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a DNA-dependent RNA polymerase (RNAP).
DNA-dependent In prokaryotic cells like E. coli, there is only one type of RNAP responsible
for transcribing genes. In eukaryotic cells, three RNAPs are involved in transcription:
In RNAPI transcribes ribosomal or rRNA,
rRNA RNAPII transcribes mRNA and
RNAPII RNAPIII transcribes transfer RNA (tRNA).
RNAPIII In vivo, mRNAs are differentially degraded by ribonucleases giving each
type of mRNA a specific half-life. The average half-life of mRNA in E.
coli, yeast, and humans are 4 minutes, 20 minutes and 10 hours,
respectively. The transient nature of mRNA provides a mechanism for genes to be
differentially expressed or regulated. This means that a gene can be
turn on when needed and turned off when not needed.
BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez ©2013
BIS101001, Spring 2013—Genes and Gene Expression, R.L. Rodriguez 6 Evidence for RNA as an Intermediate Messenger Caspersson and Brachet (1950) demonstrated that mRNA synthesis
and protein synthesis occur concomitantly in the cell.
and David Prescott’’s pulse/chase experiments using 3H-cytidine and autos
radiography showed that RNA was made in the nucleus and
transported to the cytoplasm.
transported Protein synthesis can not in the absence of RNA synthesis.
not RNA isolated from a particular cell can hybridize with ssDNA from that
cell to form a heteroduplex.
heteroduplex The base composition of RNA is generally similar to the base
composition of the DNA from which it is made.
from Base Composition
T or U
C E. coli DNA...
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This note was uploaded on 02/12/2014 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.
- Spring '08