7.2 Transcription and gene expression
Understanding:
- Gene expression is regulated by proteins
that bind to specific base sequences in DNA
- The environment of a cell and of an
organism has an impact on gene expression
- Nucleosomes help to regulate transcription
in eukaryotes
- Transcription occurs in a 5’ to 3’ direction
- Eukaryotic cells modify mRNA after
transcription
- Splicing of mRNA increases the number of
different proteins an organism can produce
Nature of science:
-
Looking for patterns, trends and
discrepancies: there is mounting evidence
that the environment can trigger heritable
changes in epigenetic factors
Applications:
-
The promoter as an example of noncoding DNA with a function
Skills:
-
Analysis of changes in DNA
methylation patterns
Regulation of gene
expression
Some proteins not regulated
– essential for survival so
are expressed all the time
Some produced at certain
times, in certain amounts
Different causes:
Variation in environmental
conditions
Cellular differentiation
Regulation of gene
expression
Normally happens at
transcription – but can
occur at posttranscription and
translation as well.
(Post-transcriptional
modification does not
occur in prokaryotes…)
Regulation of gene
expression
Promoters
Promoter-proximal
elements
Enhancers
Silencers
Promoters
Enhancers
Promoter-proximal elements
Silencers
Promoters
Non-coding DNA
Located near a gene
Binding site of RNA
polymerase – initiates
transcription
Promoters are not
transcribed
Same sequence for
most genes (general
RNA polymerase
binding)
Promoter-proximal
elements
Near to promoter
Specific sequence for each gene
Contains other regulatory elements
Enhancers
Silencers
Increase the rate of
transcription
Decrease the rate of
trasncription
Does not need to be
near promoter
Does not need to be
near promoter
Direction of transcription
1. Initiation (start at promoter)
2. Elongation (build)
3. Termination (RNA completed and breaks off at terminator)
5’
3’
3’
5’
Transcription starts
at a promoter
5’ to 3’ direction
Transcription
Create a new summary for transcription
Build on the old summary you had for SL
Include:
- Promoter
- Enhancers
- Silencers
- Initiation
- Elongation
- Termination
- Direction of transcription
Transcription
Initiation
1. DNA unwinds
2. H bonds break
3. Strands separate
4. RNA polymerase binds to promoter
Elongation
1. Enhancers speed up rate of transcription
2. Silencers slow down rate of transcription
3. Free RNA nucleotides attracted to complementary base pairs
4. Nucleotides joined by RNA polymerase in 5’ to 3’ direction
Termination
1. RNA reaches terminator and detaches
2. H bonds break
3. RNA molecule detaches from DNA
4. DNA forms double helix again (H bonds form)
Nature vs. Nurture
(environment or genes)
What do you think?
Are people’s
traits/characteristics/mannerisms/image are due to
nature or nurture?
Write down arguments for both sides and come to
your own conclusion.
Give examples.
Impact of environment (TOK)
Nature vs nurture
Genes or
environment?
Many studies on
identical and non
identical twins
raised together or
apart
More evidence for
intelligence is
inherited
Impact of environment on gene regulation
Production of skin
pigmentation during
exposure to sunlight
Melanin produced to
protect against UV
rays from the sun
Gene regulation in
response to
environment
Prior learning
DNA packaging
DNA molecules are paired with a protein called histone
Histones help to package DNA
Essential as DNA can be 4cm long, it must fit into a
microscopic nucleus
Epigenetic tags
Chemical modification of
histones is important in
whether a gene is
expressed or not.
Add:
- Acetyl group (COCH3)
- Methyl group (CH3)
- Phosphate group (PO4)
Increase or decrease the
accessibility of the gene
to transcription factors
Impact the visible
characteristics of an
individual
Post transcriptional modification
Prokaryotes: no nuclear membrane
Transcription and translation coupled
Eukaryotes: can carry out modification before mRNA
leaves nucleus
Post transcriptional modification
Pre-mRNA  mature mRNA
RNA splicing
(Introns removed and exons joined together)
RNA splicing
Allows one gene to code for multiple proteins
Must have multiple exons
Exons may or may not be included in mRNA