Genomes of Chloroplasts and Mitochondria

What is meant by uniparental (often maternal) inheritance of organelles, and how did early studies in variegated plants reveal this?

Uniparental inheritance is non‑Mendelian inheritance in which a trait is inherited from only one parent, commonly the mother.

In variegated plants, inheritance of leaf colour could not be explained by Mendel’s laws and was instead accounted for by cytoplasmic inheritance of plastids (chloroplasts), where the egg cytoplasm (and its plastids) determines progeny phenotype.

How did yeast petite mutants contribute to the discovery of mitochondrial DNA and its inheritance?

Yeast petite mutants form small colonies due to defective oxidative phosphorylation.

Their non‑Mendelian inheritance pattern was explained by stochastic segregation of defective mitochondria between daughter cells.

This phenotype was later correlated with defective mtDNA, and in 1964 Gottfried Schatz identified DNA in purified mitochondria, establishing mtDNA and its inheritance.

Why are plant mitochondrial genomes often much larger than animal mtDNAs and cpDNAs, and what is known about the origin of their extra DNA?

Plant mtDNAs can be very large, mostly due to non‑coding DNA that is not conserved across species.

Extra sequences can originate from chloroplast, nuclear, or viral DNA, and horizontal transfer from other plants, but a substantial fraction is of unknown origin, even though coding capacity is similar to smaller organelle genomes.

Describe the organisation and transcription of mammalian mtDNA, including the roles of POLRMT, TFAM, TFB2M and TEFM.

Mammalian mtDNA is transcribed from the non‑coding control region (NCR) using POLRMT, a T7‑related single‑subunit RNA polymerase.

TFAM binds upstream of promoters (LSP and HSP), bends DNA ~180°, and recruits POLRMT.

POLRMT then changes conformation to bind TFB2M, forming the initiation complex.

TEFM is required for transcription elongation.

Transcription from LSP and HSP yields two long polycistronic transcripts that are processed into mRNAs, tRNAs and rRNAs.

What are the key structural features of typical land plant cpDNAs, and how do non‑photosynthetic plants illustrate cpDNA gene loss?

Typical land plant cpDNAs have a quadripartite structure: an LSC, an SSC, and two ~20–25 kb inverted repeats (IRs), usually encoding ~100 proteins mainly for photosynthesis and organelle gene expression, with highly conserved gene content.

In non‑photosynthetic plants like E. roseum (ghost orchid), cpDNA has lost many photosynthesis‑related genes, showing that cpDNAs can undergo substantial gene loss when photosynthesis is no longer required.