Chapter 16(3) Flashcards

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1
Q

What are transposable elements?

A

DNA sequences that can move within the genome by an enzyme-driven process called transposition

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2
Q

What is an insertional inactivation?

A

When a transposable element causes a mutation if it inserts into a wild-type allele and disrupts its function

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3
Q

How was transposition discovered?

A

Barbara McClintock discovered
transposition using crosses
involving three linked genes, C,
Sh and Wx, in maize

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4
Q

What was located at the site of chromosome breakage?

A

dissociation (Ds) element

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5
Q

What element do Ds need to cause chromosome breakage?

A

Activator (Ac) element

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6
Q

What were McClintock’s 2 observations?

A
  1. McClintock observed that the
    break points of chromosome 9
    all occurred in the same position of the affected chromosome
  2. McClintock’s second observation
    was colorless kernels with varied
    patterns of purple spotting
    * She concluded that the unstable
    mutant alleles were caused by insertion of Ds into the C locus to produce a kernel lacking pigmentation
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7
Q

What did random transposition of Ds out of the gene cause?

A

caused reversion back to wild-type and the array of purple spots on the kernels

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8
Q

What are autonomous transposable elements? Example?

A

carry a transposase gene and all DNA sequences needed to carry out transposition (e.g., Ac)

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9
Q

What are non-autonomous transposable elements? Example?

A

have no transposase gene and may lack the sequences needed for transposition (e.g, Ds)

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10
Q

What two categories of transposable elements do bacterial genomes contain?

A

-IS(insertion sequence) elements
-transposons

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11
Q

What are IS elements?

A

(insertion sequence) elements are simple transposable elements containing only the genes and sequences needed for autonomous transposition

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12
Q

What is the IR sequence?

A

-They contain a transposase gene that is bracketed by a short, inverted repeat (IR) sequence
* Different IS elements have different IR sequences
* The inverted repeats are integral to transposase- mediated transposition

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13
Q

What are the role of transposons?

A

in bacteria, they carry multiple genes and confer new traits on bacteria that contain them

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14
Q

What two types are Transposons(Tn) composed of?

A

composite and simple

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15
Q

What are composite transposons?

A

-have a central region of several kb containing one or more functional genes
-The central region is flanked by complete IS elements in opposite orientation; at least one of these (sometimes both) contains a copy of the transposase gene
* The transposon Tn10 has a typical composite transposon structure

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16
Q

What are simple transposons?

A

-are flanked by very short IR
sequences of less than 50 bp
-The IR sequences do not encode transposase; this enzyme is encoded by the simple transposon itself
-There are additional genes in the central element

17
Q

What two groups are Eukaryotic transposable elements divided into?

A
  1. DNA transposons
  2. Retrotransposons
18
Q

How are DNA transposons transposed?

A

through conservative or replicative transposition

19
Q

How are retrotransposons transposed?

A

transcribed, then reverse
transcriptase produces a double-stranded DNA copy of the element, which is then inserted into the
genome (copy and paste transposition)

20
Q

What is a P element/what genome carries it?

A

-The Drosophila melanogaster genome carries several dozen copies of a transposable element
called a P element
-However, none of the Drosophila melanogaster captured before 1960s have P elements
-Today, all wild-caught D. melanogaster contain P elements

21
Q

What are retroviruses?

A

-Retroviruses infect eukaryotic cells, and have genomes composed of single-stranded RNA
- On infection the RNA is transcribed into double-
stranded DNA by reverse transcriptase, allowing the DNA to integrate into the host’s genome
-The viral genes gag and env, encoded by the integrated virus, are needed to produce new
retroviral particles; pol encodes reverse transcriptase

22
Q

Are retrotransposons related to retroviruses?

A

-Retrotransposons are related to retroviruses; all carry pol and some contain gag
-None encode env, thus they can be reverse transcribed and inserted into host DNA, but are unable to produce viral particles

23
Q

What are the gene(s) carried on retrotransposons flanked by?

A

Long terminal repeats(LTRs)

24
Q

What percent of the human genome is composed of repetitive DNA sequences derived from former transposable elements that can no longer move within the genome?

A

45%

25
Q

What two elements are relatively abundant, and are among the remaining functional transposable elements in humans?

A

Long interspersed elements
(LINEs) and short interspersed
elements (SINEs)

26
Q

What are L1 Elements?

A

-L1 elements are particularly common LINEs (about 600,000 per genome) that vary in length from 6.5 to 8.0 kb
-Full-length L1 elements encode a protein with nuclease and reverse transcriptase function, and may also encode an RNA-binding protein
-L1 elements are associated with spontaneous human mutations

27
Q

What are Alu Elements?

A

-Alu elements are the most common of the SINEs
-They vary in length from 100 to 300 bp and are flanked by 7-20 bp direct repeats
-the human genome contains about 1.2 million of these elements; these are relatively recent additions
to the genome and are found in close primate relatives but no other mammals