Molecular Evolution Study Notes

Molecular Evolution

Overview

  • Molecular evolution refers to the processes that lead to genetic changes in organisms over time.

  • Includes mechanisms such as mutations, horizontal gene transfer, and recombination.

Mutations

Definition of Mutation

  • A mutation is a change in genetic material.

Classification of Mutations

  1. Point Mutations
       - Definition: Base pair substitutions in the DNA sequence.
       - Types of Point Mutations:
         - Silent Mutation: No change in amino acid sequence.
           - Example: GGA (glycine) remains as glycine despite mutation.
         - Missense Mutation: Results in a different amino acid sequence.
           - Example: ATA becomes isoleucine (ile) instead of methionine (met).
         - Nonsense Mutation: Creates a premature stop codon, leading to truncated proteins.
           - Example: TAA leads to a stop codon instead of continuing the sequence.
           

  2. Frameshift Mutations
       - Definition: Insertion or deletion of one or more nucleotide pairs, altering the reading frame of the gene.
       

Effects of Mutations

  • Mutations can be classified as:
       - Neutral (silent),
       - Beneficial,
       - Harmful.

  • Spontaneous Mutation Rate: Approximately 10−610^{-6} mutations per million replicated genes.

Mutagens

  • Definition: Agents that increase the mutation rate by 10 to 1000 times.

  • Chemical Mutagens:
       - Nucleoside (base) analogs:
         - Incorporated into growing cells during DNA replication.
         - Can cause mutations if base-pairing properties are altered.
         - Examples include AZT used in HIV treatment.
       - Frameshift Mutagens:
         - Intercalating agents (e.g., aflatoxin, ethidium bromide) cause distortions leading to insertions or deletions during DNA replication, posing cancer risks.
     

DNA Repair Mechanisms

Methods

  1. Photolyases
       - Repair thymine dimers caused by UV radiation using energy from visible light.

  2. Nucleotide Excision Repair
       - Repairs various types of mutations, including bulky DNA adducts.

Genetic Transfer and Recombination

Types of Gene Transfer

  1. Vertical Gene Transfer:
       - Transfer occurring during reproduction between generations of cells.

  2. Horizontal (Lateral) Gene Transfer:
       - Transfer of genes between cells of the same generation, enabling genetic recombination.
         - Involves three mechanisms:
           - Transformation
           - Conjugation
           - Transduction

Mechanisms of Horizontal Gene Transfer

1. Transformation

  • Definition: Uptake of “naked” DNA by a recipient cell that must be competent to absorb it.

  • Natural competence is limited to a few genera (both Gram-positive and Gram-negative).

  • Laboratory methods can induce competence in E. coli, making it widely used in genetic engineering.

  • Historical Experiment: Griffith’s Experiment (1928)
       - Demonstrated transformation:
         1. Injected living encapsulated bacteria into a mouse: mouse died.
         2. Injected living non-encapsulated bacteria into a mouse: mouse remained healthy.
         3. Injected heat-killed encapsulated bacteria: mouse remained healthy.
         4. Mixture of living non-encapsulated and heat-killed encapsulated bacteria: mouse died, and encapsulated colonies were isolated from the dead mouse.

2. Conjugation

  • Definition: Transfer of plasmids and chromosomal DNA via direct cell-to-cell contact.

  • F+ Cells:
       - Donor cells containing the F plasmid and producing a conjugation (F) pilus (sex pilus).

  • F- Cells:
       - Recipient cells that become F+ after receiving the F factor.

  • Some F factors can integrate into the chromosome, creating Hfr (high frequency of recombination) cells.

  • R plasmids (resistance factors) can also be transferred through conjugation.

3. Transduction

  • Definition: DNA transfer from a donor to recipient cell facilitated by bacteriophages (transducing phages).

  • Types of Transduction:
       1. Generalized Transduction:
          - Occurs via the lytic cycle caused by virulent phages.
       2. Specialized Transduction:
          - Involves specific bacterial genes being transferred through the lysogenic cycle.
          
       - Process of Specialized Transduction:
         1. A phage infects a donor bacterial cell, where the bacterial chromosome is degraded and phage DNA is replicated.
         2. During viral assembly, pieces of bacterial DNA might be packaged into a phage capsid.
         3. When the phage infects a new recipient cell, it can incorporate the bacterial DNA into its chromosome, resulting in a genetically distinct recombinant cell.

Conclusion

  • Understanding these molecular processes is key for fields like genetics, biotechnology, and microbiology, as they shape evolution and diversity in microorganisms.