Transcript Selfish DNA and the wonderful world of RNA

Selfish DNA
Honors Genetics
DNA – Genes and the Rest
Genic ( Gene containing portion of DNA)  Genes are viewed as “ORFS” – Open Reading
Frames – These are determined by a start
3’ TAC and a stop ATT5’
Genes account for 25% of our DNA
 In a gene there are coding regions
Exons
 Between the Exons there are intervening
sequences called Introns
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What is the product of a Gene ?
Proteins – All types such as transmembrane
proteins , transport proteins, immune
proteins, messenger proteins, enzymes,
channels, toxins, developmental proteins
( hox), transcription factors, and many more
 RNAs – m RNA ( intermediate in protein
synthesis), t RNA required for protein
synthesis, r RNA( ribosomal RNA’s), RNAi,
antisense RNAs, micro RNAs, snurps,
spliceosome RNAs and many more
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ORF
What is a Gene?
Gene Expression
Spliceosome – editing the primary
transcript
Intron Borders
DNA to Protein
Only 1% of DNA actually defines the
structure of genes and proteins
 Out of 46 chromosomes containing 30,0050,000 genes only 1% is actively engaged in
keeping you alive
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Making a Protein
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Genes are grouped in islands on
chromosomes
Genes are in clusters on chromosomes
 What is between – the space between the
genes contains DNA that has survived
through evolution
 It is believed to be the substance of
mutations – contributing to change in the
change for both the positive and negative
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Genes were thought to be fixed
in one location on a chromosome
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17q31.2
Barbara McClintock discovered
“ jumping genes”
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Jumping genes are referred to as mobile
They have the ability to change their position and
jump from one chromosome to another
Jumping genes are also called TRANSPOSONS
The Original Jumping Genes
Transposons
1) IS - insertion sequence. Inverted repeats +
transposase gene
 2) Composite transposon. Two IS elements +
antibiotic resistance gene(s)
 3) Noncomposite transposon. Inverted
repeats + transposase gene + antibiotic
resistance gene(s)
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Transposon Action
Transposon structure
Transposable elements and the
new meaning of mutation
Retrotransposons
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Retrotransposons move by a "copy and paste" mechanism but in
contrast to the transposons described above, the copy is made
of RNA, not DNA.
The RNA copies are then transcribed back into DNA — using a
reverse transcriptase — and these are inserted into new
locations in the genome.
Many retrotransposons have long terminal repeats (LTRs) at
their ends that may contain over 1000 base pairs in each.
Like DNA transposons, retrotransposons generate direct
repeats at their new sites of insertion. In fact, it is the
presence of these direct repeats that often is the clue that the
intervening stretch of DNA arrived there by retrotransposition.
About 40% of the entire human genome consists of
retrotransposons.
Sines ( Short Interspersed
elements)
SINEs are short DNA sequences (100–400
base pairs) that represent reversetranscribed RNA
 Alus - Thousands of our Alu elements occur in
the introns of structural genes.
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Alu Action
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ALU elements have been accumulating in the human genome
throughout primate evolution, reaching a copy number of over a
million per genome. However, most of these Alu copies are not
identical and can be classified into several subfamilies (reviewed
in DEININGER and BATZER 1993 ).
These different subfamilies of Alu elements were generated
once mutations occurred within the "master" or "source" gene
that actively retroposed at different rates and time periods of
primate evolution (DEININGER et al. 1992 ).
Currently, the Alu retroposition rate is reduced by 100-fold
from its peak early in primate evolution (SHEN et al. 1991 ).
The vast majority of the Alu elements present in the human
genome inserted before the radiation of extant humans and are
therefore observed in all individuals in the human population.
Microsatellites
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Studies show that microsatellites, tandem repeat
sequences abundant in the genomes of higher
eukaryotes, contain reiterating A-rich loci, which are
involved in the higher-order organization of the
chromatin
Other studies have shown satellites consisting of about 1
million copies of a 221-bp tandem repeat unit has been
localized in the centromeres of 58 of the 64 horse
chromosomes
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Many hundreds of studies have implicated mutations in
satellites, minisatellites, and microsatellites, in diseases
which show genetic linkage, including studies on Crohn's
disease
What is Selfish DNA?
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The theory that we creatures exist for the
preservation of genes and are nothing more than
their throwaway survival machines.
Natural selection means the differential survival of
the “ fittest”
Does natural selection “ choose” the most fit
individuals or the most fit genes?
What are the characteristics of
Selfish DNA?
They have been called "junk" DNA and
"selfish" DNA.
 "selfish" because their only function seems to
make more copies of themselves and
 "junk" because there is no obvious benefit to
their host.
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Some theories on Alus
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Some of these contain sequences that when transcribed
into the primary transcript are recognized by the
spliceosome.
These can then be spliced into the mature mRNA
creating a
new exon, which will be transcribed into a new protein
product.
Alternative splicing can provide not only the new mRNA
(and thus protein) but also the old.
In this way, nature can try out new proteins without the
risk of abandoning the tried-and-true old one.
Alternative splicing
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Alternative Splicing