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Lecture 21

BIOL 112 Lecture Notes - Lecture 21: Cell Cycle Checkpoint, Reverse Transcriptase, Transposable Element

Course Code
BIOL 112
Joseph Dent

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BIOL 112 - Lecture 21: 23/March/2016
Gene Regulaon (Contd)
There are other kinds of operators;
oPosive operators which only bind to the operator
in the case of Tryptophan, if the tryptophan is
present, but not if it is absent
oNegave feedback
oAgain for tryptophan there are a bunch of genes
for synthesizing it, which are all transcribed
together as an operon
oThere is an operator between the promoter and
the genes that controls the transcripon
oSo if you do not have tryptophan, you want to
transcribe the genes and make it, but if you have a
lot/enough, then you want to turn them o" has a repressor that binds to the
operator, but the repressor binds in this case in the presence of tryptophan
Also ways of not just blocking the polymerase, but enhancing it
oEx. gene that binds to a speci'c sequence (that isn’t the operator sequence) near
the promoter, and by binding (in this case in response to CAMP (cyclic AMP)
which is a signaling molecule), you get enhances transcripon
The Eukaryoc Genome
The genomes are usually diploid, but can be tetraploid
Tetraploids are big (have twice as many chromosomes)
Any animal that has more DNA is bigger (not necessarily just tetraploids are bigger),
because the cells are bigger (not because there are more cells)
Cell size may be used to esmate genome size (for ex., of dinosaurs)
Reples tend to have large genome sizes, birds have relavely small genomes; so are
dinosaurs more like birds or reples?
Dinosaurs have cells that have bones called osteoclasts/osteocytes which are what
deposit the calcium that makes up the bone around itself, so when the organism dies,
there is a structure with a hole inside for the space that used to make up the cell thus
can look at dinosaur bones to look at the cell sizes and predict the genome size
Tyrannosaurus-rex is more like a bird (small genome)
Triceratops is more like a reple (large genome)
Genome size varies from one organism to another
These numbers are the diploid number of bp (bp = base pairs) ; it is more common to
refer to the haploid number which will be half (see image) ; humans = 6 billion bp per

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Example Queson:
The roundworm has a genome of 100 million bp long (haploid #) encoding ~20,000 genes.
Humans have a genome 3 bp long. How many genes do you think we encode?
600,000 genes (it is proporonal)
Comparave Genomics & Splicing
If there were a simpler linear relaonship
between genome size and amount of DNA,
and humans have 30 mes as much DNA as
roundworm, then humans should have 30
mes as many genes = ~600,000 genes;
however, in fact humans have ~30,000 genes
Humans have way more DNA than they need to encode all of their proteins! Why?
Coding region of the gene is broken up into bits called exons (separated by introns),
which is the part that is expressed
as a protein, so it codes for a
small piece for the 'nal protein
So before the transcribed RNA
gets to the ribosome, it
undergoes a process of splicing,
which means it cuts out the intron
sequences and splices together
the coding regions into a nice,
connuous mRNA that the
ribosome can translate e"ecvely
Splicing happens because of
proteins that catalyze the splicing reacon, which are not
just proteins, they also have an RNA component: called
snRNP (small nuclear RiboNuclear Proteins); the RNA is
important for recognizing speci'c splice sites it cuts the
mRNA and then rea@aches it to remove the intron part
which is then degraded by the snRNP proteins
Splicing is a form of RNA processing, and there are other
RNA processing forms too:
5’ G cap
Polyadenylaon ( = addion of a polyA tail)
oThe G cap is just a Guanine nucleode that gets
a@ached to the 5’ end, except it is a@ached 5’ to 5’
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