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Genetics > Lecture 11 > Flashcards

Lecture 11 Flashcards

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Biology 101 flashcards
1
Q

What are the 2 life cycles bacteriophage lambda

A

Lytic cycle and lysogenic cycle

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2
Q

What is the first step of temperate bacteriophage transduction

A

Linear chromosome circularized in bacteria by annealing complementary ends and ligation

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3
Q

Lytic pathway

A
  • Phage injects DNA
  • Phage DNA circularises
  • New phage DNA and proteins assembled into new phages
  • Cell lyses, releasing the phages
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4
Q

Lytic cycle fate

A

lambda DNA replicated by rolling stone mechanism to form concatamer of linear DNA

Concatamer cut at cos sequence by ter enzyme

Produces lambda genome sized molecules - packaged into phage particles

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5
Q

Lysogenic pathway

A

Daughter cell with prophage has phage DNA circularised

Phage DNA integrates into bacterial chromosome becoming a prophage

Bacterium reproduces to copy prophage - transmits it to daughter cells - cell division produces populations of bacteria with prophage

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6
Q

Lysogenic pathway

A
  • Site specific recombination with lambda DNA integrating at specific site in bacterial genome
  • Recombination between attB and attP site
  • attP and attB have spacer regions (O) - flanked by different integrase binding sites (B, B’, C, C’)
  • Recombination at O
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7
Q

How is integrated prophage usually excised from chromosome?

A

Recombination between core O sequences of attL (B O C’) and attR (C O B’)

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8
Q
A

Prophage excision from chromosome can occur abnormally.

Result: Some prophage DNA stays in bacterial chromosome.

Excised bacteriophage is defective, lacking some prophage genome.

Excised bacteriophage carries bacterial DNA from the region around integration site

  • Results in mixed phage lysate - contains wt lambda phage and lambda d gal+ phage
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9
Q

Lambda d gal+ phage mediate what?

A

Specialised transduction

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10
Q

Defective lambda d gal+ phage

A
  • Defective phage:
    can’t integrate at att sites
    can’t replicate and enter lytic cycle
    integrates by recombination with homologous chromosomal DNA
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11
Q

Specialised transducing phage

A

Transfer gal+ into recipient bacteria

Recombined into recipient chromosome - gal- to gal+

Specialised transduction limited to transducing genes close to att site

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12
Q

Transformation - uptake of ‘naked’ DNA

A

bacterial host with Donor DNA (a+, b+, c+)

DNA isolated

Recipient bacterium (a-, b-, c-) transformed by recombination

Transformants can be:
a-, b-, c- - No transformation from donor DNA

a+, b-, c- - a+ transformation

a+, b+, c-

a-, b+, c-

a-, b+, c+

a-, b-, c+

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13
Q

Why never a+, b+, c+, or a+, b-, c+?

A

Distance between a and c too great

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14
Q

What are transposons?

A

Pieces of DNA that move around the genome (or between chromosomal DNA and extrachromosomal elements) and insert at target sites by transposition

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15
Q

What organisms are transposons found?

A

All and form large part of genome

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16
Q

What can transposon insertion affect?

A

Genes, gene regulation, chromatin structure, genome stability, evolution

17
Q

How can transposons move?

A

Excision and integration

Replication

18
Q

3 main types of transposon

A

Insertion sequences - Bacterial DNA-only transposon - code for enzyme needed for transposition, flanked by short inverted terminal repeats

Composite transposons

Non-composite transposons

19
Q

Composite transposons

A
  • Pair of IS elements flank another gene
  • Same (Tn9) or inverted orientation (Tn5 or Tn10)
  • Often carry genes for drug resistance
  • IS10 transpose alone
  • Can be flanked by pair of IS elements but still transpose
20
Q

Non-composite transposons

A

TnA family

Large, ~5kb transposon, not dependent on IS-type elements

Independent units, genes for transposition and drug resistance

Tn3, Tn1000

Terminal inverted repeats, generate 5bp direct repeat at target site

21
Q

Transposition mechanism

A

DNA-only transposons move by cut-and-paste mechanism - element excises and inserts into target, using small amount of replication to join sites

  • Some in bacteria move by nick-and-paste - attached to donor DNA and joined to target, forms cointegrate which resolves to 2 molecules each containing a transposon
  • Few transposons can move by both
22
Q

Where does DNA cut-and-paste transposition occur?

A

Transpososome structures

23
Q

Describe transposition at molecular level

A
  1. terminal inverted repeat DNA recognised by transposase
  2. transposases oligomerize - transposon ends together
  • Transposon cleavage activated
  • Transposase/cleaved transposon complex bind target DNA - 3’ end attacks target DNA, join at staggered positions
  • ssDNA filled by host repair
24
Q

How are Hfr strains generated

A

recombination between transposons on F-plasmid and bacterial chromosome

Genetics (18 decks)

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