Biology · MU Sofia

13. Cytoplasmic (extrachromosomal) heredity in prokaryotes and eukaryotes.

Covers extranuclear inheritance, outlining how plasmids, episomes and the F factor drive bacterial conjugation and high-frequency recombination in prokaryotes. It then describes the mitochondrial genome's features, including its gene counts and the endosymbiotic origin behind nuclear control, followed by mitochondrial translation and why its genetic code diverged. The note closes with plastid DNA, covering uniparental inheritance in plants and what plastid DNA encodes. A picture and a short list of mitochondrial translation differences are included.

Inside: 1 section · 3 labelled pictures · 2 min

Topic 13 of 64 in the MU Sofia syllabus for Biology (Dental Medicine, Year 1). The free AceMed note covers it in 1 section, about 291 words (2 min of reading).

In this note

  1. Introduction

Introduction

Overview of Extranuclear Genes

  • Do not conform to Mendelian inheritance patterns.
  • Phenotype determined by maternal genotype.
  • Trait is transmitted via cytoplasmic genes in the female gamete.
  • Plasmids
  • Small circular DNA replicated independently of the bacterial chromosome.
  • Episomes can integrate into the bacterial chromosome.
  • F factor enables DNA transfer via bacterial conjugation.
  • Types of Cytoplasmic Inheritance in Prokaryotes
  • Bacterial Conjugation
  • Plasmids
  • Episomes
  • F factor
  • High Frequency Recombination (Hfr)
  • Occurs when the F factor integrates into the bacterial chromosome.
  • Enables transfer of the F factor and adjacent chromosomal DNA.
  • Mitochondrial Genome
  • Features:
  • Multiple circular copies.
  • Overlapping genes.
  • Genes translated simultaneously with non-coding sequences.
  • Contains 2 rRNA, 22 tRNA, and 13 protein-coding genes.
  • Controlled by nuclear genes due to endosymbiotic theory.
  • Evolution: Genes transferred to nucleus, reducing the number in humans compared to prehistoric organisms.

Translation in organelle

  • Cytoplasm:
  • Initiation, Elongation, and Termination phases.
  • Endoplasmic Reticulum (ER):
  • Signal Recognition Particle (SRP) and Targeting.
  • Translocation.
  • Co-translational Processing.
  • Completion and Targeting.
  • Unique Aspects of Mitochondrial Translation:
  • Different genetic code.
  • Fewer codons and tRNAs.
  • Mitochondrial ribosomes resemble bacterial ribosomes.
  • Lack of caps in organelle mRNA.
  • Methionine modification similar to prokaryotes.
  • Reasons for Mitochondrial Genetic Code Loss Over Time
  • No precise DNA repair.
  • Accumulation of mutations.
  • Presence of an autonomous genetic system in the nucleus.
  • Disadvantages of Implementing Another Genetic System
  • Energy requirement.
  • Poor repair and precision in mitochondria.
  • DNA proximity to Electron Transport Chain (ETC), prone to damage by free radicals.
  • DNA in Plastids
  • Plastids:
  • Double-membrane organelles found in plant and algal cells.
  • Responsible for manufacturing and storing food.
  • Plastid DNA Inheritance:
  • Plants inherit plastids from only one parent.
  • Plastids from the other parent are typically lost.
  • Structure of Plastid DNA:
  • Circular, linear, with multiple copies.
  • Plastid DNA Codes for:
  • rRNA, tRNA, ribosomal proteins, RNA polymerase subunits.

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