Micro Genetics

Reactive Oxygen Species (ROS)

  • Types of Reactive Oxygen Species:

    • Super-oxides:

    • Strong oxidizers.

    • Formed when oxygen gains an extra electron.

    • Peroxides:

    • Oxidizing agents.

    • Considered less toxic than super-oxides.

    • Singlet Oxygen:

    • A high-energy form of oxygen.

    • More reactive than standard molecular oxygen.

Neutralization of Reactive Oxygen Species

  • Superoxides and Peroxides:

    • Neutralized through cellular processes like respiration and photosynthesis.

  • Singlet Oxygen:

    • Neutralized primarily through photosynthesis.

Excision Repair

  • Definition:

    • A cellular process that corrects point mutations that escape proofreading.

    • Involves:

    • Removal of a damaged segment of a DNA strand.

    • Replacement with a correct segment using the undamaged strand as a template.

Ultraviolet Radiation

  • Types of Ultraviolet Radiation and Their Effects:

    • UVC radiation:

    • Highest energy UV radiation.

    • Absorbed by DNA, lipids, and proteins.

    • Completely absorbed by the ozone layer; does not reach Earth's surface.

    • UVB radiation:

    • Medium-energy radiation.

    • Damages the outer layers of skin.

    • Main cause of sunburn.

    • UVA radiation:

    • Lowest energy form of UV radiation.

    • Penetrates deepest into the skin.

    • Linked to premature aging (e.g., wrinkles) and skin cell damage.

  • Mechanism of Damage:

    • All types generate reactive oxygen species (ROS) that modify DNA bases.

    • Directly break DNA strands, leading to mutations.

Photolyase

  • Definition:

    • An enzyme that repairs DNA damage caused by UV light.

  • Mechanism:

    • Uses energy from blue light.

    • Absorbs UVA and visible light to catalyze the breaking of bonds between adjacent thymine bases, effectively repairing DNA.

SOS Repair System

  • Description:

    • A bacterial response to severe DNA damage.

    • Functions as a "last resort" mechanism to save the cell.

    • Key actions include:

    • Halting cell division.

    • Stimulating the expression of DNA repair genes.

    • Associated with a higher error rate during repair.

Ionizing Radiation

  • Types of DNA Damage Induced by Ionizing Radiation:

    • Double-Strand Breaks:

    • Both strands of the DNA helix are severed; considered the most severe form of damage.

    • Single-Strand Breaks:

    • A break occurs in only one strand of the DNA.

    • Base Damage:

    • Chemical structures of DNA bases can undergo modifications or alterations.

    • Cross-Linkages:

    • DNA strands become incorrectly bonded together.

DNA Repair Mechanisms for Ionizing Radiation Damage

  • Base Excision Repair (BER):

    • Repairs damaged or modified bases:

    • An enzyme removes the damaged base, generating a single-strand break.

    • DNA polymerase fills the gap left behind.

    • DNA ligase seals the break, restoring DNA integrity.

  • Non-Homologous End Joining (NHEJ):

    • A repair mechanism that directly ligates the broken ends of DNA without using a homologous template.

  • Homologous Recombination (HR):

    • Utilized by high-fidelity cells to repair double-strand breaks by leveraging undamaged, identical sister chromosomes as a template for repair.

Plasmids

  • Definition:

    • Circular DNA molecules that carry non-essential genes.

    • Usually contain an origin of replication.

    • Often harbor genes that confer resistance to antibiotics or other compounds.

Horizontal Gene Transfer

  • General Concept:

    • Refers to the mobile gene pool.

    • Describes the direct transfer of genetic material between organisms that are not in a parent-offspring relationship.

Major Types of Horizontal Gene Transfer

  • Conjugation:

    • Involves direct DNA transfer from one bacterium to another via physical contact.

    • Process steps:

    1. Contact is established through a structure called the sex pilus.

    2. The sex pilus retracts, bringing the two cells into close proximity, allowing membrane fusion.

    3. The plasmid DNA is nicked, and one strand is transferred through the connection.

    4. Both donor and recipient cells use DNA polymerases to synthesize a complementary strand.

    5. The bacteria then separate, both becoming competent.

  • DNA Mediated Transformation:

    • Cells uptake naked DNA fragments from their environment and incorporate them into their genome.

    • Process steps:

    1. DNA from the environment, often from dead cells, binds to the cell membrane.

    2. DNA is taken up as double-stranded DNA (dsDNA) or is partially degraded to single-stranded DNA (ssDNA).

    3. The DNA is integrated into the host genome.

    4. Only one daughter cell receives the new genes if ssDNA is incorporated.

  • Transduction:

    • Involves the transfer of bacterial DNA to a new bacterium via a bacteriophage (virus that infects bacteria).

    • Process steps:

    1. A bacteriophage infects a bacterium.

    2. New viruses are synthesized, leading to the degradation of the bacterial chromosome.

    3. Segments of the bacterial genome can be accidentally packaged into one of the new viral protein coats.

    4. The phage containing bacterial DNA infects a new bacterium.

    5. The bacterial DNA is injected into the new bacterium.

    6. This foreign bacterial DNA can be incorporated into the new bacterium's genome through homologous recombination.

Mechanisms of Horizontal Gene Transfer

  • Transformation:

    • Direct uptake of fragments of free DNA from the surrounding environment by a cell.

  • Transduction:

    • Transfer of genes from one bacterium to another facilitated by a bacteriophage.

  • Conjugation:

    • Transfer of genetic material via direct contact between cells.