A Mitochondrial and Plastid Genomes

A.1 Question 1

The mitochondrial genome’s replication ___ (fill in the blank)

  1. is regulated by cell cycle proteins in the cytoplasm.
  2. can only occur in the S phase of the cell cycle.
  3. can occur out of phase with the nuclear genome.
  4. can only occur in the M phase of the cell cycle.

A.1.1 Answer

The third answer (i.e., “3.”) should be right. This was explicitly mentioned in the lecture slides.

Neither the second or the fourth answer is right: mitochondrial DNA replication can occur out-of-phase with the cell cycle, so genome replication need not necessarily take place during these phases.

Furthermore, while mitochondrial genome replications are regulated, there is nothing to suggest that these regulations are controlled by proteins in the cell cycle.

A.2 Question 2

Choose the best statement that describes chloroplast DNA / genomes:

  1. The chloroplast genome is inherited maternally in all plants.
  2. The chloroplast genome is packaged by histones.
  3. The chloroplast organelle encodes for all proteins that the organelles need to function.
  4. Chloroplast DNA is circular.

A.2.1 Answer

The answer should be the fourth choice (i.e., “4.”).

The first answer applies to most plants, but not all plants. Furthermore, the second answer is false: plastid genomes are circular and do not require histones to be supercoiled.

While plastid genomes do encode for proteins that participate in the electron transport chain, it does not encode for every protein that the chloroplast needs to function.

A.3 Question 3

The genomes of different plastids (e.g., the chloroplast and the amyloplast of the same plant) are ___ (fill in the blank).

  1. identical in DNA sequence.
  2. non-identical in DNA sequence.
  3. derived from different ancestral symbiotic cyanobacteria.
  4. derived from different ancestral symbiotic rickettsia.

A.3.1 Answer

The second answer should be correct (i.e., “2.”). Since both organelles have different functions in the plant cell, it is highly probable that they are non-identical in their DNA sequences. For this reason, the first answer is likely to be incorrect.

Rickettsia is a bacterium that causes typhoid fever in humans. While its genome is similar to that of our mitochondria’s genomes, there is nothing to suggest that plastid genomes are an evolutionary product of rickettsia genomes (and likewise for the third answer choice)

A.4 Question 4

How many stop codons are there in the human mitochondria?

A.4.1 Answer

Two. The codons AGA and AGG are the stop codons in the human mitochondrial genome.

A.5 Question 5

Give two examples of using chloroplast genomes as a platform of genetic engineering in plants.

A.5.1 Answer

  1. Example 1: Bt toxin

    The bacterium Bacillus thuringiensis produces a toxin that is harmful to insects when ingested.

    In 1999, Kota et al. discovered that tobacco plants with Bt toxin-generating genomes incorporated into their chloroplasts had a high amount of Bt toxin on themselves. This lead to a high insect mortality rate.

  2. Example 2: glycophosphate resistance

    Glycophosphate is a potent, broad spectrum herbicide that suppresses the growth of grasses and broadleaf weeds.

    In 1998, Daniell et al. was able to successfully introduce transformed genes conferring glycophosphate resistance into the genome of tobacco plants’ chloroplasts. The resulting genetically-modified plants were able to survive in environments that had ten times the lethal concentration of glycophosphate.

A.6 Question 6

Can you use mitochondrial DNA as evidence in a paternity suit?

A.6.1 Answer

No. Mitochondrial DNA is exclusively inherited from the mother, not the father.

Hence, the mother would have to be present in the suit for the case to be successful.

A.7 Question 7

What is the possible mode of inheritance in the below pedigree chart?

Three Generation Pedigree Chart of a Trait

Figure A.1: Three Generation Pedigree Chart of a Trait

Do also explain your answer.

A.7.1 Answer

It is likely that the trait follows a mitochondrial DNA inheritance pattern. Where a mother has the trait (i.e., the black circles), her offspring also have the trait, but the same cannot be said for a father with the same trait (as his offspring does not have the trait).

The above observations are in line with the fact that mitochondrial DNA is exclusively inherited from the mother - hence the answer.

A.8 Question 8

Electron micrographs show that mitochondria in heart muscle cells have a much higher density of cristae than mitochondria in skill cells. What could be the reason?

A.8.1 Answer

Heart muscle cells require a greater amount of ATP (for contraction) than skin cells. Hence, the increased amount of cristae increases the surface area of the mitochondria.

The aforementioned allows for the insertion of more ATP synthase complexes and hence, a greater rate of ATP synthesis.

A.9 Question 9

Is this statement true or false?

Mitochondria and chloroplasts replicate their DNA in synchrony with the nuclear DNA and divide when the cell divides, thereby ensuring a constant number of organellar genomes.

Explain your answer.

A.9.1 Answer

This statement is false.

Mitochondrial and plastid genomes replicate out of phase with the cell cycle. Their replications are regulated under “constant conditions” to ensure that the amount of DNA there is afterwards is double the amount at the beginning.