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Lecture 4

BIOC 4403 Lecture Notes - Lecture 4: Selfish Dna, Group I Catalytic Intron, Mobile Genetic Elements


Department
Biochem & Molecular Biology
Course Code
BIOC 4403
Professor
Archibald John
Lecture
4

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6. Homing
the lateral (or horizontal) transfer of an
intervening sequence (e.g., an intron) to an
intron-lacking version of the gene.
Intron homing is a process whereby the
intron moves from an intron-containing
allele to an intronless allele in a
homologydependent gene conversion
event. Coconversion of flanking exon
sequences often accompanies intron
homing. <From: Mechanisms of Intron
Mobility>
7. Homing
endonucleases
-recognize and cleave 12 to 40 bp DNA
sequence motifs
(substitutions in recognition sequence can
be tolerated; less precise system than, e.g.,
type II restriction enzymes).
-some HEs also have a 'maturase' function,
whereby they help the intron fold into the
proper 3D shape required for splicing.
8. Ectopic
transposition
movement of intron to new genomic sites
9. How can
Ectopic
transposition
occur?
-reverse splicing into another RNA
(facilitated by fortuitous basepairing
between intron and RNA target (only 4-6 nt
needed)).
-reverse transcription (by an exogenous RT
enzyme).
-recombination of cDNA into genome by
recombination / repair enzymes.
1. Mobile genetic
elements
- Mobile elements are found in all domains of life, as well as viruses;
- Mobile elements are abundant and highly dynamic.
- 'parasitic' or 'selfish' genetic elements.
intervening sequences that are capable of
self-splicing and moving themselves within
and between genomes.
- Group I introns.
- Group II introns.
3. Group I introns mobile self-splicing ribozymes that
catalyze their own removal from
RNA.
4. Where are group
I introns found? - First discovered in 1982 in
ribosomal RNA of the ciliated
protozoan Tetrahymena (see L2).
- Group I introns exist in protein,
rRNA and tRNA genes in diverse
eukaryotic nuclear, mitochondrial,
and plastid genomes, as well as in
bacteria.
5. What do group I
introns look like
and how do they
work?
-200-500 nt in length with complex
secondary / tertiary structure.
-Some (but not all) group I introns
contain an ORF encoding a
'homing endonuclease' (HE) [see
following slide].
-Intron removed by two
transesterification reactions (first
initiated by OH group of
exogenous guanosine).
Lecture 4. Mobile Introns and Inteins
DNA elements that encode proteins that mediate
the movement of the element within a genome and
between genomes.
2. Mobile introns
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