Download The Molecular Genetics of Gene Expression

8
The Molecular Genetics of
Gene Expression
Transcription Elongation
Fig. 8.6c
Transcription Initiation
• Promoter = nucleotide sequence 20-200 bp long—is the
initial binding site of RNA polymerase and transcription
initiation factors
Fig. 8.8
Transcription Termination
Fig. 8.9
Prokaryotic Transcipts
DNA
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What’s different about
transcription in
eukaryotes?
•Multiple RNA polymerases
•5’ capping
•Splicing, 1 gene/transcript
•PolyA tail
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Multiple RNA
Polymerases
• RNA polymerases are large, multisubunit complexes
whose active form is called the RNA polymerase
holoenzyme
• Bacterial cells have only one RNA polymerase
holoenzyme, which contains six polypeptide chains
• Eukaryotes have several types of RNA polymerase
• RNA polymerase I transcribes ribosomal RNA.
• RNA polymerase II - all protein-coding genes as well as
the genes for small nuclear RNAs
• RNA polymerase III - tRNA genes and the 5S component
of rRNA
5’ Cap
Following initiation a 7- methylguanosine is added to the
5’-end of the primary transcript = cap
Splicing
• RNA splicing occurs in nuclear particles known as
spliceosomes
• The specificity of splicing comes from the five small
snRNP—RNAs denoted U1, U2, U4, U5, and U6,
which contain sequences complementary to the
splice junctions
Splicing
Adding a PolyA tail
Eukaryotic RNA processing events
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Translation
• The translation system consists of five major components:
 Messenger RNA: mRNA is needed to provide the coding sequence
of bases that determines the amino acid sequence in the resulting
polypeptide chain
 Ribosomes are particles on which protein synthesis takes place
 Transfer RNA: tRNA is a small adaptor molecule that translates
codons into amino acid
 Aminoacyl-tRNA synthetases: set of molecules catalyzes the
attachment of a particular amino acid to its corresponding tRNA
molecule
 Initiation, elongation, and termination factors
Ribosomes
tRNA
2 dimensional
3 dimensional
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Translation Elongation
1)
2)
3)
Translation Initiation
1.) Small
subunit binds
to a ribosome
binding site
2.) methionine
charged tRNA
binds to the Psite on the
ribosome
3.) the large
subunit tops it
off…. This
brings you to
the first step
of elongation
Translation Termination
Translation
•
The mRNA is translated in the 5’-to-3’ direction. The polypeptide is synthesized from the
amino end toward the carboxyl end
•
Most polypeptide chains fold correctly as they exit the ribosome: they pass through a
tunnel in the large ribosomal subunit that is long enough to include about 35 amino
acids
Emerging from the tunnel, protein enters into a sort of cradle formed by a protein
associated with the ribosome: it provides a space where the polypeptide is able to
undergo its folding process.
The proper folding of more complex polypeptides is aided by proteins called chaperones
and chaperonins
•
•
Translation
• The mRNA in bacteria is often polycistronic (encodes serveral
genes), each protein coding region is preceded by its own
ribosome-binding site and AUG initiation codon
• The genes contained in a polycistronic mRNA often encode the
different proteins of a metabolic pathway.
What’s different about
translation in eukaryotes?
•Initiation does not occur at a Shine Delgarno
sequence. The ribosome assembles at the 5’ cap and
translocates to the initiation codon
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Genetic Code
• The genetic code is the list of all codons and the amino acids
that they encode
• Main features of the genetic code were proved in genetic
experiments carried out by F.Crick and collaborators:
• Translation starts from a fixed point
• There is a single reading frame maintained throughout the
process of translation
• Each codon consists of three nucleotides
• Code is nonoverlapping
• Code is degenerate: each amino acid is specified by more
than one codon
Genetic Code
• Most of the codons were
determined from in
vitropolypeptide synthesis
• Genetic code is universal =
the same triplet codons
specify the same amino
acids in all species
• Mutations occur when
changes in codons alter
amino acids in proteins