Biogenesis of Mitochondria

Mitochondria are essential organelles responsible for cellular respiration and energy production. They primarily arise through the growth and division of pre-existing mitochondria or from promitochondria. Microscopic evidence strongly supports this theory.

Observations under a phase-contrast microscope have revealed mitochondria in the process of dividing. Additionally, electron microscopy studies on green algae Micromonas—which contains a single mitochondrion—have detected a membrane partition dividing the mitochondrion into two. However, these observations do not entirely exclude the possibility that mitochondria may also arise through other mechanisms.

Experimental Evidence: David Luck’s Study on Neurospora

In the 1960s, David Luck designed an experiment using the filamentous fungus Neurospora crassa to investigate whether mitochondria could form through mechanisms other than division. His experiment involved the following steps:

1. Incorporation of a Radioactive Phospholipid Precursor

Neurospora colonies were incubated in ³H-choline, a radioactive precursor for phospholipids, allowing it to be incorporated into mitochondrial membranes.
This ensured that existing mitochondria were labeled with radioactivity.

2. Growth in Non-Radioactive Medium

The labeled cultures were then transferred to a medium without ³H-choline and allowed to grow for varying periods.
This step prevented any new mitochondria from incorporating the radioactive precursor, allowing researchers to track the fate of pre-existing mitochondria.

3. Autoradiographic Analysis

Microscopic autoradiography was used to examine mitochondrial radioactivity.
Silver grains were counted to determine how radioactivity was distributed among mitochondria over time.

Possible Outcomes and Interpretations

Three different patterns of radioactivity distribution could emerge based on different hypotheses of mitochondrial biogenesis:

1. Growth & Division of Existing Mitochondria (Supported Hypothesis)

If new mitochondria form by the growth and division of pre-existing mitochondria, each doubling in the number of mitochondria after the removal of ³H-choline should result in a 50% reduction in radioactivity per mitochondrion.
This is because labeled phospholipids from pre-existing mitochondria are distributed among newly formed mitochondria.

2. De Novo Synthesis of Mitochondria

If mitochondria can form from scratch (de novo synthesis), then newly synthesized mitochondria should lack radioactivity.
Only the pre-existing mitochondria would remain radioactive, retaining the original label over time.

3. Mitochondria Arising from Other Cellular Membranes

If mitochondria develop from other cellular membranes, the pre-existing radioactive mitochondria would retain their radioactivity.
New mitochondria should initially be radioactive because they originate from labeled membranes, but as time passes and the absence of ³H-choline leads to a decline in radioactive membrane components, the average mitochondrial radioactivity should gradually decrease.

Experimental Findings and Conclusion

The results of the study followed the first pattern:

With each round of mitochondrial division, the radioactivity per mitochondrion decreased by approximately 50%.
This strongly supported the hypothesis that new mitochondria arise by the growth and division of pre-existing mitochondria rather than through de novo synthesis or the transformation of other cellular membranes.

Mitochondrial Formation in Yeast: A Special Case

In some cases, mitochondria seem to reappear in cells that initially lack them. This phenomenon is observed in yeast, which can survive under both aerobic and anaerobic conditions:

Under anaerobic conditions, yeast relies on glycolysis for energy production. Since oxidative phosphorylation is unnecessary in the absence of oxygen, mitochondria gradually disappear.
When exposed to oxygen, mitochondria reappear.

Was This Evidence of De Novo Mitochondrial Formation?

Initially, the reappearance of mitochondria was believed to be evidence of de novo formation. However, electron microscopy studies provided an alternative explanation:

Small double-membrane-enclosed vesicles (~1 µm in diameter) were detected in anaerobically grown yeast cells.
These vesicles lacked cristae and cytochromes but contained mitochondrial DNA (mtDNA).
Upon exposure to oxygen, their inner membranes formed cristae, synthesizing cytochromes and other respiratory chain components.

Discovery of Promitochondria

Since these vesicles had the potential to differentiate into fully functional mitochondria, they were named promitochondria.

Significance of Promitochondria

Promitochondria act as precursors to mitochondria, allowing cells to adapt to changing oxygen availability.
Rather than forming mitochondria de novo, cells preserve mitochondrial genetic material and basic membrane structures in a dormant state.
Upon oxygen exposure, these dormant vesicles develop into fully functional mitochondria, ensuring a quick adaptation to aerobic metabolism.

Conclusion

Mitochondria primarily arise through the growth and division of pre-existing mitochondria, as demonstrated by David Luck’s experiment.
In anaerobic conditions, mitochondria may disappear but are not lost entirely. Instead, they exist in an intermediate promitochondrial state.
Upon oxygen exposure, promitochondria develop into fully functional mitochondria, debunking the idea of de novo mitochondrial synthesis in yeast.

This reinforces the semi-autonomous nature of mitochondria, which contain their own DNA and division machinery, yet depend on the host cell for essential components.

MCQs on Biogenesis of Mitochondria

1. How do new mitochondria primarily arise?

A) By the fusion of pre-existing mitochondria
B) By the growth and division of pre-existing mitochondria
C) By de novo synthesis from other organelles
D) By nuclear DNA encoding entire mitochondria

✅ Answer: B) By the growth and division of pre-existing mitochondria

2. Which microscopic evidence supports mitochondrial division?

A) Observation of mitochondria splitting under a phase-contrast microscope
B) Detection of mitochondrial disappearance under anaerobic conditions
C) Formation of mitochondria from other cellular membranes
D) Direct visualization of mitochondrial DNA in the nucleus

✅ Answer: A) Observation of mitochondria splitting under a phase-contrast microscope

**3. What was the primary aim of David Luck’s experiment on Neurospora?**

A) To determine if mitochondria arise from the nuclear membrane
B) To confirm that mitochondria contain their own DNA
C) To investigate whether mitochondria arise by growth & division or de novo synthesis
D) To study the effect of oxygen on mitochondrial function

✅ Answer: C) To investigate whether mitochondria arise by growth & division or de novo synthesis

**4. Why was ³H-choline used in David Luck’s experiment?**

A) To act as a radioactive tracer incorporated into mitochondrial DNA
B) To track phospholipid incorporation into mitochondrial membranes
C) To stain mitochondria for easier microscopic visualization
D) To inhibit mitochondrial function and observe the effects

✅ Answer: B) To track phospholipid incorporation into mitochondrial membranes

5. According to the growth and division hypothesis, what happens to mitochondrial radioactivity after each division?

A) It remains constant in all mitochondria
B) It completely disappears after one cycle
C) It decreases by 50% per division
D) It increases as new mitochondria form

✅ Answer: C) It decreases by 50% per division

6. If mitochondria were formed de novo, what would be observed in the autoradiography experiment?

A) New mitochondria would be non-radioactive
B) All mitochondria would maintain their original radioactivity
C) New mitochondria would initially be radioactive but lose it over time
D) Mitochondrial division would cease entirely

✅ Answer: A) New mitochondria would be non-radioactive

7. What evidence suggested that mitochondria do not form de novo in yeast?

A) The presence of mitochondrial-like vesicles containing mtDNA in anaerobic conditions
B) The complete disappearance of mitochondria in anaerobic yeast
C) The inability of yeast to switch between aerobic and anaerobic conditions
D) The direct transformation of the nucleus into mitochondria upon oxygen exposure

✅ Answer: A) The presence of mitochondrial-like vesicles containing mtDNA in anaerobic conditions

8. What are promitochondria?

A) Fully developed mitochondria with a unique membrane structure
B) Dormant mitochondrial precursors that can develop into functional mitochondria
C) Mitochondria that undergo fusion instead of division
D) Mitochondria that lack DNA and rely entirely on the nucleus

✅ Answer: B) Dormant mitochondrial precursors that can develop into functional mitochondria

9. What happens to yeast mitochondria under anaerobic conditions?

A) They grow larger and become more efficient
B) They disappear completely and cannot be regenerated
C) They transform into promitochondria and lose their cristae
D) They actively divide to maintain cellular respiration

✅ Answer: C) They transform into promitochondria and lose their cristae

10. What is the significance of mitochondrial biogenesis?

A) It allows cells to maintain energy production by replicating mitochondria as needed
B) It prevents cellular adaptation to different environmental conditions
C) It shows that mitochondria are entirely independent of the cell
D) It proves that mitochondria originate only from nuclear DNA

✅ Answer: A) It allows cells to maintain energy production by replicating mitochondria as needed

11. Which of the following best describes the role of mitochondria in a eukaryotic cell?

A) Protein synthesis
B) Lipid metabolism
C) ATP production through oxidative phosphorylation
D) DNA replication

✅ Answer: C) ATP production through oxidative phosphorylation

12. Why was autoradiography used in David Luck’s experiment?

A) To visualize mitochondrial DNA directly
B) To track the distribution of radioactive phospholipids in mitochondrial membranes
C) To measure the size of mitochondria over time
D) To detect mitochondrial fusion events

✅ Answer: B) To track the distribution of radioactive phospholipids in mitochondrial membranes

13. What structure divides mitochondria during their replication?

A) Cytoplasmic vesicles
B) Membrane partitioning
C) Nucleus-derived membranes
D) Endoplasmic reticulum

✅ Answer: B) Membrane partitioning

14. What was the key conclusion from David Luck’s study on mitochondrial biogenesis?

A) Mitochondria can form de novo from cellular membranes
B) Mitochondria arise primarily by the growth and division of pre-existing mitochondria
C) Mitochondria are synthesized entirely by the nucleus
D) Mitochondria disappear permanently under anaerobic conditions

✅ Answer: B) Mitochondria arise primarily by the growth and division of pre-existing mitochondria

15. Which organelle shares a similar biogenesis pattern with mitochondria?

A) Golgi apparatus
B) Lysosomes
C) Chloroplasts
D) Ribosomes

✅ Answer: C) Chloroplasts

16. Which component of mitochondria contains its genetic material?

A) Inner membrane
B) Cristae
C) Matrix
D) Outer membrane

✅ Answer: C) Matrix

17. Why do mitochondria have their own DNA?

A) They evolved from endosymbiotic bacteria
B) They are independent of the cell nucleus
C) They are formed de novo from nuclear DNA
D) Their genetic material is inherited from the father

✅ Answer: A) They evolved from endosymbiotic bacteria

18. Which of the following is NOT a characteristic of promitochondria?

A) Double-membrane structure
B) Presence of mitochondrial DNA
C) Well-developed cristae
D) Ability to differentiate into functional mitochondria

✅ Answer: C) Well-developed cristae

19. What happens to promitochondria when anaerobic yeast is exposed to oxygen?

A) They remain unchanged
B) They synthesize cytochromes and develop cristae
C) They dissolve and are replaced by new mitochondria
D) They fuse to form larger structures

✅ Answer: B) They synthesize cytochromes and develop cristae

20. What evidence suggests mitochondria evolved from bacteria?

A) Presence of circular DNA similar to bacterial DNA
B) Mitochondria are found in prokaryotic cells
C) They have a single membrane
D) They synthesize their own ribosomes independently

✅ Answer: A) Presence of circular DNA similar to bacterial DNA

21. What would happen if mitochondrial DNA replication stopped?

A) The mitochondria would continue to function normally
B) The mitochondria would lose the ability to produce energy efficiently
C) The cell would increase nuclear DNA replication
D) New mitochondria would form from the endoplasmic reticulum

✅ Answer: B) The mitochondria would lose the ability to produce energy efficiently

22. Which of the following best explains why mitochondria divide?

A) To replace damaged mitochondria and meet cellular energy demands
B) To store excess ATP molecules
C) To increase cell size
D) To remove toxins from the cell

✅ Answer: A) To replace damaged mitochondria and meet cellular energy demands

23. What is the function of cristae in mitochondria?

A) They store genetic material
B) They increase surface area for ATP production
C) They assist in protein synthesis
D) They regulate mitochondrial division

✅ Answer: B) They increase surface area for ATP production

24. How does mitochondrial biogenesis support cellular adaptation?

A) By allowing cells to generate more energy under high-demand conditions
B) By reducing the need for nuclear DNA
C) By making the cell independent of oxygen
D) By preventing cell division

✅ Answer: A) By allowing cells to generate more energy under high-demand conditions

25. How do mitochondria replicate?

A) By binary fission, similar to bacteria
B) By budding off from the nuclear membrane
C) By merging with the Golgi apparatus
D) By being synthesized entirely by ribosomes

✅ Answer: A) By binary fission, similar to bacteria

26. Which enzyme is crucial for mitochondrial DNA replication?

A) DNA polymerase alpha
B) DNA polymerase beta
C) DNA polymerase gamma
D) RNA polymerase

✅ Answer: C) DNA polymerase gamma

27. Mitochondrial division is regulated by which protein?

A) Dynamin-related protein 1 (Drp1)
B) Actin
C) Tubulin
D) Histone H1

✅ Answer: A) Dynamin-related protein 1 (Drp1)

28. The endosymbiotic theory suggests mitochondria originated from which type of bacteria?

A) Cyanobacteria
B) Alpha-proteobacteria
C) Spirochetes
D) Gram-positive bacteria

✅ Answer: B) Alpha-proteobacteria

29. In which part of the mitochondria does oxidative phosphorylation occur?

A) Outer membrane
B) Inner membrane
C) Intermembrane space
D) Matrix

✅ Answer: B) Inner membrane

30. What role does mitochondrial fission play in cellular health?

A) It helps in the distribution of mitochondria during cell division
B) It increases mitochondrial size
C) It prevents mitochondrial DNA replication
D) It stops ATP production

✅ Answer: A) It helps in the distribution of mitochondria during cell division

31. Which of the following is TRUE about mitochondrial inheritance?

A) It follows a Mendelian pattern
B) It is inherited maternally
C) It is inherited paternally
D) It is randomly distributed from both parents

✅ Answer: B) It is inherited maternally

32. What is the primary function of mitochondrial fusion?

A) To separate damaged mitochondria
B) To increase ATP production by sharing contents between mitochondria
C) To reduce the number of mitochondria in a cell
D) To synthesize new mitochondrial DNA

✅ Answer: B) To increase ATP production by sharing contents between mitochondria

33. What is the main function of cardiolipin in mitochondria?

A) It stabilizes mitochondrial membranes and respiratory complexes
B) It helps in mitochondrial DNA replication
C) It functions in mitochondrial protein synthesis
D) It acts as a carrier for mitochondrial RNA

✅ Answer: A) It stabilizes mitochondrial membranes and respiratory complexes

34. Which condition is associated with defective mitochondrial biogenesis?

A) Parkinson’s disease
B) Tuberculosis
C) Diabetes insipidus
D) Huntington’s disease

✅ Answer: A) Parkinson’s disease

35. Which of the following signals can trigger mitochondrial biogenesis?

A) High ATP levels
B) Low AMP/ATP ratio
C) Increased activity of PGC-1α (Peroxisome proliferator-activated receptor gamma coactivator 1-alpha)
D) High glucose availability

✅ Answer: C) Increased activity of PGC-1α (Peroxisome proliferator-activated receptor gamma coactivator 1-alpha)

36. Which structure is responsible for importing proteins into mitochondria?

A) Nuclear pore complex
B) TOM (Translocase of the Outer Membrane) and TIM (Translocase of the Inner Membrane) complexes
C) Ribosomes
D) Endoplasmic reticulum

✅ Answer: B) TOM (Translocase of the Outer Membrane) and TIM (Translocase of the Inner Membrane) complexes

37. What happens if mitochondrial DNA mutations accumulate?

A) The cell becomes more efficient at energy production
B) Mitochondrial function declines, leading to diseases such as Leigh syndrome
C) The cell starts producing more mitochondria
D) The mitochondria fuse to compensate for the loss

✅ Answer: B) Mitochondrial function declines, leading to diseases such as Leigh syndrome

38. Which organ has the highest number of mitochondria per cell?

A) Liver
B) Heart
C) Skin
D) Pancreas

✅ Answer: B) Heart

39. The mitochondrial genome primarily encodes proteins for which cellular process?

A) Glycolysis
B) Electron transport chain and ATP synthesis
C) DNA replication
D) Protein degradation

✅ Answer: B) Electron transport chain and ATP synthesis

40. What is mitophagy?

A) The fusion of mitochondria to increase ATP production
B) The breakdown and removal of damaged mitochondria
C) The replication of mitochondrial DNA
D) The uptake of mitochondria by lysosomes

✅ Answer: B) The breakdown and removal of damaged mitochondria

41. Why do muscle cells have a high number of mitochondria?

A) They have low energy demands
B) They rely on glycolysis instead of oxidative phosphorylation
C) They require a continuous supply of ATP for contraction
D) They do not contain nuclei

✅ Answer: C) They require a continuous supply of ATP for contraction

42. What is the function of cytochrome c in mitochondria?

A) It helps in lipid metabolism
B) It transfers electrons in the electron transport chain
C) It synthesizes ATP directly
D) It regulates mitochondrial fission

✅ Answer: B) It transfers electrons in the electron transport chain

43. Which signaling pathway is most involved in mitochondrial biogenesis?

A) PGC-1α/Nrf2 pathway
B) MAPK pathway
C) mTOR pathway
D) NF-κB pathway

✅ Answer: A) PGC-1α/Nrf2 pathway

44. What is the significance of mitochondrial uncoupling proteins (UCPs)?

A) They inhibit ATP production
B) They regulate mitochondrial membrane potential and heat production
C) They promote mitochondrial fission
D) They facilitate protein transport into mitochondria

✅ Answer: B) They regulate mitochondrial membrane potential and heat production

45. What happens when the mitochondrial outer membrane becomes permeable?

A) ATP production increases
B) Cytochrome c is released, triggering apoptosis
C) Mitochondrial DNA replication accelerates
D) Mitochondrial biogenesis is blocked

✅ Answer: B) Cytochrome c is released, triggering apoptosis