Cytoplasmic Inheritance
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