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Chapter 16

HMB265H1 Chapter Notes - Chapter 16: Exonuclease, Peptide, Transposase

Human Biology
Course Code
Maria Papaconstantinou

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HMB265 Lecture 16 READINGS - Transposable Elements
(Pg. 264-265; 317-325)
Unequal crossing-over and transposable elements
Transposable elements (TEs):
- DNA segments that can jump/transpose from place to place in genome
- If jumps into gene can disrupt gene function = mutation
- Certain TEs insert themselves into particular genes _ not others
- = mutation rates vary from gene to gene
Replicative transposition:
- TEs move after making a copy of themselves
- Copy gets inserted into different chromosomal location, initial version stays
Conservative transposition:
- TEs just leave their original position
- = here have exception to general rule that rate of reversion slower than rate of forward
9.3 Transposable Genetic Elements
Transposition: Type of sequence rearrangement w/ significant genomic impact
- Move small pieces of DNA (TEs) from one position to another
- Marcus Rhoades + B McClintock
- Findings contradicted classical recombination that genes located at fixed position
Transposable elements are classified according to how they move
- TE = any piece of DNA that evolves ability to move from place to place within genome
- Can have “selfish” TEs (don’t do anything for host, just carry information that allow self-
- Can have helpful TEs
- EX: TE maintain length of Drosophila chromosomes
- Most TEs length = 50 - 10 000 bp
- Each TE can be present in genome 1000s of times
- TEs consist of 12.5% of Drosophila genome
Two groups of TEs
- Classify based on how they move in genome
[1] Retrotransposons
- Transpose via reverse transcription of RNA intermediate
- Undergo replicative transposition
- = copy + paste transposition
[2] DNA transposon
- Move DNA directly w/o RNA intermediate
- Undergo conservative transposition
- = cut + paste transposition
Focus On GENETICS: Transposable Elements in Corn
- Barbara McClintock found transposable element in one strain of corn
- = Ds (Dissociation) TE
- Insert Ds into certain location on chromosome 9 (mutable site)
- Causes chromosomal break at that position
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- But only happens in presence of another unlinked genetic element that activates
chromosomal break = Ac (Activator)
- If Ac present, Ds can jump to other chromosoma locations = Ds causes breakage at new
- Breakage of chromosome 9 occurs in numerous cells that continue to divide and grow
- = as kernel gets larger, have variable-sized sectors w/ recessive corn phenotypes
- Ac can also cause new mutations on its own
- Corn kernel is dark red, plump, stachy
b/c have dominant alleles
- When have Ac, get breakage at Ds
locus = acentric chromosomal fragment w/
dominant alleles lost = get colourless,
shrunken, waxy patch
- Colourless kernel b/c insertion of Ds
element in coding region = inactivate
gene function
- Add Ac = excision of Ds element from C
gene = restore C gene function
- Even in absence of Ds can have
spotted phenotype when Ac element
inserts into coding region of C gene
Nearly half of the human genome
consists of transposable elements
- 44% of human genome = TEs
- Mostly retrotransposons
- = LINEs (long interspersed elements)
- Human genome has ~20 000 copies of main human LINE (Ll)
- = SINEs (short interspersed elements)
- Human genome has ~300 000 copies of main human SINE (Alu)
Retrotransposons move via RNA intermediates
Transposition of retrotransposon
[1] Transcription of retrotransposon by RNA polymerase into RNA
[2] RNA encodes a reverse-transcriptase-like enzyme
[3] Copies RNA into single strand of cDNA
[4] Uses cDNA strand as template to make double-stranded cDNA
= original copy of retrotransposon in same place
find more resources at oneclass.com
find more resources at oneclass.com
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