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Lecture 5

BIOB11H3 Lecture Notes - Lecture 5: Tata-Binding Protein, Branch Point, Transcription Factor Ii H

Biological Sciences
Course Code
Dan Riggs

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BIOB11 – Lecture 5
HnRNA Synthesis & Processing
-For rRNA that is made by polymerase I it is initially a 45s unit that then gets cut into
three discrete molecules of 28s, 18s, and 5.8s
-mRNA are synthesized as precursors and then get cut and processed to make small
Some ‘spatial’ things to consider
-Nucleolous (nucleoli): within nucleus where rRNA genes clustered; there can be than one
of them more than one = nucleoli
oActive transcription of tandemly repeated genes
oWhere ribosomal RNA synthesis occurs
-Cytoplasm: contains > 10 million ribosomes
-Nucleus: site of transcription of hnRNA; approximately 70% of all newly synthesized
oAs hnRNA is made, it is quickly broken down; most of it is actually discarded and
Approximately 5% of total RNA is mRNA
RNA polymerase II: transcription of ‘genes’ (mRNAs)
-Are these made as precursors and processed similar to rRNAs?
-Pulse strategy used
oConclusion: mRNAs are synthesized as precursors
hnRNA = heterogenous nuclear RNA
- hnRNA heterogeneous nuclear RNA is the precursor; it gets cut and processed to make
smaller mRNAs there are about 30,000 genes of which require many transcripts of
various sizes and so it is called hnRNA because these are all made up of different sizes
depending on the gene it is encoding and made in the nucleus; Not all of them make it as
Conversions of hnRNA to mRNA – Pulse chase experiment on hnRNA
-In the pulse chase experiment: they labeled cells for short periods of time with Phosphate
(32) and then purified, and centrifuged the RNA to separate the molecules by size in
fractions with, the lower fractions containing larger segments
-They determined Optical density (a measure of the absorbance rate) and the radioactivity
(to see where most of the RNA was)
- Pulse phase: This is right at the start; most of the hnRNA are contained in the large
fractions as shown by the red radioactivity
oThe optical density is always going to be the same because fractions that contain
certain size will always have the same absorbance rate
- Chase: once this occurred after three hours, it was seen that most of the hnRNA had gone
from the large fractions to much smaller fractions indicating that they were Mrna
-From this, it has been determined that hnRNA molecules are processed to yield much
shorter mRNAs – hnRNA have a lot of different genes at various sizes but after

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BIOB11 – Lecture 5
processing it moves to a much smaller molecular size, going from an average of 15KB to
an average of 2KB

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BIOB11 – Lecture 5
-Keep in mind for figures 11-13 and 11-17:
oAt any point in time the majority of the RNA that is being synthesized is hnRNA,
however 90% of the stable RNA in the cell is rRNA
This is because of the half life of hnRNA and mRNA are relatively short
‘Half-life’ of hnRNA/mRNA: minutes to hours (avg. 40 min)
‘Half-life’ of rRNAs: days to months and so accumulation occurs
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