Download Pengaturan Ekspresi gen 1. Struktur gen prokaryot dan eukaryot

Pengaturan Ekspresi gen
1. Struktur gen prokaryot dan
eukaryot
Perbedaan organisasi gen antara prokaryot dan eukaryot
Prokaryot
1. Simple regulatory process
Eukaryot
1. Complex regulatory process; at multiple
locations &levels
2. Genes are contiguous segments of DNA 2. Genes are often split- not contiguous
that are colinear with the mRNA that is
segments of coding sequences & often
translated into a protein.
interrupted by intervening sequences
3. mRNAs are often polycistronic.
3. mRNAs are monocistronic.
4. All RNA species are synthesized by a
single RNA polymerase.
4. Three different RNA polymerases are
responsible for the different classes of RNA
molecules
5. mRNA is simulataneously
transcribed and translated.
5. mRNA is processed before transport to the
cytoplasm where it is translated. Caps and
tails are added and internal parts
6. Operons are present- groups of genes
that function to produce proteins needed
by the cell.
6.Operons are absent
Gen prokaryot
Perbandingan,mRNA prokaryot dan eukaryot
(polycistronic dan monocistronic)
Start and stop signals for RNA synthesis by a
bacterial RNA polymerase.
Template strand (lower), whereas the upper strand
corresponds in sequence to the RNA that is made (note
the substitution of U in RNA for T in DNA).
(A) The polymerase begins transcribing at the start site. Two
short sequences (shaded red), about -35 and -10
nucleotides from the start, determine where the
polymerase binds; close relatives of these two
hexanucleotide sequences, properly spaced from each
other, specify the promoter for most E. coli genes.
(B) (B) A stop (termination) signal. The E. coliRNA
polymerase stops when it synthesizes a run of U
residues (shaded blue) from a complementary run of A
residues on the template strand, provided that it has just
synthesized a self-complementary RNA nucleotide
sequence (shaded green), which rapidly forms a hairpin
helix that is crucial for stopping transcription. The
sequence of nucleotides in the self-complementary
region can vary widely.
Shine Dalgarno
• The Shine-Delgarno sequence is a purine-rich sequence (GAGGGG)
found in the initiator region of prokaryotic mRNA. It is located about 10
nucleotides upstream the initiator codon AUG.
• The Shine-Delgarno sequence binds to a complementary region near the
3’-end of the 16S rRNA (small subunit of the ribosome). The number of
base pairs between these mRNA and rRNA ranges from three to nine.
• This region therefore has influence on where the translation process
starts. Protein synthesis actually begins with the interaction of the ShineDelgarno sequence of the mRNA with the rRNA of the ribosome.
Sintesis RNA
In bacteria, transcription and translation are often coupled.
Interchangeable RNA polymerase subunits as
a strategy to control gene expression in a
bacterial virus.
The bacterial virus SPO1, which infects the bacterium B.
subtilis, uses the bacterial polymerase to transcribe its early
genes immediately after the viral DNA enters the cell. One of
the early genes, called 28, encodes a sigmalike factor that
binds to RNA polymerase and displaces the bacterial sigma
factor. This new form of polymerase specifically initiates
transcription of the SPO1 "middle" genes. One of the middle
genes encodes a second sigmalike factor, 34, that displaces
the 28 product and directs RNA polymerase to transcribe the
"late" genes. This last set of genes produces the proteins that
package the virus chromosome into a virus coat and lyse the
cell. By this strategy, sets of virus genes are expressed in the
order in which they are needed; this ensures a rapid and
efficient viral replication.
Interchangeable RNA polymerase subunits as a strategy
to control gene expression in a bacterial virus.
RNA polymerase: 1 error /104 bp
DNA polymerase: 1 error/107 bp
Eucaryote gene
Steps leading from gene to protein in eucaryotes and bacteria. The
final level of a protein in the cell depends on the efficiency of each
step and on the rates of degradation of the RNA and protein
molecules.
(A) In eucaryotic cells the RNA molecule produced by transcription
alone (sometimes referred to as the primary transcript) would
contain both coding (exon) and noncoding (intron) sequences.
Before it can be translated into protein, the two ends of the RNA are
modified, the introns are removed by an enzymatically catalyzed
RNA splicing reaction, and the resulting mRNA is transported from
the nucleus to the cytoplasm. Although these steps are depicted as
occurring one at a time, in a sequence, in reality they are coupled
and different steps can occur simultaneously. For example, the RNA
cap is added and splicing typically begins before transcription has
been completed. Because of this coupling, complete primary RNA
transcripts do not typically exist in the cell.
(B) In procaryotes the production of mRNA
molecules is much simpler. The 5 end of an mRNA
molecule is produced by the initiation of
transcription by RNA polymerase, and the 3 end is
produced by the termination of transcription. Since
procaryotic cells lack a nucleus, transcription and
translation take place in a common compartment.
In fact, translation of a bacterial mRNA often
begins before its synthesis has been completed .
DNA in the nucleus
One function of the nuclear envelope may
be to protect the long, fragile DNA
molecules from the mechanical forces
generated by the cytoplasmic filaments in
eucaryote
RNA processing in nucleus. RNA splicingseveral different proteins.
A comparison of the structures of procaryotic
and eucaryotic messenger RNA molecules.
Although both mRNAs are synthesized with a triphosphate group at
the 5' end, the eucaryotic RNA molecule immediately acquires a 5'
cap, which is part of the structure recognized by the small
ribosomal subunit. Protein synthesis therefore begins at a start
codon near the 5' end of the mRNA (see Figure 6-24). In procaryotes,
by contrast, the 5' end has no special significance, and there can be
multiple ribosome-binding sites (called Shine-Dalgarno sequences)
in the interior of an mRNA chain, each resulting in the synthesis of a
different protein.
Bacterial messenger RNAs are commonly polycistronicthat is, they
encode multiple proteins that are separately translated from the
same mRNA molecule. Eucaryotic mRNAs, in contrast, are typically
monocistronic, with only one species of polypeptide chain being
translated per messenger molecule
monocistronic and polycistronic.
a cistron is defined as a genetic unit that encodes a
The organization of genes on a
typical vertebrate chromosome.
Proteins that bind to the DNA in regulatory
regions determine whether a gene is
transcribed; although often located on the
5' side of a gene, regulatory regions can
also be located in introns, in exons, or on
the 3' side of a gene. Intron sequences are
removed from primary RNA transcripts to
produce messenger RNA (mRNA)
molecules.
Negative and positive control of
alternative RNA splicing.
(A) Negative control, in which a repressor
protein binds to the primary RNA transcript
in tissue 2, thereby preventing the splicing
machinery from removing an intron
sequence.
(B) Positive control, in which the splicing
machinery is unable to efficiently remove a
particular intron sequence without
assistance from an activator protein.
Control of alternative splicing
Regulation of transcription
Transcription of a gene by RNA polymerase can be regulated by at least
three types of proteins:
Specificity factors alter the specificity of RNA polymerase for a given
promoter or set of promoters, making it more or less likely to bind to them.
Repressors bind to non-coding sequences on the DNA strand, impeding
RNA polymerase's progress along the strand, thus impeding the expression
of the gene.
Activators enhance the interaction between RNA polymerase and a
particular promoter, encouraging the expression of the gene.
In prokaryotes, repressors and activators bind to regions called operators
that are generally located near the promoter.
In eukaryotes, transcriptional regulation tends to involve combinatorial
interactions between several transcription factors, which allow for a
sophisticated response to multiple conditions in the environment. This
permits spatial and temporal differences in gene expression. Eukaryotes
also make use of enhancers, distant regions of DNA that can loop back to
the promoter.
DNA in the nucleus
One function of the nuclear envelope may
be to protect the long, fragile DNA
molecules from the mechanical forces
generated by the cytoplasmic filaments in
eucaryote
RNA processing in nucleus. RNA splicingseveral different proteins.