14. Cytoplasmic (extrachromosomal) inheritance in prokaryotes and eukaryotes.
Introduces cytoplasmic inheritance through genetic material outside the nucleus, mainly maternal, and the plasmagenes that carry it. It then covers bacterial plasmids, including R-plasmids and the Bacillus anthracis example, the F factor, episomes, conjugation and Hfr cells. For eukaryotes it describes mitochondrial and chloroplast DNA, the mitochondrial genome contents, their distinct genetic code, DNA repair limits and endosymbiotic origin.
Topic 15 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 619 words (4 min of reading), with 25 flashcards.
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Cytoplasmic (extrachromosomal) inheritance in prokaryotes and eukaryotes.
In Prokaryotes
In Eukaryotes
Cytoplasmic (extrachromosomal) inheritance in prokaryotes and eukaryotes.
Cytoplasmic inheritance refers to traits passed through genetic material outside the nucleus
These traits do not follow Mendel’s laws and are mostly inherited from the mother
The gene responsible is located in the cytoplasm of the egg cell, not in the nucleus
The structures that carry these genes are called plasmagenes
Plasmagenes are self-replicating and work independently of chromosomes
In Prokaryotes
Plasmids
Plasmids are small circular DNA molecules that exist outside the bacterial chromosome
They can replicate independently of the main chromosome
Found in bacteria, archaea, and some yeasts
Plasmid numbers vary between one and several hundred per cell
Functions of Plasmids
Often carry antibiotic resistance genes, known as R-plasmids
R-plasmids can be easily transferred between bacteria, spreading resistance
Some plasmids carry genes for bacterial virulence
Example: Bacillus anthracis (causes anthrax) has two plasmids:
pH01 (produces toxin)
pH02 (produces protective capsule)
Both plasmids are needed for the bacterium to be infectious
Episomes and Conjugation
Some plasmids, called episomes, can integrate into the bacterial chromosome
The F factor (fertility plasmid) allows DNA transfer between bacteria
Bacteria with the F factor are called F positive or F plus (F+)
F+ cells form a sex pilus that connects to F negative (F−) cells
A copy of the F plasmid is transferred during conjugation
After transfer, both cells become F+ and can pass on the plasmid
Hfr Cells
If the F plasmid becomes integrated into the bacterial chromosome, the cell becomes an Hfr cell (high-frequency recombination)
Hfr cells can transfer parts of their chromosome to another bacterium
The transfer starts from the F plasmid and continues into the adjacent bacterial genes
This process spreads chromosomal genes rather than just the plasmid
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