Pengaturan Ekspresi gen 1. Struktur gen prokaryot dan eukaryot

Perbedaan organisasi gen antara prokaryot dan eukaryot 1. Simple regulatory process 1. Complex regulatory process; at multiple locations &levels 2. Genes are contiguous segments of DNA that are colinear with the mRNA that is translated into a protein. 2. Genes are often split- not contiguous segments of coding sequences & often 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). 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) 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 Shine-Delgarno 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 (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 splicing-several 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 single polypeptide

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 splicing-several different proteins.