virus, and potentially spread to other cells.

What are some examples of viral proteins that inhibit apoptosis?

Viruses employ various strategies to inhibit apoptosis. Examples include viral homologs of Bcl-2 proteins (like Epstein-Barr virus's BHRF1), caspase inhibitors (like cowpox virus's CrmA), and proteins that inhibit p53 (like adenovirus E1B-55K or Hepatitis B virus's HBx).

Are there similarities and differences between apoptosis in animals and plants?

Programmed cell death in plants shares molecular similarities with animal apoptosis but also differs. Plants lack immune systems for removing cell debris and instead use vacuole rupture with self-degrading substances. They also lack phagocytic cells, relying on internal mechanisms for clearance.

What is the significance of the 'Warburg hypothesis' in relation to apoptosis and cancer?

The Warburg hypothesis suggests that cancer cells often shift their metabolism towards glycolysis, even in the presence of oxygen. This metabolic shift is sometimes correlated with the pathological inactivation of apoptosis in cancer cells, indicating a link between metabolic changes and the evasion of cell death.

How does the intrinsic pathway utilize mitochondria in the process of apoptosis?

The intrinsic pathway relies heavily on mitochondria. Stress signals cause mitochondria to release key proteins, such as cytochrome c, into the cell's cytoplasm. This release is a critical step that triggers the activation of caspases, the executioners of apoptosis.

What is the function of the apoptosome in the intrinsic apoptotic pathway?

The apoptosome is a protein complex formed when cytochrome c binds to Apaf-1 in the presence of ATP. This complex then cleaves and activates caspase-9, which is an initiator caspase, thereby propagating the apoptotic signal downstream to executioner caspases.

What are the key differences between Type I and Type II cells in the context of Fas-mediated apoptosis?

In Type I cells, Fas-mediated signaling directly activates caspases for rapid cell death. In Type II cells, the initial Fas signaling is weaker and requires amplification through a feedback loop involving mitochondria to release more pro-apoptotic factors and activate caspases effectively.

How do Inhibitor of Apoptosis Proteins (IAPs) function?

IAPs are a family of proteins that directly inhibit caspases, thereby preventing or suppressing apoptosis. They act as crucial negative regulators, and their inactivation, often mediated by proteins like SMACs released from mitochondria, is necessary for apoptosis to proceed.

What is the role of nitric oxide (NO) in apoptosis?

Nitric oxide (NO) can play a complex role in apoptosis. It has been shown to induce apoptosis by affecting mitochondrial membrane potential, potentially making it more permeable. NO can act as a signaling molecule that either initiates or inhibits apoptosis depending on the cellular context and subsequent pathways.

What is the 'falling off' meaning of apoptosis related to?

The Greek origin of 'apoptosis' relates to the 'falling off' of petals from flowers or leaves from trees. This imagery reflects the controlled, programmed nature of cell death, where a cell detaches and is removed without causing damage.

What is the significance of the Bcl-2 gene discovery in the study of apoptosis?

The discovery in 1988 that the BCL2 gene, implicated in follicular lymphoma, encodes a protein that inhibits cell death was a pivotal moment. It highlighted the role of specific genes in controlling apoptosis and linked dysregulation of cell death to diseases like cancer, bringing the field out of obscurity.

How does the p53 protein regulate the cell cycle in response to DNA damage?

When DNA damage occurs, p53 protein levels increase. It can halt the cell cycle, typically at the G1 phase, providing time for DNA repair. If the damage is irreparable, p53 then triggers apoptosis to eliminate the compromised cell.

What are some examples of drugs that can be used to treat diseases by targeting apoptosis?

Therapeutic agents like Herceptin, Iressa, and Gleevec can induce apoptosis in cancer cells by blocking upstream growth and survival signaling pathways. Additionally, treatments can involve increasing death receptor ligands or antagonizing anti-apoptotic pathways.

What is the 'Inverse Warburg hypothesis' mentioned in relation to neurodegenerative diseases?

The 'Inverse Warburg hypothesis' is mentioned in the context of neurodegenerative diseases, suggesting an inverse epidemiological comorbidity between these conditions and cancer. It implies a potential link between altered cellular metabolism and the propensity for either cancer development or neurodegeneration, possibly related to apoptosis regulation.

How does the virus Oropouche (OROV) induce apoptosis in HeLa cells?

Oropouche virus (OROV) induces apoptosis in HeLa cells, leading to their degeneration. Studies show this involves DNA fragmentation and the release of cytochrome C from mitochondria into the cytosol, indicating activation via the intrinsic apoptotic pathway, triggered by intracellular stimuli from the viral infection.

What is the 'Lock-in and apoptosis' strategy developed for HIV eradication?

The 'Lock-in and apoptosis' strategy involves using a synthesized compound (L-Hippo) to bind to the HIV protein PR55Gag, suppressing viral budding. This traps the virus within the cell, allowing the cell to undergo natural apoptosis, thus potentially eradicating HIV from reservoir cells.

What are some of the consequences of excessive apoptosis (hyperactive apoptosis)?

Excessive or uncontrolled apoptosis can lead to detrimental effects such as atrophy and tissue damage. It is implicated in various diseases, including neurodegenerative conditions like Alzheimer's and Parkinson's, and can contribute to the progression of infections like HIV by causing excessive loss of immune cells.

What is the role of CD4 glycoprotein in HIV progression related to apoptosis?

HIV proteins can decrease the amount of CD4 glycoprotein on the surface of T-helper cells. This reduction is part of the mechanisms by which HIV contributes to the depletion of these vital immune cells through apoptosis, ultimately leading to a compromised immune system and AIDS.

How does Canine Distemper Virus (CDV) induce apoptosis?

Canine Distemper Virus (CDV) can induce apoptosis, typically via the extrinsic pathway, by activating caspases that disrupt cellular functions. While in normal cells it activates caspase-8 followed by caspase-3, in HeLa cells, CDV appears to induce apoptosis via the intrinsic pathway, bypassing caspase-8.

What is the significance of the 'Warburg hypothesis' in the context of apoptosis and cancer metabolism?

The Warburg hypothesis describes the metabolic shift in cancer cells towards glycolysis. This phenomenon is sometimes linked to the pathological inactivation of apoptosis in cancer cells, suggesting that altered metabolism might contribute to the evasion of programmed cell death, a key characteristic of malignancy.

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Defining Apoptosis

Programmed Cell Death

Apoptosis, derived from Ancient Greek meaning 'falling off', is a highly regulated form of programmed cell death essential for multicellular organisms and some single-celled eukaryotes. It involves a cascade of biochemical events leading to characteristic cellular changes, including blebbing, shrinkage, nuclear fragmentation, and DNA fragmentation, culminating in the formation of apoptotic bodies.

On average, adult humans lose 50 to 70 billion cells daily through apoptosis, with children losing 20 to 30 billion. This controlled process is distinct from necrosis, which results from acute injury.

Controlled vs. Traumatic Death

Unlike necrosis, which is a traumatic response to cellular injury, apoptosis is a precise, genetically determined process. It ensures that dying cells are packaged into membrane-bound apoptotic bodies, which are then efficiently cleared by phagocytes without causing inflammation or damage to surrounding tissues.

This controlled removal is vital for development, such as the separation of digits in embryos, and for tissue homeostasis.

Cellular Morphology

The morphological hallmarks of apoptosis include:

Cell Shrinkage: Retraction of cellular extensions and breakdown of the cytoskeleton.
Nuclear Condensation (Pyknosis): Chromatin aggregates against the nuclear envelope.
Nuclear Fragmentation (Karyorrhexis): The nucleus breaks into discrete bodies.
DNA Fragmentation: DNA is cleaved into regularly sized fragments, often visualized as a 'ladder' on gel electrophoresis.

These changes are mediated by activated caspases, a family of proteases.

Etymology and History

Linguistic Roots

The term "apoptosis" originates from Ancient Greek (apóptōsis), meaning "falling off," akin to leaves falling from a tree. This term was suggested by Professor James Cormack, a Greek language expert at the University of Aberdeen.

Historically, the concept was first described by Carl Vogt in 1842 and later by Walther Flemming in 1885. However, it was John Kerr, Alastair Currie, and Andrew Wyllie who, in a seminal 1972 paper, distinguished it from traumatic cell death and formally termed it "apoptosis," establishing it as a fundamental biological phenomenon.

Scientific Recognition

The field gained significant momentum with the discovery of the BCL2 gene in 1988, which encoded a protein that inhibited cell death, linking apoptosis to human diseases like cancer. This breakthrough, along with subsequent research identifying key genes controlling the process, led to the 2002 Nobel Prize in Physiology or Medicine awarded to Sydney Brenner, H. Robert Horvitz, and John Sulston for their work on apoptosis regulation in model organisms.

Activation Mechanisms

Extrinsic Pathway

The extrinsic pathway is initiated by external signals. Extracellular ligands, such as Tumor Necrosis Factor (TNF) or Fas Ligand (FasL), bind to specific cell-surface death receptors (e.g., TNFR1, Fas/CD95). This binding triggers the recruitment of adaptor proteins (like TRADD and FADD) to form the Death-Inducing Signaling Complex (DISC). The DISC then activates initiator caspases (e.g., caspase-8), which subsequently activate effector caspases, initiating the cell death cascade.

Intrinsic Pathway

The intrinsic pathway, also known as the mitochondrial pathway, is triggered by intracellular stress signals (e.g., DNA damage, oxidative stress, growth factor withdrawal). These stresses lead to changes in the mitochondrial outer membrane, mediated by proteins like Bax and Bak. This results in the release of pro-apoptotic factors, such as cytochrome c, from the mitochondrial intermembrane space into the cytosol. Cytochrome c binds with Apaf-1 and ATP to form the apoptosome, which activates initiator caspase-9, leading to effector caspase activation.

Common Components

Both pathways converge on the activation of caspases, a family of cysteine proteases crucial for executing apoptosis. Initiator caspases cleave and activate effector caspases (caspase-3, -6, -7), which dismantle cellular components. Proteins like Bcl-2 family members (Bcl-2, Bcl-xL, Bax, Bak) act as critical regulators, balancing cell survival and death signals. SMACs (Second Mitochondria-derived Activator of Caspases) also play a role by inhibiting apoptosis-blocking proteins (IAPs).

Key Players in Apoptosis

Caspases

Caspases are cysteine-dependent aspartate-specific proteases that act as the executioners of apoptosis. They exist as inactive procaspases and are activated through proteolytic cleavage. Initiator caspases (caspase-8, -9) are activated by death signals, which then cleave and activate effector caspases (caspase-3, -6, -7). These effector caspases degrade vital cellular proteins, leading to the characteristic morphological changes of apoptosis.

Bcl-2 Family

The Bcl-2 family proteins are central regulators of the intrinsic apoptotic pathway. They exist in pro-apoptotic (e.g., Bax, Bak, Bid, Bad) and anti-apoptotic (e.g., Bcl-2, Bcl-xL) forms. The balance between these proteins determines mitochondrial outer membrane permeability and the subsequent release of cytochrome c. BH3-only proteins, like Bid and Bad, often act as sensors of cellular stress, initiating the cascade.

Other Key Molecules

Cytochrome c: Released from mitochondria, it binds to Apaf-1 to form the apoptosome, activating caspase-9.

Apaf-1: Apoptotic protease activating factor 1; essential for apoptosome formation.

SMAC/DIABLO: Released from mitochondria, they antagonize IAPs (Inhibitors of Apoptosis Proteins), thereby promoting caspase activity.

AIF (Apoptosis-Inducing Factor): Mediates caspase-independent apoptosis by translocating to the nucleus and causing DNA condensation.

Apoptosis in Disease

Cancer & Defective Pathways

Dysregulation of apoptosis is a hallmark of cancer. Cancer cells often evade apoptosis through mechanisms such as overexpressing anti-apoptotic proteins (like Bcl-2 or XIAP), inactivating tumor suppressors (like p53), or mutations in apoptotic pathway components. This resistance allows damaged cells to survive, proliferate, and metastasize. Conversely, some cancer therapies (chemotherapy, radiation) work by inducing apoptosis in cancer cells.

Viral Infections

Viruses can manipulate apoptosis for their own propagation. Some viruses induce apoptosis in infected cells to facilitate viral spread via apoptotic bodies. Others encode proteins that inhibit apoptosis, protecting infected cells and allowing viral replication. Examples include viruses like HIV, which can trigger CD4+ T-cell apoptosis, leading to immune deficiency, and canine distemper virus (CDV), which can induce apoptosis via extrinsic or intrinsic pathways.

Plant vs. Animal Apoptosis

While plants also undergo programmed cell death, the process differs from animal apoptosis. Plants lack phagocytic cells for removing apoptotic bodies; instead, dying cells self-degrade via vacuolar rupture. The absence of a complex immune system also shapes plant cell death mechanisms. Despite these differences, molecular similarities exist, leading to ongoing debate about whether the term "apoptosis" is fully applicable.

Research & Methods

Distinguishing Cell Death

Differentiating apoptosis from necrosis is crucial for understanding cellular responses. Techniques include:

Morphological Analysis: Light and electron microscopy to observe characteristic apoptotic changes (pyknosis, karyorrhexis, blebbing).
Biochemical Assays: Detecting DNA fragmentation (DNA laddering), caspase activation (e.g., Western blotting), cytochrome c release, and phosphatidylserine exposure on the cell surface (using flow cytometry).
Live Cell Imaging: Time-lapse microscopy to observe dynamic cellular events.

Pathway Knockouts

Gene knockout studies in model organisms have been instrumental in elucidating the roles of specific proteins in apoptosis. By disrupting genes for caspases (e.g., caspase-8, -9, -3), regulatory proteins (e.g., Bax, Apaf-1), or signaling molecules (e.g., TNF), researchers can observe the resulting phenotypes. These studies reveal the complex interplay of pathways and highlight the existence of compensatory mechanisms or alternative cell death routes when key components are absent.

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References

HeLa cells are an immortalized cancer cell line used frequently in research. The cell line was established by removing cells directly from Henrietta Lacks, a cancer patient.

A full list of references for this article are available at the Apoptosis Wikipedia page

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The Elegant Exit

💡 Dive in with Flashcard Learning! 💡

Defining Apoptosis ℹ️

💀 Programmed Cell Death

⚖️ Controlled vs. Traumatic Death

🔬 Cellular Morphology

Etymology and History 📜

🗣️ Linguistic Roots

💡 Scientific Recognition

Activation Mechanisms ⚙️

➡️ Extrinsic Pathway

mitochondria Intrinsic Pathway

🔗 Common Components

Key Players in Apoptosis 🧬

🔪 Caspases

⚖️ Bcl-2 Family

⚛️ Other Key Molecules

Apoptosis in Disease ⚠️

♋ Cancer & Defective Pathways

🦠 Viral Infections

🌳 Plant vs. Animal Apoptosis

Research & Methods 🔬

🧪 Distinguishing Cell Death

🧬 Pathway Knockouts

Teacher's Corner 🧑‍🏫

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📜 Important Notice

Dive in with Flashcard Learning!

Defining Apoptosis

Programmed Cell Death

Controlled vs. Traumatic Death

Cellular Morphology

Etymology and History

Linguistic Roots

Scientific Recognition

Activation Mechanisms

Extrinsic Pathway

Intrinsic Pathway

Common Components

Key Players in Apoptosis

Caspases

Bcl-2 Family

Other Key Molecules

Apoptosis in Disease

Cancer & Defective Pathways

Viral Infections

Plant vs. Animal Apoptosis

Research & Methods

Distinguishing Cell Death

Pathway Knockouts

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice