Transcript Gene Expression

What you should know about gene
expression
• Phenotype is determined by the proteins
produced as the result of gene expression.
• Only a fraction of the genes in a cell are
expressed.
• Gene expression is influenced by intraand extra-cellular environmental factors.
• Gene expression is controlled by the
regulation of both transcription and
translation.
Phenotype/Genotype
• A cell’s genotype is
determined by the
sequence of DNA bases in
its genes
• Phenotype is determined
by the proteins produced
as the result of gene
expression
Selective Gene Expression
• Although a specialised cell has a
complete set of the organism's genes,
only those needed for its specialised
functions are switched on. All other
genes are switched off.
• So only a fraction of genes in a cell are
expressed
• The process of differentiation
proceeds in a stepwise fashion and
involves changes in gene expression.
• Gene expression is the activation of a
gene that results in the formation of
a protein.
• Gene expression is
influenced by intraand extra-cellular
environmental factors.
Gene activity in different
specialised cells
Cell type
Activity of genes coding for production of:
antibodies
lysosome
enzymes
enzymes for synthesis of
urea
pepsin
phagocyte
off
on
off
off
liver
off
off
on
off
stomach
lining
off
off
off
on
lymphocyte
on
off
off
off
Gene expression is controlled by the regulation
of the processes of TRANSCRIPTION and
TRANSLATION (protein synthesis).
DNA Base -> Amino Acid -> Protein
• 4 different organic bases, which make up
the genetic code (DNA)
• Bases are taken in groups of 3,
gives rise to 64 different
combinations (4³)
• Amino acids are produced according
to the sequence of every 3 bases on DNA
• Each individuals’ base triplets are arranged in
a specific order for coding for the particular
proteins needed by that individual
Gene expression- Do you know ?
• Phenotype is determined by the proteins
produced as the result of gene expression.
• Only a fraction of the genes in a cell are
expressed.
• Gene expression is influenced by intraand extra-cellular environmental factors.
• Gene expression is controlled by the
regulation of both transcription and
translation.
RNA,
Transcription and Translation
What you should know about RNA
• RNA is single stranded
• RNA contains uracil instead of thymine RNA
contains ribose instead of deoxyribose sugar.
• mRNA carries a copy of the DNA code from the
nucleus to the ribosome.
• Ribosomal RNA (rRNA) and proteins form the
ribosome.
• Each transfer RNA (tRNA) carries a specific
amino acid.
Structure of RNA
• Differences between DNA and RNA
RNA:
• Single Stranded
• Bases: Adenine, Uracil, Guanine and
Cytosine
• Ribose Sugar
Types of RNA
• Messenger RNA (mRNA) – carries the
genetic information (in base triplets
called codons) from the nucleus to the
cytoplasm for protein synthesis
• Transfer RNA (tRNA) – Carries amino
acids on the anti-codon to the
appropriate codon to form a polypeptide
RIBONUCLEIC ACID
• DNA carries the code (“recipe”) for
making proteins
• The “equipment” for protein synthesis is
in the cytoplasm
• mRNA acts as a go-between molecule,
carrying the “recipe” to the
“equipment”.
Transcription
Transcription- what you should
know
• Describe the transcription of DNA into primary and
mature RNA transcripts and include the role of RNA
polymerase and complementary base pairing.
• The introns of the primary transcript of mRNA are noncoding and are removed in RNA splicing.
• The exons are coding regions and are joined together to
form mature transcript. This process is called RNA
splicing.
TRANSCRIPTION
• The process of making mRNA from the template
provided on DNA
• DNA to act as template
• Free RNA nucleotides
• Enzymes including RNA polymerase
• ATP for energy
• When mRNA is formed, it has triplets of bases
along it. These are called codons:
CODON
•
AGCUUACGGAUG
TRANSCRIPTION
•
Transcription - copies the information in DNA into an RNA molecule
- occurs in the nucleus.
1) RNA polymerase enzyme binds to
the promoter region of the DNA,
it moves along the DNA, unwinding
the double helix & breaking hydrogen
bonds holding the base pairs together
2) Free RNA nucleotides join onto the 3’
end of the growing mRNA & bond with
their complementary base pairs on
the DNA (Remember- A-U, G-C)
3) The RNA that has been produced at
this stage is known as the primary
transcript
• The primary transcript now undergoes splicing
• Hydrogen bonds
between DNA bases
re-form
• DNA coils up into
double helix
SPLICING
Introns and exons
• The primary transcript of RNA is composed of introns
and exons.
• The introns are non-coding regions of genes and so do not
appear in the mRNA in eukaryotic cells. The exons are
coding regions of genes and so do appear in the mRNA.
• The introns of the primary transcript of mRNA are
removed in RNA splicing.
RNA splicing
• In RNA splicing, the primary transcript is cut at the
boundaries between the introns and exons.
• The introns are removed and the exons are joined
together, to leave a continuous sequence of nucleotides.
• The mRNA can then leave the nucleus via a nuclear pore
and enter the cytoplasm to
move onto the next stage of
protein synthesisTRANSLATION.
DNA to Protein
Transcription- Do you know ?
• How to describe the transcription of DNA into primary
and mature RNA transcripts and include the role of RNA
polymerase and complementary base pairing.
• That the introns of the primary transcript of mRNA are
non-coding and are removed in RNA splicing.
• The exons are coding regions and are joined together to
form mature transcript. This process is called RNA
splicing.
Translation
Translation –what you should know
• Describe the translation of mRNA into a polypeptide by
tRNA at the ribosome.
• tRNA folds due to base pairing to form a triplet anticodon
site and an attachment site for a specific amino acid.
• Triplet codons on mRNA and anticodons translate the
genetic code into a sequence of amino acids.
• Start and stop codons exist.
• Codon recognition of incoming tRNA, peptide bond
formation and exit of tRNA from the ribosome as a
polypeptide is formed.
TRANSLATION
• Changing the code on mRNA into a
sequence of amino acids.
• Occurs in the cytoplasm
• Involves ribosomes
The ribosome
• found in the cytoplasm either
floating freely or attached to the
rough endoplasmic reticulum.
• Ribosomes floating freely are used
to synthesise proteins for use within
the cell; those attached to the
endoplasmic reticulum synthesise
proteins for export or inclusion in
the membrane.
• Ribosomes are formed from proteins
and a third type of RNA known as
ribosomal RNA (rRNA).
David S. Goodsell RCSB PDB
• During translation they allow the
mRNA and tRNA to come together.
Amino acid
attachment
site
tRNA
• Complementary base pairing
occurs between nucleotides within
the strand of tRNA, producing
tRNA’s distinctive structure. This
structure exposes a triplet
anticodon site and attachment
site for a specific amino acid.
• The triplet anticodon site is
complementary to the triplet
codon site on the mRNA.
Triplet
anticodon
TRANSLATION
1) During translation
the mRNA passes
through the
ribosome.
AMINO
ACID
tRNA
2) The ribosome
binds to the mRNA so
that the start codon
is in position
RIBOSOME
U C G
CODON
A G C A U U A G C C C U A G A G G
mRNA
ANTICODON
TRANSLATION
3) The codons are recognised
by tRNA, each carrying a
particular amino acid.
U A A
U C G
A G C A U U A G C C C U A G A G G
TRANSLATION
4) The appropriate tRNA
brings its amino acid to the
ribosome as it moves along the
mRNA.
U C G U A A
A G C A U U A G C C C U A G A G G
TRANSLATION
U C G
U A A
U C G
A G C A U U A G C C C U A G A G G
TRANSLATION
5) Adjacent amino acids then
join with a peptide bond.
PEPTIDE
BOND
U C G
G G A
U C G
A G C A U U A G C C C U A G A G G
U A A
TRANSLATION
U A A
U C U
G G A
A G C A U U A G C C C U A G A G G
U C G
TRANSLATION
6) The tRNA then leaves the ribosome.
U C G
U C U
A G C A U U A G C C C U A G A G G
G A A
TRANSLATION
7) This process continues until
a stop codon is reached and
the polypeptide is released.
POLYPEPTIDE
CHAIN
G G A
A G C A U U A G C C C U A G A G G
U C U
DNA – mRNA – tRNA - Protein
Complete the sequences of mRNA and tRNA then using
Torrance pg 39 find out the names of the amino acids.
DNA - TTACGGCAATGCGGTACCGTTGGGGGCAG
mRNA Codons (set of 3 bases)
tRNA Anti-codons (set of 3 matching bases)
Protein -
DNA – mRNA – tRNA - Protein
DNA - TTACGGCAATGCGGTACCGTTGGGGGCAG
mRNA - AAUGCCGUUACGCCAUGGCAACCCCCGUC
Codons (set of 3 bases)
tRNA - UUACGGCAAUGCGGUACCGUUGGGGGCAG
Anti-codons (set of 3 matching bases)
Protein - aspn– ala – val – thr – pro – try – glu – pro - pro
Overview
Fate of Protein
• The protein is
carried in a vesicle
from the RER to the
Golgi where it is
processed and
packaged into a
vesicle which fuses
with the cell
membrane.
Translation –You should know
• The process of translation of mRNA into a polypeptide
by tRNA at the ribosome.
• tRNA folds due to base pairing to form a triplet anticodon
site and an attachment site for a specific amino acid.
• Triplet codons on mRNA and anticodons translate the
genetic code into a sequence of amino acids.
• Start and stop codons exist.
• Codon recognition of incoming tRNA, peptide bond
formation and exit of tRNA from the ribosome as a
polypeptide is formed.
Protein synthesis storyboard
• You are going to produce a storyboard
showing the process of protein synthesis in a
number of steps.
• For each step you should draw a diagram
and give a short description of what is
happening.
• Use as many boxes as you require to
describe the full process.
• The steps should be displayed in the correct
order, with all the main molecules mentioned.
Protein synthesis storyboard