② Genome maps can be constructed through transformation: the closer the genes are located, the more likely they are to undergo simultaneous transformation.

⑶ Transduction: The process by which a bacteriophage transfers genes from one host bacterium to another.

① Typically, lytic phages are used, but lysogenic phages can be used in specific cases.

② 1^st. A phage infects a bacterium, which then becomes the donor cell.

③ 2^nd. The phage DNA replicates, and the cell synthesizes large amounts of proteins encoded by the phage genes.

○ During this time, phage proteins prevent the host cell from synthesizing proteins encoded by its own DNA.

○ The host cell’s DNA is fragmented.

④ 3^rd. When new phage particles are assembled, fragments of bacterial DNA containing specific host genes may sometimes be packaged into the phage capsid.

⑤ 4^th. When a phage carrying this bacterial DNA infects a recipient bacterium, recombination occurs between the donor and recipient cells. This process is homologous recombination, similar to crossing-over.

⑥ 5^th. As a result, the genotype of the recombinant cell may differ from both the donor and the recipient cells.

⑦ Genome maps can be constructed through transduction.

⑷ Conjugation: The phenomenon in which genetic material is directly transferred between two temporarily adjacent bacteria.

① Plasmid: A small circular DNA molecule used by bacteria to share antibiotic resistance information.

② F plasmid

○ F Plasmid: Composed of 25 genes, most of which are required for sex pilus formation (e.g., tra genes).

○ A cell possessing the F plasmid is referred to as a donor cell (F⁺), while a cell without it is referred to as a recipient cell (F^-).

○ F⁺ cells transfer the F plasmid to F^- cells, converting them into F⁺ cells.

③ General donor strain conjugation: Genetic material is transferred from a donor cell to a recipient cell via a sex pilus.

○ 1^st. The F⁺ cell forms a conjugation bridge with the F^- cell, and one strand of the plasmid is cleaved.

○ Condition for recipient cell: Must be rec⁺.

○ 2^nd. Using the unbroken strand as a template, the donor cell synthesizes a new complementary strand.

○ 3^rd. In the meantime, the cleaved strand detaches and one end enters the F^- cell.

○ 4^th. In the recipient cell, the plasmid strand is connected to itself to form a complete circular plasmid.

○ 5^th. The recipient cell becomes an F⁺ cell.

○ The conjugation bridge usually breaks before the entire chromosome and remaining F factor are fully transferred.

○ It is when the recipient cell retains some of the donor cell’s DNA.

○ Since the F plasmid is transferred last, it is rare for an F^- cell to become an F⁺ cell.

○ Drawback: The frequency of this process is extremely low.

Figure 2. Conjugation between bacteria

Figure 3. General donor strain conjugation

③ High frequency recombinant strain conjugation

○ High-frequency recombination (Hfr) donor cell: A bacterial strain in which the F factor is integrated into the chromosomal DNA (Bac).

○ The F plasmid is randomly inserted into the genome.

○ Resolves the low-frequency issue of F⁺-F^- conjugation.

○ Transfers both the F plasmid and parts of the chromosome, leading to bacterial DNA recombination.

○ The F factor is transferred last, and the unwinding direction (clockwise or counterclockwise) varies depending on its relative position.

④ Interrupted mating technique: Applied in constructing genome maps.

Figure 4. Interrupted mating technique process

Figure 5. Interrupted mating technique result

○ 1^st. Hfr strain is str^s leu⁺ thr⁺ azi^r ton^r lac⁺ gal⁺.

○ Condition: Antibiotic sensitivity, prototrophy (non-auxotrophy)

○ 2^nd. F-strain is str^r leu^- thr^- azi^s ton^s lac^- gal^-.

○ Condition: Antibiotic resistance, auxotrophy

○ 3^rd. Cultured in medium with streptomycin but without threonine and leucine: Only the strains with str^r leu⁺ thr⁺ survive.

○ 4^th. azi^r survives additionally 8 minutes after conjugation: azi^r is next to str^r leu⁺ thr⁺.

○ 5^th. ton^s survives additionally 10 minutes after conjugation: ton^s is next to str^r leu⁺ thr⁺ azi^r.

○ 6^th. lac⁺ survives additionally 16 minutes after conjugation: lac⁺ is next to str^r leu⁺ thr⁺ azi^r ton^s.

○ 7^th. gal⁺ survives additionally 25 minutes after conjugation: gal⁺ is next to str^r leu⁺ thr⁺ azi^r ton^s lac⁺.

3. Mobile DNA

⑴ Mobile DNA can induce mutations.

⑵ Type 1. Transposon

① Definition: “Cut and paste” mechanism.

○ The most studied transposons include Tn5, Tn9, and Tn10.

② Component 1. Transposable element

○ The actual DNA segment responsible for mobility.

③ Component 2. Insertion sequence (IS)

○ Less than 2,500 base pairs.

○ Features

○ Insertion sequences are located at both ends of the transposon.

○ An antibiotic resistance gene may be located in the central region.

○ A pair of insertion sequences is cut simultaneously.

○ 2-1. Transposase gene

○ Catalyzes the insertion of the gene into another location.

○ 2-2. Inverted repeat sequence (IR)

○ Sequences located at both ends of the insertion sequence.

○ Identical to each other but oriented in opposite directions.

○ Example

5’-ATCCGGT- ··· - [transposase gene] - ··· -ACCGGAT-3’

3’-TAGGCCA- ··· - [transposase gene] - ··· -TGGCCTA-5’

○ The example above is an insertion sequence, with 5’-ATCCGGT-3’ and 5’-ACCGGAT-3’ being the inverted repeat sequences.

○ 20 ~ 40 nucleotides

Figure 6. The location of insertion sequences and inverted repeat sequences on a transposon

④ AC-DS system: McClintock’s research

Figure 7. McClintock’s research

○ Arrow: Indicates the potential for transposition.

○ Ac (Activator): Encodes transposase, enabling it to transpose on its own.

○ Ds (Dissociator): A factor that can be transposed by Ac but does not encode transposase gene.

○ Ds transposes under the influence of Ac, overlaying the genetic code of gene C in various locations.

○ Ac transposes independently, overlaying the genetic code of gene C in various locations.

○ The size of the spots on speckled corn kernels is related to when transposition happened during development.

⑶ Type 2. Retrotransposon

① Definition: “Copy and paste” mechanism.

② mRNA → cDNA → inserted into another DNA location.

⑷ Type 3. LINE (Long Interspersed Element) (e.g., L₁)

⑸ Type 4. SINE (Short Interspersed Element) (e.g., Alu)

4. Plasmodesmata

⑴ Plasmodesmata: Cell junctions between plant cells that allow the exchange of signals and substances.

⑵ The cell membranes of adjacent cells are connected.

⑶ Plant viruses move to adjacent cells through plasmodesmata.

Input: 2015.7.12 00:08

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Chapter 5-1. Horizontal Transfer of Genes

1. Virus

2. Bacterial recombination

3. Mobile DNA

4. Plasmodesmata

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