Microbial Genetics

 In bacteria genetic transfer (recombination) can happen three ways:  Transformation  Transduction  Conjugation  The result is a recombinant cell that has a genome different from either the donar or the recipient.

A segment of DNA is transferred from one bacterial cell to another by a bacterial virus called “bacteriophage” or phage.

Once the bacteriophage” attaches to the bacterial cell wall, it injects its nucleic acid into host cell.

A phage enzyme is produced that breaks down the host DNA into small fragments

Phage DNA is replicated and phage coat proteins are produced

During the formation of mature phage particles, a few phage heads surround fragments of bacterial DNA instead of phage DNA.

The phage particles carrying the bacterial DNA infects another bacterial cell, transferring the bacterial DNA to a new cell.

The bacterial DNA being injected to a new host cell

When the bacterial DNA introduced into a new host cell, it can become integrated into bacterial chromosome, thereby transferring genes to the recipient.

Microbial Genetics Genetic Variations and Mutations

 Roughly 99.9 % of human genomes (3.2 billion bases) are the same between any two people. Amazing!!!!!!!!!!!  The remaining tiny fraction of the genome, 0.1 % (several million bases)-makes a person unique. This small amount of variation determines how a person looks, or the diseases he or she develops.

 Heritable variation within and between populations of organisms

 Where does genetic variation come from?  Mutations (ultimate sources of all genetic variations)  Recombination of chromosomes that occurs during sexual reproduction

 Permanent change in the DNA sequence  Outcome depends on:  What gene(s) is (are) affected  where in the gene the change occurs, (i.e., in the coding or non-coding region)  the exact nature of the change.

Most mutations have no known effect at all because they occur in non-coding regions of the DNA In addition, there are some mutations that do occur in coding regions of DNA, yet they have no known effect All these are silent mutations

 Some of the mutations that occur in the coding regions of genes have "harmless" effects.  They can, for example, change the way a person "looks." Some people have blue eyes, others brown; some are tall, others short; and some faces are oval, others round.

 There are a group of mutations in coding regions that result in harmful effects.  They cause disease because changes in the genome's instructions alter the functions of important proteins that are needed for health.  For example, diabetes, cancer, heart disease, and hemophilia all result from mutations that cause harmful effects.

 There are genetic mutations that have "latent" effects. These variations, found in coding regions, are not harmful on their own, However, such mutations cause some people to be at higher risk for some diseases such as cancer, but only after exposure to certain environmental agents. They may also explain why one person responds to a drug treatment while another does not.

 Finally, there are genetic mutations that have “Helpful" effects. These variations, usually induced by scientist either to study a particular gene or correct abnormal gene. This is called “gene therapy”.

 Spontaneous Mutations:  occur in the natural environment without the addition of mutagens (agents that cause mutations)  Occur randomly and spontaneously  Induced Mutations:  Mutations that are created by the addition of mutagens

 Two types:  DNA Mutations: affect one base pair in the DNA  Chromosomal Mutations: affect entire section of DNA on the chromosome

 Point mutation (single nucleotide polymorphism): a mutation that alters ONE base of DNA sequence  Types of point mutations:  Substitution  Deletion  Insertion

 Most common type of point mutation  Mistake during DNA replication, incorrect base incorporated into DNA

Point mutation: Substitution Silent mutation The substitution results in a codon that codes for the SAME amino acid as the original. Therefore the protein structure and function is not altered.

Point mutation: Substitution Missense mutation A base substitution results in a different codon. Therefore a different amino acid is coded for. This can alter the structure and function of the overall protein. like in Sickle Cell Anemia.

Point mutation: Substitution Nonsense mutation The substitution results in the original codon being converted into a STOP codon This does not “make sense” to the translation machinery so translation STOPS This results in a truncated (shortened) protein

 Insert or delete a nucleotide- very disastrous  Shifts codons of DNA when transcribed into RNA (also called frame shift mutation)  All nucleotides downstream of mutation will be grouped into improper codons, and wrong amino acids will be added  Protein will be non-functional

 Chromosomal mutations: permanent changes in the DNA that alter the chromosome itself Types Deletion Duplication Inversion Translocation

 The loss of a portion of a chromosome  Cri-du-chat syndrome  Infant cries like a cat  Respiratory problems, early death Deletion in chromosome 5

 A portion of a chromosome is duplicated (copied) more than once  Can be detrimental if it occurs within a gene region

 A segment of a chromosome is broken in two places, reversed, and ligated back together  Detrimental if it occurs in the middle of a gene

 A piece of a chromosome is broken off and joined to a DIFFERENT chromosome  They can cause problems during metaphase of meiosis I with homologous pairing  Can change the expression of genes  Some leukemias can result from translocations

 Mutations are induced by either certain chemical mutagens or physical mutagens  Sometimes scientists intentionally mutate DNA to study it  Mutagens are agents or substances can cause mutations.

 Example: nitrous acid (HNO2)  Converts adenine so it no longer pairs with thymine  Instead pairs with cytosine

 Compounds that resemble bases closely

Repair of thymine dimers