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Lateral Transfer. Donating Genes Mutation often disrupts the function of a gene Gene transfer is a way to give new functions to the recipient cell Thus,

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Presentation on theme: "Lateral Transfer. Donating Genes Mutation often disrupts the function of a gene Gene transfer is a way to give new functions to the recipient cell Thus,"— Presentation transcript:

1 Lateral Transfer

2 Donating Genes Mutation often disrupts the function of a gene Gene transfer is a way to give new functions to the recipient cell Thus, gene transfer is a quicker way for a population to generate versatility and diversity

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4 Gene Transfer Can involve the transfer of an entire plasmid Can also be parts of chromosomes Recombination is needed to place the parts into a stable chromosome Recombination is the exchange of genes (or parts of genes) between two DNA molecules (or parts of the same molecule)

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6 Lateral Transfer Antibiotic resistance Virulence factors Metabolic enzymes, i.e. Pseudomonas species have plasmids for degradation of petroleum hydrocarbons Other non-essential (but very useful) functions The DNA that is swapped is often a plasmid

7 Plasmids that have made the rounds R100 – genes for resistance to sulfonamides, streptomycin, tetracycline, chloramphenicol, mercury found in Escherichia, Klebsiella, Salmonella (enterics) Penicillinase-producing plasmid in Neisseria may be from Streptococcus E.coli O157:H7 has a Shigella plasmid

8 (mercury resistance) (sulfonamide res.) (streptomycin res.) (chloramphenicol res.) (tetracycline res.) See also fig. 8.28b

9 Transformation Naked DNA is transferred from one cell to another In nature, it often follows the death and lysis of one cell Transformation must occur before the DNA is completely degraded

10 Transformation A whole plasmid or chromosomal fragments can be transferred The fragments must be integrated by recombination Stable transformation  the new DNA is inherited by the progeny

11 Fig. 8.24

12 Competence The DNA must pass through the cell wall and cell membrane Not all bacteria are naturally competent – some Neisseria, Streptococcus, Staphylococcus are They can be made so in the lab with Ca ++ or with electric shock

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14 Some History 1928 – Frederick Griffith works with S. pneumoniae One strain has a capsule and is virulent Another strain has no capsule and does not cause disease The virulence of the first strain can be eliminated by heat-killing the cells

15 Fig. 8.23

16 Griffith’s Experiments Injecting heat-killed encapsulated strain along with the avirulent strain causes disease He even isolated live, encapsulated S. pneumoniae from the diseased mice The avirulent strain was transformed by some genetic material from the virulent strain (Griffith did not know that is was DNA)

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18 Conjugation Requires that cells make direct contact The two cells are often of opposite mating type Can be interspecies Gram-negative produces sex pili Gram-positives use other surface molecules for attachment

19 Fig. 8.25

20 Conjugation A plasmid is transferred from an F + cell to an F - cell One stand of the plasmid is transferred Replication occurs in each cell

21 F+ X See also fig. 8.26a

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23 Fig. 8.26b & c

24 Agrobacterium tumefaciens Transfers genes to plants! Lateral transfer is not limited to prokaryotes Can eukaryotes transfer their DNA? Unknown, except by transformation

25 Crown gall tumor

26 S.B. Gelvin (2005) Nature 433:

27 Recent Studies “Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota.” Nature. 4/8/10. “The shared antibiotic resistome of soil bacteria and human pathogens.” Science. 8/31/12.

28 Transduction The DNA is carried by an intermediary The intermediary is a bacteriophage (virus) Normally, the phage carries its own DNA Occasionally, it randomly picks up some bacterial DNA during infection

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31 Transduction Phage injects its DNA into host cell Cell replicates phage DNA and translates phage proteins Cellular DNA is fragmented Some phage incorporated pieces of the bacterial DNA In some cases, the next round of infection will involve transfer of bacterial DNA

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33 Fig. 8.27

34 Transduction Often mediated by a prophage during the lysogenic phase of infection During lysogeny, the phage DNA is recombined into the host chromosome for any # of generations (the phage DNA is the prophage)

35 Transduction

36 Phage Conversion Some pathogens rely on prophage DNA for their virulence Corynebacterium diptheriae Clostridium botulinum Vibrio cholerae Streptococcus pyogenes

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38 Natural Selection One of the keys to natural selection is the existence of a diverse or versatile population Lateral transfer of genes is a potent way to generate versatility (more so than mutation) Lateral transfer takes full advantage of already occurring diversity Remember, this transfer often occurs across species

39 Transposons – Mobile DNA Originally termed “jumping genes” by Barbara McClintock in the 1950s Transposons are DNA sequences They can move from one place to another – on the same chromosome, from chromosome to chromosome, between plasmid and chromosome, into prophage DNA, etc.

40 Transposons Always contain insertion sequences and a gene encoding a transposase enzyme Transposase opens a piece of DNA and seals it around the transposon (which gets inserted into the break) Transposons can also contain additional genes, i.e. those for antibiotic-resistance The transposon may disrupt other genes when jumping

41 Frequency of to per generation Red = insertion sequences tnp = transposase gene Transposon kan = kanamycin-resistance gene str = streptomycin-resistance gene bleo = bleomycin-resistance gene

42 Transposition One copy can become mutated in future generations See also fig. 8.29


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