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Introns,
Exons,
Mutations, and
“Junk”
DNA
Turns out that eukaryotic DNA is more complicated than we’ve
implied thus far…
5’ cap and 3’
poly-A tail are
added to
protect the
RNA
•Eukaryotic genes contain non-amino-acid coding DNA (introns)
•After transcription, mRNA introns are cut out
•The exons are reattached to form “mature” mRNA
•Exons are rearranged to form different proteins (alt. splicing)
•This allows 30,000 genes to produce 120,000 diff. proteins.
Trivia:
•1-1.5% of human DNA is exons
•24% is introns
•Introns make up around 90% of the DNA in a gene
•The rest is “junk” DNA- non-coding portions of
DNA that are not a part of genes
Why do we have junk DNA?
•Not entirely clear at this time
•May be due to viral insertions throughout evolution…
•May protect genes during crossing-over events…
•May be due to transposons (jumping genes)…
Mutations: Good or Bad?
Creates new alleles. No mutations, no evolution.
-Only mutations in germ-line tissues (gametes) are
passed on to offspring
-Germ-line mutations have larger impact on evolution
That said, few mutations are helpful. Most mutations
either have no effect on the organism or are harmful.
http://news.bbc.co.uk/media/images/38123000/jpg/_38123945_frog300.jpg
Kinds of Mutations
•Point Mutations: altering one
or a few base pairs in a gene
•Frame Shift: when a mutation
causes subsequent bases to be
out of place- disastrous
•Chromosome Mutations: when
part of a chromosome inverts,
or relocates to another
chromosome- disastrous