Transcript Gene Expression - houstonisd.org

Gene Expression
 Remember, every cell in your body
contains the exact same DNA…
…so why does a muscle cell have different
structure and function than a nerve cell?
Review: DNA codes for…
Proteins being produced can be turned
off and on just like a light switch
based on the needs of a cell.
Chromosomes
A structurally organized single piece of
coiled DNA containing many different
genes. Each gene will code for ONE single
PROTEIN.
Gene 1
Gene 2
Gene 3
Gene 4
Gene 5
Gene 6
Regulated Gene Expression
Regulated = control the speed/amount
Gene Expression = using the information
from a gene to express a protein
Regulated Gene Expression = how a cell
controls the speed or amount of a gene
being expressed as a protein
Methods of regulation
In Eukaryotes:
In Prokaryotes:
1. Regulating
1. Regulating
transcription (DNA
 mRNA) by:
1. Activator Proteins
2. Promoters
transcription (DNA
 mRNA) by:
1. Operators
2. Promoters
3. Repressor Protein
2. Splicing of mRNA
Exons - Expressed
Regulation in Eukaryotes
#1 Regulating Transcription (DNA  mRNA)
 If a protein is needed by the cell, a gene coding
for that protein needs to be expressed,
TRANSCRIPTION WILL OCCUR.
 If a protein is NOT needed by the cell,
TRANSCRIPTION WILL STOP.
Eukaryotic Transcription Factors
- Activator Protein - a molecule that turns ON
transcription
- Transcription Factors – molecules that allow RNA
Polymerase to bind to DNA and begin transcription
- Promoter – sequence
on DNA before the
Gene where Activator
Proteins and
Transcription
Factors can bind to…
Eukaryotic:
Transcription will TURN ON
 When Activators and Transcription
Factors are both bound to the Promoter
site, RNA polymerase will bind to DNA
and begin transcription.
 The gene will be transcribed and the
protein will be translated/expressed.
Eukaryotic: Splicing RNA
After Transcription: Portions of the transcribed mRNA is
(spliced) cut out
Exons – the portions of the gene on mRNA that are cut, translated,
and EXPRESS proteins
Intron – the portions of the gene on mRNA that do not code for
proteins and are NOT expressed
The spliced Exons will then code for a Protein!
Regulation of Prokaryotic Genes
 Operon – multiple genes that are grouped together on
one consecutive segment of DNA that is transcribed/copied
onto ONE single mRNA
 Promoter – sequence in an operon where RNA polymerase
binds to and begins transcription
 Operator – sequence in an operon where a repressor can
bind or unbind to
 Repressor – a protein that can bind to the operator to
PREVENT Transcription from occurring
Lac Operon - Prokaryotes
Lac Operon: a DNA sequence of three genes (Lac Z, Lac Y, &
Lac A) that codes for proteins that break down lactose
When lactose is present…
- Repressor is NOT BOUND to the operator
- RNA polymerase can bind to the PROMOTER and
TRANSCRIPTION OCCURS
- Expression of lactase proteins occurs
When lactose is absent…
Repressor IS BOUND to the operator
RNA polymerase can NOT bind to the PROMOTER and
TRANSCRIPTION Does NOT Occur
Expression of lactase proteins is REPRESSED! (switch is OFF)