Mitochondrial DNA and Protein Synthesis
1. Role of mtDNA Mitochondrial DNA (mtDNA) codes for ribosomal RNAs, transfer RNAs (tRNAs), and small mitochondrial polypeptides. Several pieces of evidence support the conclusion that mtDNA serves as a template for the transcription of mitochondrial RNAs (mtRNAs):
- Mitochondria contain their own RNA polymerase, which can be distinguished from nuclear RNA polymerase by differing susceptibility to inhibitors.
- Ethidium bromide inhibits mtDNA synthesis but not nuclear DNA synthesis, supporting the conclusion that independent RNA synthetic pathways exist in the nucleus and mitochondria.
- Mitochondria synthesize proteins using a unique set of ribosomes, tRNAs, and protein factors.
- The mRNAs transcribed by mtDNA are translated into polypeptides within mitochondria using protein-synthesizing machinery that differs from that present in the cytosol.
2. Ribosomes Mitochondrial ribosomes differ from cytoplasmic ribosomes in size and chemical composition, as shown in the following table:
| Source | Intact Ribosomes | Ribosomal Subunits | Ribosomal RNAs |
|---|---|---|---|
| Mitochondria (mammals) | 55-60S | Large – 45S, Small – 35S | 16S, 12S |
| Mitochondria (yeast) | 75S | Large – 53S, Small – 35S | 21S, 14S |
| Mitochondria (plants) | 78S | Large – 60S, Small – 40S | 26S, 18S, 5S |
| Cytoplasm (Eukaryotes) | 80S | Large – 60S, Small – 40S | 28S, 18S, 5.8S, 5S |
3. Transfer RNAs (tRNAs) Mitochondria contain tRNAs and aminoacyl-tRNA synthetase. The aminoacyl-tRNA synthetase is coded by nuclear DNA, but the tRNAs transcribed from mitochondria differ in sequence from the corresponding tRNAs found in the cytosol. Mammalian mtDNA codes for only 22 tRNAs following wobble rules. Another unusual feature of mitochondrial tRNA is the presence of the tRNA for formylmethionine, previously thought to be found only in bacteria. This suggests similarities between protein synthesis in mitochondria and bacteria.
4. Protein Synthesis Factors Protein synthesis factors of mitochondria are structurally distinct from their counterparts in the cytosol. Several mitochondrial protein synthesis factors are functionally interchangeable with those obtained from bacterial cells.
5. Altered Genetic Code Examining the base sequence of mitochondrial DNA has revealed that mRNA is translated into polypeptide chains using a genetic code that is slightly different from the universal code. Differences in codon usage can even occur among the mitochondria of different organisms.
| S.N. | Codon | Universal Code System | Mammals | Yeast |
| 1 | AUA | Ile | Met | Met |
| 2 | UGA | Stop | Trp | Trp |
| 3 | AGA, AGG | Arg | Stop | Arg |
| 4 | CUU, CUC, CUA, CUG | Leu | Leu | Thr |
Mitochondria synthesize polypeptides whose genes reside in mtDNA. Nuclear genes cooperate with mitochondrial genes in making mitochondrial proteins.
6. Process of mtDNA Protein Synthesis The synthesis of proteins within mitochondria follows a unique process involving the following steps:
- Transcription:
- mtDNA is transcribed within the mitochondria by mitochondrial RNA polymerase to produce mRNAs, tRNAs, and rRNAs.
- These transcripts undergo minimal processing before translation.
- Translation Initiation:
- The mRNA associates with mitochondrial ribosomes, which differ from cytoplasmic ribosomes.
- The small ribosomal subunit binds to the mRNA and recognizes the start codon with the help of mitochondrial initiation factors.
- The initiator tRNA carrying formylmethionine (fMet) binds to the start codon.
- Elongation:
- Aminoacyl-tRNAs are delivered to the ribosome by mitochondrial elongation factors.
- Peptide bonds are formed between adjacent amino acids, elongating the polypeptide chain.
- The ribosome moves along the mRNA, incorporating additional amino acids.
- Termination:
- When the ribosome encounters a stop codon, mitochondrial release factors promote the release of the completed polypeptide.
- The ribosomal subunits dissociate, allowing for the initiation of another round of translation.
- Protein Processing and Import:
- Some synthesized proteins remain within mitochondria, while others are transported for functional integration.
- Chaperone proteins assist in proper folding and assembly of mitochondrial proteins.
- Certain proteins encoded by nuclear DNA are imported into the mitochondria to function alongside those synthesized in situ.
1. Which of the following is coded by mitochondrial DNA (mtDNA)?
a) Ribosomal RNAs (rRNAs)
b) Transfer RNAs (tRNAs)
c) Small mitochondrial polypeptides
d) All of the above
Answer: d) All of the above
2. How is mitochondrial RNA polymerase different from nuclear RNA polymerase?
a) It is inhibited by ethidium bromide
b) It has a different susceptibility to inhibitors
c) It does not require a template for transcription
d) It only synthesizes proteins
Answer: b) It has a different susceptibility to inhibitors
3. What is the size of intact mitochondrial ribosomes in mammals?
a) 80S
b) 75S
c) 55-60S
d) 78S
Answer: c) 55-60S
4. Which of the following is an unusual feature of mitochondrial tRNAs?
a) They follow the universal genetic code
b) They are identical to cytosolic tRNAs
c) They include a tRNA for formylmethionine
d) They are coded by nuclear DNA
Answer: c) They include a tRNA for formylmethionine
5. What is the function of aminoacyl-tRNA synthetase in mitochondria?
a) It synthesizes mRNA
b) It helps in protein folding
c) It charges tRNAs with the correct amino acids
d) It binds ribosomes to the mRNA
Answer: c) It charges tRNAs with the correct amino acids
6. Which of the following codons is translated differently in mitochondrial genetic code as compared to the universal code?
a) AUG
b) UGA
c) CCC
d) GGG
Answer: b) UGA
7. In mammalian mitochondria, what does the codon AUA code for?
a) Leucine
b) Methionine
c) Isoleucine
d) Arginine
Answer: b) Methionine
8. How does mitochondrial protein synthesis differ from cytoplasmic protein synthesis?
a) It uses a unique set of ribosomes and tRNAs
b) It takes place in the nucleus
c) It does not involve mRNA
d) It does not require ribosomes
Answer: a) It uses a unique set of ribosomes and tRNAs
9. What is the first amino acid in mitochondrial protein synthesis?
a) Glycine
b) Alanine
c) Formylmethionine (fMet)
d) Serine
Answer: c) Formylmethionine (fMet)
10. What happens during the termination phase of mitochondrial protein synthesis?
a) The peptide bond formation continues indefinitely
b) The ribosome dissociates, and the completed polypeptide is released
c) Another ribosome immediately attaches and starts translation
d) The mRNA is degraded before translation is complete
Answer: b) The ribosome dissociates, and the completed polypeptide is released
11. Which of the following is NOT a function of mitochondrial protein synthesis factors?
a) Assisting in elongation
b) Assisting in initiation
c) Assisting in termination
d) Synthesizing nuclear proteins
Answer: d) Synthesizing nuclear proteins
12. Which of the following is NOT synthesized inside mitochondria?
a) Some ribosomal RNAs
b) Some mitochondrial tRNAs
c) Most mitochondrial proteins
d) DNA polymerase
Answer: d) DNA polymerase
13. What is the role of mitochondrial chaperone proteins?
a) Assisting in proper protein folding
b) Breaking down mitochondrial mRNA
c) Synthesizing new mitochondrial proteins
d) Transporting nuclear DNA to mitochondria
Answer: a) Assisting in proper protein folding
14. How do nuclear genes contribute to mitochondrial protein synthesis?
a) They encode mitochondrial tRNAs
b) They encode some mitochondrial protein synthesis factors
c) They directly synthesize all mitochondrial proteins
d) They have no role in mitochondrial protein synthesis
Answer: b) They encode some mitochondrial protein synthesis factors
15. Which of the following supports the idea that mitochondria share an evolutionary link with bacteria?
a) The presence of formylmethionine in mitochondrial protein synthesis
b) The use of 80S ribosomes in mitochondria
c) The complete dependence of mitochondria on nuclear DNA
d) The presence of introns in mitochondrial genes
Answer: a) The presence of formylmethionine in mitochondrial protein synthesis
16. Which enzyme is responsible for transcribing mtDNA?
a) RNA polymerase I
b) RNA polymerase II
c) Mitochondrial RNA polymerase
d) Reverse transcriptase
Answer: c) Mitochondrial RNA polymerase
17. Which of the following statements about mitochondrial ribosomes is true?
a) They are identical to bacterial ribosomes
b) They are larger than cytoplasmic ribosomes
c) They have different rRNA compositions than cytoplasmic ribosomes
d) They do not participate in protein synthesis
Answer: c) They have different rRNA compositions than cytoplasmic ribosomes
18. How many tRNAs are coded by mammalian mtDNA?
a) 20
b) 22
c) 30
d) 18
Answer: b) 22
19. What is the function of mitochondrial elongation factors?
a) Recognizing the start codon
b) Delivering aminoacyl-tRNAs to the ribosome
c) Terminating translation
d) Folding proteins into their final structure
Answer: b) Delivering aminoacyl-tRNAs to the ribosome
20. In yeast mitochondria, what does the codon CUU code for?
a) Leucine
b) Threonine
c) Arginine
d) Methionine
Answer: b) Threonine
21. Why is ethidium bromide used in mitochondrial studies?
a) It inhibits cytoplasmic protein synthesis
b) It specifically inhibits mitochondrial DNA replication
c) It increases mitochondrial ATP production
d) It stimulates ribosomal function
Answer: b) It specifically inhibits mitochondrial DNA replication
22. What type of genetic code variation is observed in mitochondria?
a) The same as the nuclear genetic code
b) The same across all organisms
c) Slight modifications from the universal code
d) A completely different code than nuclear DNA
Answer: c) Slight modifications from the universal code
23. What happens if a mitochondrion loses its mtDNA?
a) It continues to function normally
b) It loses its ability to synthesize certain proteins
c) It becomes more efficient in energy production
d) It starts synthesizing proteins using nuclear ribosomes
Answer: b) It loses its ability to synthesize certain proteins
24. Which of the following proteins is required for mitochondrial protein synthesis but encoded by nuclear DNA?
a) Mitochondrial RNA polymerase
b) Mitochondrial elongation factors
c) Aminoacyl-tRNA synthetases
d) All of the above
Answer: d) All of the above
25. What is the role of mitochondrial release factors in translation?
a) Initiating transcription
b) Promoting polypeptide chain elongation
c) Recognizing stop codons and releasing the polypeptide
d) Assisting in aminoacyl-tRNA synthesis
Answer: c) Recognizing stop codons and releasing the polypeptide
26. Why is mitochondrial protein synthesis considered semi-autonomous?
a) Because mitochondria completely rely on nuclear DNA for proteins
b) Because mitochondria synthesize some of their own proteins but rely on nuclear genes for others
c) Because mitochondria can function independently without any external support
d) Because mitochondrial ribosomes are identical to cytoplasmic ribosomes
Answer: b) Because mitochondria synthesize some of their own proteins but rely on nuclear genes for others
27. How do mitochondrial ribosomes compare to bacterial ribosomes?
a) They are structurally similar but not functionally interchangeable
b) They are identical in all aspects
c) They are larger and more complex than bacterial ribosomes
d) They cannot synthesize proteins
Answer: a) They are structurally similar but not functionally interchangeable
28. Why do mitochondria need chaperone proteins?
a) To help in proper protein folding and assembly
b) To degrade misfolded proteins
c) To transport ribosomes into mitochondria
d) To increase ATP production
Answer: a) To help in proper protein folding and assembly
29. What happens to mitochondrial mRNAs after transcription?
a) They are exported to the cytoplasm for translation
b) They undergo minimal processing before being translated in mitochondria
c) They are stored in the nucleus until needed
d) They are immediately degraded
Answer: b) They undergo minimal processing before being translated in mitochondria
30. Why are some mitochondrial proteins synthesized in the cytoplasm and then imported into mitochondria?
a) Because mitochondria lack the necessary ribosomes
b) Because mtDNA does not code for all mitochondrial proteins
c) Because mitochondrial ribosomes are inefficient
d) Because nuclear genes regulate all mitochondrial functions
Answer: b) Because mtDNA does not code for all mitochondrial proteins
31. Where is mitochondrial DNA (mtDNA) located?
a) In the nucleus
b) In the cytoplasm
c) Inside the mitochondria
d) Inside the Golgi apparatus
Answer: c) Inside the mitochondria
32. What shape is mitochondrial DNA?
a) Linear
b) Circular
c) Double-stranded linear
d) Single-stranded
Answer: b) Circular
33. How is mitochondrial DNA inherited in most animals?
a) From both parents equally
b) Only from the father
c) Only from the mother
d) Through random distribution
Answer: c) Only from the mother
34. What is a major function of mitochondria?
a) Protein packaging
b) Energy production via ATP synthesis
c) Lipid storage
d) DNA replication for cell division
Answer: b) Energy production via ATP synthesis
35. What is the function of mitochondrial ribosomes?
a) DNA replication
b) Protein synthesis within the mitochondria
c) Exporting proteins to the nucleus
d) Transporting ATP out of the mitochondria
Answer: b) Protein synthesis within the mitochondria
36. How many proteins are encoded by human mitochondrial DNA?
a) 10
b) 13
c) 20
d) 30
Answer: b) 13
37. What process primarily occurs in mitochondria?
a) Glycolysis
b) Oxidative phosphorylation
c) DNA transcription
d) Cytoplasmic translation
Answer: b) Oxidative phosphorylation
38. Which enzyme is involved in mitochondrial DNA replication?
a) DNA polymerase I
b) DNA polymerase III
c) DNA polymerase gamma
d) RNA polymerase
Answer: c) DNA polymerase gamma
39. What is one key difference between mitochondrial and nuclear genomes?
a) Mitochondrial DNA has more genes than nuclear DNA
b) Mitochondrial DNA is linear, while nuclear DNA is circular
c) Mitochondrial DNA is inherited maternally, whereas nuclear DNA is inherited from both parents
d) Mitochondrial DNA is transcribed in the nucleus
Answer: c) Mitochondrial DNA is inherited maternally, whereas nuclear DNA is inherited from both parents
40. Which of the following mitochondrial functions is regulated by nuclear DNA?
a) ATP production
b) Mitochondrial DNA replication
c) Mitochondrial protein synthesis
d) All of the above
Answer: d) All of the above
41. Which molecule is essential for mitochondrial protein synthesis?
a) Ribosomal RNA (rRNA)
b) mRNA
c) Transfer RNA (tRNA)
d) All of the above
Answer: d) All of the above
42. Which amino acid does the mitochondrial start codon code for in mammals?
a) Leucine
b) Valine
c) Formylmethionine (fMet)
d) Arginine
Answer: c) Formylmethionine (fMet)
43. What is the role of mitochondrial release factors in translation?
a) They initiate protein synthesis
b) They recognize stop codons and terminate translation
c) They assist in peptide bond formation
d) They bind amino acids to tRNAs
Answer: b) They recognize stop codons and terminate translation
44. What is the impact of mutations in mtDNA?
a) They do not affect mitochondrial function
b) They can cause mitochondrial diseases and affect energy production
c) They are easily repaired like nuclear DNA mutations
d) They only affect ribosome synthesis
Answer: b) They can cause mitochondrial diseases and affect energy production
45. How do mitochondria import proteins synthesized in the cytoplasm?
a) Through simple diffusion
b) Using specialized transport complexes like TOM and TIM
c) Through direct ribosomal attachment
d) By synthesizing all necessary proteins internally
Answer: b) Using specialized transport complexes like TOM and TIM
46. Which of the following codons is recognized differently in mitochondrial genetic codes?
a) UGA – Stop codon in universal code, but codes for Tryptophan (Trp) in mitochondria
b) AUG – Methionine in both nuclear and mitochondrial codes
c) CCC – Codes for Proline in both systems
d) GGG – Codes for Glycine in both systems
Answer: a) UGA – Stop codon in universal code, but codes for Tryptophan (Trp) in mitochondria
47. What is the significance of formylmethionine (fMet) in mitochondrial translation?
a) It initiates translation like in bacterial systems
b) It signals the termination of translation
c) It prevents translation errors
d) It is used only in cytoplasmic translation
Answer: a) It initiates translation like in bacterial systems
48. Which ribosomal RNA (rRNA) is found in mammalian mitochondrial ribosomes?
a) 16S and 12S
b) 18S and 5.8S
c) 21S and 14S
d) 28S and 5S
Answer: a) 16S and 12S
49. Why do mitochondria have their own DNA?
a) Because they were once free-living bacteria that became part of eukaryotic cells (Endosymbiotic theory)
b) Because they are completely independent from the cell
c) Because they lack nuclear control
d) Because they need to produce their own ATP
Answer: a) Because they were once free-living bacteria that became part of eukaryotic cells (Endosymbiotic theory)
50. What is the role of mitochondrial chaperone proteins?
a) Assisting in proper protein folding and assembly
b) Breaking down mitochondrial mRNA
c) Transporting ATP across membranes
d) Destroying misfolded proteins
Answer: a) Assisting in proper protein folding and assembly