Structure and Replication of Genomes

Introduction to Microbial Genetics

Microbial genetics is the study of inheritance and inheritable traits as expressed in an organism’s genetic material. Understanding microbial genetics is fundamental to microbiology, as it explains how traits are passed on and how genetic information directs cellular functions.

• Genetics: The scientific study of heredity and variation in organisms.

• Genome: The entire genetic complement of an organism, including all its genes and nucleotide sequences.

• Genotype: The complete collection of genes in an organism.

• Phenotype: The physical and functional traits of an organism, resulting from gene expression (e.g., hair and eye color in humans).

• Gene: A molecular segment of a chromosome that constitutes a unit of heredity and directs cellular functions.

Structure of Nucleic Acids

Nucleic acids, such as DNA and RNA, are polymers of nucleotides and serve as the genetic material in all living organisms.

• Each nucleotide is composed of:

  • Phosphate group

  • Pentose sugar (deoxyribose in DNA, ribose in RNA)

  • Nitrogenous base (adenine, thymine, cytosine, guanine in DNA; uracil replaces thymine in RNA)

• The length of DNA is expressed in base pairs (bp).

• DNA is typically double-stranded and forms a double helix, stabilized by hydrogen bonds between complementary bases (A-T and G-C).

• RNA is usually single-stranded and contains uracil instead of thymine.

Example: In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

Structure of Prokaryotic Genomes

Prokaryotic genomes are organized differently from eukaryotic genomes, reflecting their simpler cellular structure.

• Prokaryotic chromosomes:

  • Main portion of DNA, associated with proteins and RNA.

  • Prokaryotic cells are haploid (contain a single chromosome copy).

  • Chromosome is typically a circular DNA molecule located in the nucleoid region.

• Plasmids: Small, circular DNA molecules that replicate independently of the chromosome.

  • Not essential for normal metabolism, growth, or reproduction.

  • Can confer survival advantages, such as antibiotic resistance.

  • Types of plasmids:

    • Fertility plasmids (F plasmids): Carry genes for conjugation (transfer of DNA between bacteria).

    • Resistance plasmids (R plasmids): Carry genes for antibiotic resistance.

    • Bacteriocin plasmids: Carry genes for toxins that kill other bacteria.

    • Virulence plasmids: Carry genes that enhance pathogenicity.

Structure of Eukaryotic Genomes

Eukaryotic genomes are more complex and compartmentalized than prokaryotic genomes.

• Nuclear chromosomes:

  • Typically more than one chromosome per cell.

  • Chromosomes are linear and sequestered within the nucleus.

  • Eukaryotic cells are often diploid (two chromosome copies per cell).

• Extranuclear chromosomes:

  • Found in mitochondria and chloroplasts.

  • Resemble prokaryotic chromosomes (circular DNA).

  • Code for a small fraction of the cell’s genes (about 5%).

  • Some fungi, algae, and protozoa also carry plasmids.

Example: Human cells have 46 linear chromosomes in the nucleus and circular mitochondrial DNA.