Prokaryotic Chromosome and Mitochondria .

Prokaryotic Chromosome

• Nucleoid: A complex of DNA and RNA molecules with nucleoproteins, but without a membrane.
• DNA: Circular and not attached to histone proteins. Typically one circular molecule, but a few molecules are possible.
• Plasmids: Small, circular molecules containing a small part of genetic information that contributes to sensitivity to various toxic substances like antibiotics. Some can be mobile, transferring from one bacterium to another, and can join/leave the nucleoid, playing a role in genetic recombination.
• Stretched DNA of E. coli is approximately 1400 \, \mu m long, while the cell is only about 1 to 5 \, \mu m in length.
• The nucleoid's structure is maintained by DNA-binding proteins.

Supercoiling

• Negative Supercoiling: Occurs in the opposite direction of the DNA double helix.
• DNA Topoisomerases: Control the level of negative supercoiling. DNA gyrase introduces supercoils, while DNA topoisomerase I prevents supercoiling from reaching unacceptably high levels.
• Factors influencing supercoiling include topoisomerase gene expression, the ratio of ATP to ADP concentration, and processes like transcription that unwind DNA.

Cell Envelope

• Contains domains for DNA origin and DNA-binding proteins.

Mitochondrial Chromosome

• Mitochondria are self-replicating organelles, independent of the cell cycle.
• They are believed to have originated from incorporated α-purple bacteria.
• Mitochondria transferred many essential genes to the nuclear chromosomes.

Mitochondria Structure

• Outer Membrane: Composed of equal amounts of phospholipids and proteins. Permeable to nutrient molecules, ions, ATP, and ADP molecules, facilitated by integral proteins called porins.
• Inner Membrane: Complex structure folded into cristae to increase surface area, contributing to chemical reactions like ATP production. Strictly permeable only to oxygen and regulates the transfer of metabolites.
• Intermembrane Space: Similar to the cytosol.
• Matrix: Complex mixture of proteins and enzymes important for the synthesis of ATP molecules, and contains mitochondrial ribosomes, tRNAs, and mitochondrial DNA (mtDNA).

mtDNA

• Circular, negatively supercoiled, similar to bacterial chromosomes, and located in the matrix.
• May contain 4-10 mtDNA copies.
• Three topoisomerases (Top1mt, Top3α, and Top2β) are found in mitochondria, all encoded in the nuclear genome.

Features and Functions of Mitochondria

• Self-replication.
• Cytoplasmic inheritance: transmission to daughter cells via cytoplasm.
• Aerobic ATP production: 13 times more efficient than anaerobic ATP production in the cytosol.
• Ion homeostasis and storage of calcium ions.
• Biogenesis of steroids: important roles in the biosynthesis of steroid sex hormones.
• Apoptosis: contain several pro-apoptotic molecules that activate cytosolic proteins to induce apoptosis.

Summary

• The nucleus is the controlling center of the cell, and its structure is strictly linked with its function.
• Changes in the structure influence the function and may lead to disease.
• Packing of nucleosomes leads to the formation of chromatin.
• The packing level of chromatin depends on function.
• Formation of sister chromatids results from DNA replication.
• Metaphase chromosome can be observed during cell division.
• Karyotype is species-specific.
• Normal human karyotype is 46,XX or 46,XY.
• Prokaryotic and mitochondrial chromosomes show structural similarity.