Transcript Chapter 15 - jl041.k12.sd.us

Gene Expression and
Regulation
I. Gene Regulation in Eukaryotes
A. Transcriptional Controls
1. Gene amplification
2. Chemical Modification
A. Post-transcriptional Controls
B. Translational Controls
A. Transcriptional Control
1. Gene Amplification: A gene sequence can
be repeated many times on a DNA
sequence…resulting in multiple mRNA
transcripts, therefore more gene
expression.
TRANSCRIPTION
Different gene regions of a DNA molecule
mRNA
Transcript
processing
rRNA
tRNA
proteins
mature
mRNA
TRANSLATION
synthesis of
polypeptide
chain on
the platform
of an intact
ribosome
subunits for
ribosomes
convergence
of RNAs
mature
tRNA
Pools of
amino acids,
tRNAs, and
ribosomal
subunits in
the
cytoplasm
FINAL PROTEIN
Fig. 15.5, p. 245
for use in cell or for export
A. Transcriptional Control
2. Chemical Modification: A DNA segment
may be wrapped tightly with histones,
preventing gene expression.
In mammalian females, the
extra “X” is tightly bound,
resulting in the presence
of a Barr Body in the
nucleus.
Mosiac Effect and X inactivation
B. Post Transcription
Splicesomes: Enzymes that rearrange
segements of introns, creating new
combinations of proteins.
C. Translational Controls
mRNA transcript will be digested when it
reaches the cytoplasm (a good thing, or
once a gene were turned on it would be
forever expressed). A transcript has a cap
added to the 5’ end and a poly-A tail added
to the 3’ end. If these tails/caps are long, it
will take the enzymes in the cytoplasm a
greater amount of time to digest the
coding region of the transcript.
unit of transcription in a DNA strand
exon
intron
exon
intron
exon
3’
5’
transcription into pre-mRNA
poly-A
tail
cap
5’
3’
(snipped out)
(snipped out)
5’
3’
mature mRNA transcript
Fig. 14.9, p. 229
II. Gene Regulation in Prokaryotes
Gene Regulation in Prokaryotes:
Prokaryotes have only one DNA molecule (circular
and not protected by nuclear envelope) and this
DNA molecule is not bound up with histones.
Thus, gene regulation in prokaryotes is unique.
One of the best known pathways of gene
recognition is the lac Operon, a regulatory
pathway by which bacteria are able to produce
the enzyme to digest lactose only when
necessary (when lactose is present in the
environment).
A. Operon and DNA
Operon is a regulatory system that controls
DNA transcription in prokaryotes.
Operon contains a promoter (the specific
nucleotide sequence that tells a cell to
begin or start transcription), an operator (a
segment of DNA that can be used to turn
gene expression on or off) and more than
one gene.
B. Actors in Lac Operon
Regulatory or Repressor Protein – Binds
with operator (segment of DNA) to prevent
a gene from being transcribed.
Substrate/Inducer – Interacts with protein to
prevent it from adhering to DNA.
Operator – Section of DNA
Promoter – Section of DNA.
Operon – The entire system.
regulator gene
transcription,
translation
repressor
protein
gene 1
(codes for
b-galactosidase)
gene 2
(codes for
premease)
gene 3
(codes for
trans-acetylase)
promoter
operator
(binding site
(binding
for RNA
site for
polymerase) repressor)
lactose operon
Fig. 15.4a, p. 243
RNA
polymerase
mRNA transcript
translation into three polypeptide
chains for three different enzymes
lactose
b-galactosidase
permease
trans-acetylase
Fig. 15.4b, p. 243
Without Lactose:
Describe the chain of events that occurs in a
bacterial colony when no lactose is
present.
With Lactose:
Describe the chain of events that occur
when lactose is present in a bacterial
culture:
Positive Control: Glucose Digestion
Bacterial cells digest glucose before lactose.
If glucose is present, cAMP is converted to
ATP.
cAMP, when present, bonds to promoter
region and helps RNA polymerase begin
transcription of lactose digestion genes.