Genetics of organelles 1

Question 1

Is the following statement true: Mitochondria are transferred from mother cell to daughter cell. The number of mitochondria is relatively static.

Answer 1

False.

Mitochondria fuse and split dynamically. In different cell types, the number of mitochondria differs. This means that mitochondrial DNA is in constant shuffling.

Question 2

What’s contained in the mitochondrial genome?

Answer 2

1. Simple RNApol (likely deriving from a phage)
2. Bacterial RNApol
3. Genes for oxidative phosphorylation
4. rRNAs
5. tRNAs

There are no intergenic sequences!!

Question 3

How is the mitochondrial genome replicated and transcribed?

Answer 3

1-. Rolling circle-replication using an RNA primer (+DNApol). First, the heavy strand is synthesized, when it’s made the first loop, the light strand is synthesized.

2. There are three promoters of which two produce the same transcript. The light- and heavy-strand transcripts are handled separately leading to some clumsiness. Not all genes will be expressed at the same time.
3. The genome is trans-spliceable. Example: NAD2 requires one exon deriving from the light chain, the remaining exons are present in the heavy chain.
4. There are some cases where nuclear- and mitochondrial DNA need to create a common gene product (see another slide).

Question 4

Why is complex 1 of the electron transport chain a lethality factor?

Answer 4

39 subunits derive from the nuclear genome, 7 subunits derive from the mitochondrial genome. If the mitochondrial- and nuclear subunits aren’t compatible, you suffer embryonic death.

Question 5

Fun fact about the spliceosome:

Answer 5

The few introns which are present in the mitochondrial DNA often self-splice themselves due to self-complementarity. This is thought to be the first self-replicating molecule and the foundation of our current spliceosome.

There are three groups of ribozymes which cause self-splciing of which group 1 and 2 share mechanisms with the spliceosome.

Question 6

What’s RNA editing?

Answer 6

RNA editing occurs in trypanosomes (a parasite). In vertebrates, RNA editing affects a few bases. In squids, there’s lots of it (pan-editing).

RNA editing happens in both mitochondria and the nuclear genome. pre-mRNAs are bound perfectly complementary to an RNA. The complementary RNA acts like a gRNA for the editosome which deaminates/aminates certain bases.

A –> I

C –> U

(Look into this more if you’ve got time).

Question 7

When designing a transcript which you’d want to translate in the mitochondria, what do you need to take into consideration?

Answer 7

The mitochondrial tRNAs are different from the genomic ones. Most mitochondrial tRNAs encode the same amino acid. The difference between mitochondrial- and nuclear translation is attributed to separate evolutionary lineages.