BIOL 240 - Topic 9 Review PDF

Topic 9.1 Bacteria as Subjects of Genetic Research -------------------------------------------------- ### Microbial Genetics - Microbial genetics grew from microbiology - 1940-1950s (hayday) - Led to molecular biology - Required dveleopment of model systems for genetic investiga...

Topic 9.1 Bacteria as Subjects of Genetic Research -------------------------------------------------- ### Microbial Genetics - Microbial genetics grew from microbiology - 1940-1950s (hayday) - Led to molecular biology - Required dveleopment of model systems for genetic investigations - *E.Coli* and *Salmonella typhimurium* - **Giants in Microbial Genetics** - Barbara McClintock - Ester Lederberg - Edward Tatum - 70% of genes we know what they do, 30% we don't know - **Used E.Coli K12 as the model bacteria to study bacterial genetics** - **Level 1 organism** - No toxins - No adhesion factors - No iron-transport systems - No capsule - No plasmid - Smaller genome - Rough colony type (partial LPS) #### Bacterial Genetics - Study of bacteria genetics has come a long way - Early researchers focused on microbes of practical importance (i.e. pathogens) - Researchers today also focus on a greater understanding of the genetic potential of microbes - **Trevor Charles (prof at UW): make bioplastic from genes in bacteria; plant interaction** #### Organization of Bacterial Genomes - Single chromosome and plasmids (don't need to have plasmids) - Termed the **replicon** (all the things that replicate along with the cells) - Plasmid copy number in the cell is closely regulated (replicate to a certain number of copies) - Bacteriophage DNA may also be present #### **Plasmids** A green and red circle with black text Description automatically generated with medium confidence - Typically **smaller** than **genomes** - Usually **do not encode housekeeping genes (Not going to find DNA polymerases, RNA polymerases, ribosomal amino acids)** - Antibiotic resistance genes common - Copy number governed by plasmid-encoded genes -- **number of copies of plasmid inside a bacterial cell** is controlled by **genes on the plasmid itself** - Plasmids can have genes that **regulate how often they replicate**. - Some plasmids are \"high copy\" (many copies per cell), while others are \"low copy\" (few copies per cell). - These genes ensure the plasmid maintains the right number of copies without overloading or under-supplying the cell. - Plasmids with **similar replication controls** = **incompatible (Inc)** - **If there is 2 plasmids in a cell that have the same copy number control mechanism, 1 will always outcompete the other** ### Mutations #### **Genetic Terminology** - **Wild Type (OG strain):** - Strain most likely found in nature - Original isolate - Source of deriving mutants - **Mutant:** strain carrying a mutation, relative to the wild type (gain or lose a function) - **Mutation**: change in a gene that disrupts/alters function - **Allele (variation of a gene)** - May gain or loss or change a function - **Auxotroph (can't make something that they need)** - Mutant that is **unable to make a particular compound** - Often a **mutation in amino acid biosynthesis** - **Prototroph (can make something that it needs)** - Strain **capable of making all required organic compounds** - **General gene designation rules** - Three letter abbreviation in italics, followed by a capital letter to separate genes in the same pathway (***lacY* vs. *lacZ***) - **Proteins:** same three letter designation but first letter capitalized with no italics (i.e. LacZ) - **Genotype (what is in the DNA that causes the mutation):** - Description of alleles within an organism - Generally reflects **differences** from wild type - **Example:** - *hisC:* gene involved in histidine synthesis - *hisC^-^:* carries a loss of function mutation in *hisC* gene, genetically wild type for every other gene - **Phenotype (what has been observed, certain characteristics):** - **Observable properties of a strain** - HisC^-^: strain unable to grow in absence of histidine **\ ** #### **Bacteria are Ideal genetic Research Candidates -- Observe Difference b/t Wild Type and Mutants** - One chromosome for easy detection of mutations - In early studies, nutritional mutants were used - Allowed **study of one gene based on its inability to use or produce a particular nutrient** - Below example: **Packaged deal: both nutrients are required for survival. If one is missing, the cell can't grow, regardless of whether it can make the other.** **Medium** **Met+ Pro+** **Met- Pro+** **Met+ Pro-** ---------------------------- --------------- --------------- --------------- Methionine + Proline Growth Growth Growth No Methionine + No Proline Growth No Growth No Growth Methionine no Proline Growth Growth No Growth No methionine proline Growth No Growth Growth **\ (growth depends on the -- gene)** #### **Studying Mutants** - Changes in genes are often visible by **changes in phenotype or growth patterns** - Exopolysaccharide mutant phenotype - Auxotrophic mutant phenotype - Carbon utilization mutant phenotype **\ ** #### **Method of Selecting Mutants** - **Selection (genotype)**: - Isolation of cells with particular genotype on basis of **growth under certain conditions** - Example: - Can select for His+ on the basis of growth (grow a medium without histidine, only cells that are His+ will survive bc they can make their own histidine) - **Cannot** directly select for **His-** as they **will not grow without histidine.** Harder to isolate through selection since they die in this environment - **In short:** only mutants with desired traits service under specific conditions - **Screening (phenotype)**: - Identification of cells with a phenotype (observable trait, colour, shape, **inability to grow)** - Colour, morphology, "no growth" - **Can identify His- by screening** - To find **His⁻ cells** (can't make histidine), you grow cells on a medium **with histidine** where both His⁺ and His⁻ can grow. - Then, you test for \"**no growth**\" on a medium **without histidine**. Cells that **fail to grow are His⁻ mutants**. - **In short:** visually or experimentally identify mutants (like His⁻) based on their **observable characteristics** (e.g., lack of growth). **Method** **Medium** **His+** **His-** --------------- -------------- ---------------------- ---------------------------------------------- **Selection** No Histidine Growth (**isolate**) No Growth **Screening** Histidine Growth Growth No Histidine Growth No Growth (change in behaviour, **isolate)** #### **Selection** - Selectable mutations generally grant a **growth advantage under specific conditions** - i.e. conditions that **kill wild type cells** - useful in genetic research - **non-selectable mutations** - confer **no advantage or confers a disadvantage** - detection requires **screening** of large number of colonies #### **Phenotype Selection** - use a growth medium that will inhibit microbes **lacking desired genes** - **antibiotic selection** is commonly used (**select mutants that do not respond to antibiotics)** #### Screening - **more tedious than selection** - selection: cell do all the work - can facilitate screening with **replica plating** - **identification of auxotrophs** #### **Phenotype Screening** - duplicate plates are created - one lacks a particular nutrient - a mutation has occurred where **a colony grows on full support plate but** **does not grow on partially support plate** **Single plate:** visible phenotype (red vs. not red) **Two plates:** phenotype not visible, need to compare growth on selective vs. complete media #### **Replicate Plating** - method of creating duplicate plates 1. Imprint master plate with colonies on velvet 2. Place different plates with different nutritional medium on velvet to stamp it 3. Incubate and compare growth on replica plates #### **Patching** - Transferring colonies to a gridded plate - Usually more accurate and reproducible than standard velvet replica plating - More systematic way of determining the mutant (see it more clearly) 1. Pick colonies from master plate with toothpick (in complete growth medium) 2. Place each picked up colonies into each grid, incubate (in both defined and complex growth medium) 3. Compare growth on test plate **Phenotype** **Selection vs. Screening** ------------------------ ----------------------------- Auxotroph Screening Temp Sensitive Screening Cold Sensitive Screening Drug Resistant Selection Rough Colony Screening Nonencapsulated Screening Nonmotile (can't move) Screening Pigmentless Screening Sugar fermentation Screening Virus resistant Selection - **Selection Needed:** Drug-resistant and Virus-resistant (mutants survive in inhibitory conditions). - **Screening Needed:** All others, because their traits (e.g., growth patterns, color, morphology) must be visually observed. Topic 9.2 -- Mutants, Mutations, and Strains -------------------------------------------- ### Mutations - DNA replication occurs without many errors -- but mistakes can occur - **Mutations are not always bad, can further evolution** - **Alter coding portion of gene,** resulting in an **aberrantly functioning protein** - proteins that are structurally flawed, mislocalized, expressed at the wrong time, or present in the wrong amounts #### **Types of Mutations** - **silent**: **no change in amino acid sequence** of protein, **third position of a codon** - **missense**: change in a codon that results in **coding for a different amino acid** in the eventual protein - **non**-**sense**: change that forms a stop codon where one should not be found - **frameshift:** result of **insertions or deletions of nucleotides,** changing how ribosome **reads** an mRNA molecule and can alter amino acid sequences of proteins **Here's the table summarizing the different types of mutations:** **Type of Mutation** **Change in Amino Acid Sequence?** **Codon Changes?** **Overall Results** ---------------------- ------------------------------------ ------------------------------------------------- ------------------------------------------------------------------------------------------------------------ Silent No Yes (usually in the third position of a codon). The amino acid sequence of the protein remains unchanged, so there is no effect on protein function. Missense Yes Yes Results in a different amino acid being incorporated into the protein, potentially altering its function. Nonsense Yes Yes (creates a stop codon). Causes premature termination of translation, leading to a shorter, often nonfunctional protein. Frameshift Yes Yes (due to insertion or deletion). Alters the entire amino acid sequence downstream of the mutation, often producing a nonfunctional protein. **Key Takeaways:** - **Silent mutations do not change the protein\'s function.** - **Missense mutations** may or may not affect function, **depending on the importance of the changed amino acid**. - **Nonsense mutations** are usually **severe, truncating the protein**. - **Frameshift mutations** have **widespread effects,** often **resulting in nonfunctional proteins.** #### **Reversion -- make sure mutant remains mutant throughout experiment** - **Reversion**: a mutation that **corrects** a metabolic abnormality back to the wild-type form - Problematic when trying to determine mutation rates of a chemical or DNA exchange rates between two microbes - To avoid this problem, **double and triple auxotroph mutant strains** were studied instead: - Decreases possibility of **spontaneous reversion mutation** - **(double down on the amount of mutations, make sure it won't revert back)** #### **Spontaneous Mutations** - TLDR: **mutations can happen randomly without any outside influence** - Experiments by **Esther Lederberg** **used replica plating to illustrate spontaneous mutation without selective pressure** - Cells that were **never exposed to streptomycin developed resistance,** showing that **mutations can arise in absence of selective agents.** **\ ** - **Luria and Delbruck** showed **variable resistance to phage infection arises** in bacteria **without selective pressure (**As you divide, error occurs) #### Evolution in a Test Tube - **Richard Lenski 1988:** *E.coli* Revolution of **75 days**, showed **increase in fitness** - Cultures given an extended generational time without selective pressures, compared to original cultures that had not had those conditions - Ability to growth in culture enhanced overtime - Evidence of both evolution and of ideal nature of microbes for genetic studies - Stress response gene hit by this - Can't use citrate - Red \>\> white (red is the evolved culture) ### Restriction Enzymes and Cloning #### Restriction Enzymes (REs) - **(are essentially defense mechanism of bacteria, cut DNA)** - **Cut DNA at specific recognition site (**specific sequences of DNA called recognition sites, **even number of bases**) - **Recognition sites** are usually **palindromic** (read same forward and backward, proteins cut in from both sides, in the same location) - Similar ends of cut DNA can be paired together and ligated - These ends can stick back together with other matching DNA pieces using an enzyme called DNA ligase, which glues them together **How many times will EcoRI cut a 5 megabase bacterial genome?** 1/ 4\^(n) \(n) is the number of bases 1/4096 6 bases, this is the chance of finding a recognition site Round to 1/5000 bases, how many out fo 1/5M bases? Cut 1000x any avg megabase genome **Zero times EcoRI cuts E.Coli genome,** because **EcoRI** defend itself against foreign genomes, would not attach its own genome - Methylate every single base so that it doesn't get cut by EcoRI #### Modification Enzymes - **Restriction** **enzymes** are always **paired** with **modification** **enzymes** - Often in a single operon (**produced together**) - Referred to as R/M systems - **Protects cell's own DNA with this system** - Recognize the same site as the paired striction enzyme - Methyltransferase (modification enzyme) activity protects DNA from endonuclease activity (adds a methyl group to specific DNA sequences in the cell's genome) **The restriction enzyme cuts foreign DNA (e.g., from viruses), but the modification enzyme ensures the cell's own DNA stays safe by marking it with methyl groups.** **Steps to Mix DNA using EcoRI:** - Digest with restriction endonuclease EcoRI - Mix digested DNAs and incubate - Treat annealed DNA fragments with DNA ligase - **Use this approach to cut and paste DNA into some destination** #### Cloning Vectors - REs allows researchers to stitch together fragments of useful DNA into recombinant molecules - Recombinant molecules can be used to clone a bacterial gene of interest - Vectors are used to insert a recombinant DNA molecule into a recipient host bacterial cell: - Plasmids - Phages - Cosmids #### Plasmid Cloning Vectors - First used in the 1970s by **Cohen** at Stanford - **Cut fragments from two plasmids carrying antibiotic resistance genes with the same RE** - **The transformed strain exhibited traits from both plasmids** - Basically: took two different plasmids, each carrying a gene for antibiotic resistance, both plasmids were cut using the **same restriction enzyme (RE),** created **matching ends on DNA fragment** - Can select for these AB-resistant genes and glue DNA back together using ligase, inserted into bacterial cell (**transformation)** - **Exhibit traits from both plasmids, resistant to both antibiotics** - **Only circular ligation products transform** #### Desirable Plasmid Traits for Easier Gene Cloning - Origin of replication (**oriV**), (which host can accommodate and replicate this plasmid) - Selectable marker gene (i.e. AmpR, TcR) (resistant); only stuff resistant can grow - Multiple cloning site (put restriction enzyme all in one place, always cut it in the same place) - Small size - High copy number (oriV) #### **Cloning** 1. Cut both vector and the genomic DNA with BamHI 2. Mix cleaved vector fragment and genomic DNA fragments and ligate a. AmpRTcR (doesn't add the genomic DNA) b. Linear (no growth, **only cells with circular plasmids grow)** c. AmpRTcS -- Tc sensitive now d. AmpRTcs with amylase gene -- success! 3. Transform ligation products into *E.Coli* and spread onto plate. 4. Then **select for ampicillin resistance (screening)** 4. imprint ampicillin-resistant colonies onto velvet 5. Screen for tetracycline sensitivity to identify recombinant clones 6. Incubate plates and compare 7. Patch tetracycline-sensitive colonies on agar with starch. Incubate and flood with iodine **Step** **What Happens** **Why It's Done** ---------- ----------------------------------------------------------------------- ---------------------------------------------------------------------------------------- **1** Grow bacteria on a plate with **ampicillin**. To select bacteria carrying the plasmid with **ampicillin resistance**. **2** Press a velvet pad onto the ampicillin plate to **pick up colonies**. To copy the bacteria onto another plate for testing. **3** Transfer colonies to a second plate with **tetracycline**. To test if bacteria are sensitive to tetracycline (indicates recombinant DNA). **4** Incubate both plates and compare growth. Colonies that grow on ampicillin but **not tetracycline** are **recombinants**. **5** Place recombinant colonies on a starch-containing plate. To test if the recombinant bacteria express the desired enzyme (e.g., α-amylase). **6** Incubate and flood the starch plate with iodine. To check for **clear zones**, which indicate enzyme activity (starch hydrolysis). **7** Look for clear zones around colonies. Colonies with clear zones produce the desired enzyme and are the correct recombinants. **Key Idea (2 screening processes):** - **Steps 1-4**: Identify bacteria with recombinant plasmids using antibiotic resistance. - **Steps 5-7**: Confirm those recombinants are producing the desired enzyme using starch hydrolysis. Cells with alpha-amylase activity create clear zones around their colonies when iodine is added, **indicating they can break down starch.** #### Screening for Transformed Cells - **X-gal system** = visual blue/white colony growth (with insert, no B-galactosidase activity! Tell tell sign) - Both come together (LacZ alpha and LacZ w from host combined forms functional B-galactosidase; forms blue colony, DO NOT WANT, plasmid closing in on itself); non-functional, white colony, has inserted DNA - **E-coli is not best host for cloning (different host better expression)** #### **Shuttle vector Plasmids** - Replication in certain hosts will be restricted by the origin of replication - **Shuttle-vector plasmids** have multiple types of origins of replication - **Shuttle vector plasmids** are special because they have **multiple origins of replication,** allowing them to work in **different types of host cells** (i.e. bacteria and yeast) **serve as a shuttle between different organisms** - this expands the range of host cell types the plasmids can be inserted into #### Phage Vectors - mix viral DNA with fragment of interest - lysogenic lambda phage can carry \~ 20kb fragments - "cos" is abbreviation of "cohesive end site" - Same number of bases replaced, will get packaged - **Each clearing zone has a recombinant gene of interest (isolated gene)** #### Cosmids - Phage genomes that omit nearly **all the phage DNA, leaving more room for the fragment** - only the **critical phage cos packaging recognition sites** remain - other elements include a **multiple cloning site** and an **antibiotic selection marker** - cosmids can typically carry 35-45 kb fragments - if add in 30-40 kb in the cos site, can be places in phage head and infect in that way (MOSTLY YOUR DNA) ### DNA Transfer #### Transformation - introduce extracellular DNA directly into an organism - doesn't require cell-to-cell contact - some bacteria are naturally competent for transformation - other bacteria can be artificially induced to become competent - treatment with calcium cations - electroporation #### **Naturally Competent Bacteria -- Transformation (review in video) -- take up naked DNA from environment** 1. DNA from a donor cell attaches to special **receptor proteins** on the surface of the recipient bacterium. (These proteins recognize and bind the donor DNA to start the transformation process.) 2. One strand of the donor DNA is pulled into the recipient cell through a channel (made of pore proteins). The other strand is broken down (degraded) outside the cell. The remaining single strand inside the cell is coated by single-stranded DNA-binding proteins and RecA protein to protect it. (This ensures only a single, stable strand of donor DNA enters the recipient.) 3. The incoming single-stranded donor DNA is combined with the recipient\'s chromosome through a process called recombination. The recipient's original DNA strand is replaced and degraded. (This integrates the new DNA into the recipient's genome, transforming the bacterium with new genetic information.) #### **Artificial Ways to Induce Bacteria to become Competent** - Treatment with calcium cations (low efficiency) - Electroporation (put electrical charge across) -- electroporator, **electric** **field** **transient** **pores**, cuvettes - **Both make membrane fluid, put the DNA in by heating or cooling (freeze)** **\ ** **Specialized and Generalized Transduction! From readings** - **Conjugation:** - **Process**: Transfer of plasmids from donor (male) to recipient (female) bacterial cells. - **Mechanism**: Mediated by tra genes, with some interacting with the oriT region to initiate DNA transfer, and others forming pili to aid in DNA transfer. - **Integration and Transfer**: Conjugative plasmids can integrate into the bacterial chromosome and transfer part of it. Donor cells have sex pili encoded by plasmid-encoded tra genes, bringing donor and recipient cells together. - **DNA Transfer**: One strand of the plasmid is nicked, unwound, and transferred to the recipient cell. The transferred strand is replaced in the donor by replication, and a complementary strand is synthesized in the recipient. - **High-Frequency Recombination (Hfr) Strains:** - **Integration**: Plasmids like F can integrate into the chromosome at insertion sequences (IS) via homologous recombination. - **Gene Transfer**: Hfr strains can transfer particular genes at characteristic frequencies, with genes nearest the oriT transferred at the highest frequency. - **Gene Mapping**: Interrupted mating experiments can demonstrate sequential gene transfer and map genes. - **Detection of Chromosome Conjugation:** - **Markers and Selection**: Use of antibiotic resistance and auxotrophic markers to detect transconjugant bacteria. - **Selection Conditions**: Conditions must favor the growth of desired transconjugants while inhibiting donor and recipient growth. - **F\' Plasmid:** - **Excision**: Integrated F plasmids can excise and carry part of the chromosome, resulting in F\' plasmids that carry chromosomal genes. - **Transposition:** - **Process**: Genes move from one genome part to another via transposable elements, including insertion sequences (IS) and transposons. - **Mechanism**: Transposase enzymes recognize terminal inverted repeat sequences, enabling transposition via site-specific recombination. - **Mutagenesis Tool**: Transposons can disrupt gene functions, useful for introducing antibiotic resistance and selecting colonies with desired phenotypes. - **Transduction:** - **Generalized Transduction**: Lytic bacteriophages can accidentally package host DNA, creating transducing particles that transfer DNA to recipient cells, enabling recombination. - **Specialized Transduction**: Lysogenic bacteriophages integrate into the host genome and can imperfectly excise, carrying specific host genes into transducing particles. All particles in this case are transducing particles, containing both virus and host genes. ### Conjugation - Transfer of DNA from cell to cell via **direct contact/sex pilus formation** - **Mechanism:** - F plasmid carries genes to form a sex pilus and bridge between two cells - F plasmid can be copied and sent across the bridge (**origin of transfer first**) into a recipient cell - Turns an F- cell into an F+ cell capable of conjugating with another F- cell **(Donor -\> Receptor)** - Note the *tra* genes... **(transfer from donor to recipient)** - (FYI, **F** stands for fertility) - oriV present for **replication** - oriT present for **original of transfer (mating bridge)** - **F factor -- the F+ strain, can mate with F- strain** 1. F+ donor cells using sex pilus as arms to grab the F- recipient cell; form **mating bridge, single strand nick made at oriT** 2. **One strand of F plasmid transferred to recipient;** replication occurs in both cells to result in **double-stranded DNA molecules** 3. Completion of F-plasmid transfer results in two F+ bacteria - **F plasmid can integrate into the host chromosome by homologous recombination** - F plasmid is an **episome (DNA that can integrate into the chromosome or exit autonomously)** - Creates new opportunities to **pass genetic information from one cell to another** - **Hfr (high frequency of recombination) strain DNA transfer** - Incorporated F plasmid sends the host cell DNA next to its incorporation site across the mating bridge overtime - Can be used to **map** locations of genes in the host chromosome Step 1 (left) Step 2 (right)\ - **In short: F plasmid embed itself into the chromosome,** passing over the plasmid gene with the E-Coli chromosome **high frequency of recombination** - **Then as the integration goes, map out the genetic marker based on time passed** #### Generation of F' Plasmids - An incorporated F plasmid excises itself (DNA can also be sent out) - Excision is inaccurate, and **some host DNA is excised** as well - When F' plasmid conjugates, sends **host cell DNA to recipient** - **can mate with other F- cells** #### Uses of Conjugation - **triparental conjugation** - conjugation can still occur using **recombinant plasmid with the genes** - **more room in the recombinant plasmid for a desired DNA fragment** 1. Preparation: The recombinant plasmid is constructed in the lab, containing the desired DNA fragment. 2. Donor-Intermediate Conjugation: Strain A (donor) transfers the helper plasmid to Strain C (intermediate). This helper plasmid has the genes required for conjugation, such as those encoding for the mating bridge. 3. Intermediate-Recipient Conjugation: Strain C (now containing both the helper plasmid and the recombinant plasmid) forms a conjugation bridge with Strain B (recipient). 4. DNA Transfer: The recombinant plasmid from Strain C is transferred to Strain B through the conjugation bridge. ### Transposition - Movement of DNA via **mobile genetic elements** - **Transposable elements can move within and between genomes** - First detected in **corn by Barbara McClintock** #### Transposition Can be Categorized as: - **Insertion sequence:** encode only the proteins needed for transposition - **Transposons**: contain other genes in addition to those needed for transposition #### Mechanisms of Transposition: - Requires **transposase and resolvase genes** - **Replicative transposition:** copies the element and moves the copy to another location (**copy paste**) (**need both transposase and resolvase to copy and paste**) - **Non-Replicative Transposition: cuts and pastes the elements into a new location (just need transposase gene)** Transposons: 1. Not site specific 2. For repeated DNA pieces - Transposition can be used to **disrupt functional genes and observe phenotypic changes** - **Suicide vector plasmid** carrying the **transposon** - Recipient cell gets the plasmid, **but plasmid can't replicate** - **Transposition can still occur at random** - **Screening and/or selection for desired disruption follows** ### Transduction - **Virus accidently packages a fragment** of host cell DNA (= **transducing particle)** - **virus delivers that fragment instead of viral DNA** to the **next cell** - virus is usually **unable to replicate** become **lack of viral genome** - **homologous recombination must occur** Right (general transduction: lytic) Specialized transduction: lysogenic - historically, co-transduction frequency was used to map bacterial genomes - genes closer to a **known marker gene** would be transduced with that marker more frequently than ones farther away - can also be used to modify bacteria - experimental modification of a common vaginal tract microbe to express and secrete a chemokine with anti-HIV activity - work has only been performed in vitro so far **Types of Transduction:** 1. **Generalized Transduction**: - **Mechanism**: During the lytic cycle, a bacteriophage accidentally packages fragments of the host cell\'s DNA instead of its own viral DNA. - **Outcome**: When this phage infects another bacterium, it injects the host DNA, which can then recombine with the recipient\'s genome. - **Purpose**: This facilitates the horizontal transfer of genes between bacteria, promoting genetic diversity. 2. **Specialized Transduction**: - **Mechanism**: During the lysogenic cycle, the bacteriophage integrates its DNA into the host genome. If the phage excises imperfectly, it can carry adjacent host genes with it. - **Outcome**: The phage with host genes infects a new bacterium, transferring specific genes along with viral DNA. - **Purpose:** Allows for the precise transfer of specific genes, contributing to genetic variation and adaptation.