Transcript 7.2 Transcription and gene expression (HL ONLY

7.2 Transcription & Gene
Expression
RNA and Protein Synthesis
Essential Idea
Information stored as a code in DNA is
copied onto mRNA.
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Understanding
• Transcription occurs in a 5’ to 3’ direction.
• Nucleosomes help to regulate transcription in
eukaryotes.
• Eukaryotic cells modify mRNA after transcription.
• Splicing of mRNA increases the number of different
proteins an organism can produce.
• 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.
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IB Assessment Statement
State that transcription is carried out in a 5’→ 3’
direction.
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Direction of Transcription:
The 5' end of the nucleotide is added to the 3' of the
already existing mRNA chain.
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/transcription.swf
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The sense strand has the same base sequence as
the transcribed mRNA except that the base thymine
is replaced by the base uracil.
The anti-sense strand acts as the template for the
transcription of mRNA.
The RNA nucleotides are polymerised along the sugar
phosphate backbone by RNA polymerase.
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IB Assessment Statement
Explain the process of transcription in prokaryotes,
including the role of the promoter region, RNA
polymerase, nucleoside triphosphates and the
terminator.
http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter15/mrna_synthesis__transcription_.html
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Transcription in Prokaryotes
• The Promoter region allows the binding of RNA
polymerase. The RNA polymerase is then able
to:
o
o
o
Find the anti-sense strand.
Find the start for transcription.
Know the direction of transcription.
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Transcription in Prokaryotes
The Promoter region allows the binding of RNA The hydrogen bonds between
the bases of the DNA helix are opened up by DNA helicase.
The bases of the anti-sense strand ('3 to 5' for DNA) are exposed progressively.
RNA nucleotides complementary base pair with the anti-sense nucleotide
bases.
The free nucleotides (nucleoside triphosphates) are based on RNA. The sugar
is the pentose ribose and there are four different nitrogen bases.
The nucleotides are adenine, guanine, cytosine and uracil.
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Transcription in Prokaryotes
The RNA polymerase forms covalent bonds between the nucleotides.
Free energy is released from the oxidation of the nucleoside triphosphates to
form the bond.
The bonds are formed by joining the 5' of the free nucleotide to the 3' end of the
nucleotide already part of the mRNA chain.
The RNA polymerase works along the nucleotides completing the pentosephosphate backbone.
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Transcription in Prokaryotes
The mRNA builds up with the RNA polymerase moving along the
anti-sense strand joining the nucleotides.
As with the other biochemical processes considered in the
syllabus there are additional factor involved in transcription.
These are not required for the examination
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Transcription in Prokaryotes
The RNA polymerase reaches the terminator and
the RNA polymerase stops.
The mRNA is complete
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Transcription in Prokaryotes
Various factors result in the RNA polymerase being released and will return to
catalyze another mRNA.
The mRNA itself is released from the antis-sense strand.
The mRNA strand in prokaryotes can be use straight away unlike the eukaryotic
mRNA which requires further modification (see next slide)
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IB ASSESSMENT STATEMENT
State that eukaryotic RNA needs the
removal of introns to form mature mRNA
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RNA Editing
mRNA Editing
The introns are cut out
of RNA molecules.
Exon Intron
DNA
Pre-mRNA
The exons are the
spliced together to
form mRNA.
mRNA
Cap
Tail
mRNA Editing
re-mRNA has been produced
through transcription of the
anti-sense strand as
described for prokaryotic
transcription.
(a) The non coding introns are
spliced out of the mRNA.
The introns are broken down in
the nucleus.
(b) The remaining mRNA is
called mature mRNA and is
exported from the nucleus to
the cytoplasm for translation
into the polypeptide
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Animation on mRNA editing
http://bcs.whfreeman.com/thelifewire/content/chp14/1
401s.swf
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Translation
Messenger RNA is
transcribed in the nucleus,
and then enters the cytoplasm
where it attaches to a
ribosome.
Nucleus
mRNA
How are genes regulated? – TWO
METHODS
1.Histone proteins in nucleosomes help to
regulate transcription in eukaryotes by
modifying it structure chemically which
enhance or inhibit the expression of genes
(transcription)
2.Regulatory Proteins (i.e.hormones)that
bind to specific DNA Sequences which
enhance or inhibit the expression of genes
(transcription)
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Gene Regulation & Expression
Nucleosomes help to regulate transcription in
eukaryotes.
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METHOD 1: Nucleosomes & Gene
Expression (the regulation of transcription)
• Nucleosomes have
proteins called histones.
• Chemical modification
of these histones are an
important factor is
determining whether a
gene is expressed or
NOT.
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Method 1: Chemical Modification of Histones & Gene
expression
Types of modifications of histones:
1. Addition of acetyl group to a histone tail
2. Addition of a methyl group to a histone tail
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Method 1: Chemical Modification of Histones & Gene
expression
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Method 1: Chemical Modification of Histones & Gene
expression
Chemical Modification of histones can either activate or
deactivate genes by increasing or decreasing the accessibility
of the gene to transcription factors
•
Acetyl group: neutralizes the positive charge
on histones, making DNA less tightly coiled–>
increases transcription
•
Methyl group: maintains positive charge on
histones, making DNA tightly coiled –>
decreases transcription
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Method 1: Chemical Modification of Histones & Gene
expression
Method 2: Gene expression is regulated by proteins that bind to
specific base sequences in DNA.
DNA SEQUENCES (in which regulatory proteins can bind)
• Enhancers: regulatory sequences on DNA which increase the
rate of transcription when proteins bind to them.
• Silencer : sequences on DNA which decrease the rate of
transcription when proteins bind to them.
• Enhancers & Silencers are unique for each gene.
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Gene Expression by regulatory proteins.
https://www.youtube.com/watch?v=KKR28Y
_L4CA
https://www.youtube.com/watch?v=rs6UkVa
OPzo
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Epigenetics: The environment of a cell and of
an organism has an impact on gene
expression.
http://learn.genetics.utah.edu/content/epigenetic
s/intro/
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Environmental Affects of Genetic Expression
The term epigenetics refers to heritable changes in gene expression
(active versus inactive genes) that does NOT involve changes to the
underlying DNA sequence;
Epigenetic change is a regular and natural occurrence but can also be
influenced by several factors:
 Including age,
 the environment/lifestyle,
and disease state.
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Environmental Affects of Genetic Expression
Epigenetic modifications can manifest as commonly as the manner in which cells
terminally differentiate to end up as skin cells, liver cells, brain cells, etc. Or,
epigenetic change can have more damaging effects that can result in diseases like
cancer.
At least TWO systems in which gene expression is modified including:
1. DNA methylation
2. histone modification
New and ongoing research is continuously uncovering the role of epigenetics in a
variety of human disorders and fatal diseases.
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END OF SECTION