Gene expression From Gene to Protein Translation RNA DNA Transcription and Splicing Protein
The genetic information of all organisms is stored in long strains of DNA (desoxyribonucleic-acid). Genes are the functional subunits of the genome. They are arranged in a succession on the DNA. Usually one gene encodes one protein. The DNA sequence determines the sequence of amino acids of the resulting protein.
Transcription The way from DNA to RNA
Transcription Transcription is the first step of genexpression. The template for transcription is DNA. The product of this process is messenger RNA (mRNA). RNA polymerase is the enzyme performing transcription. Transcription proceeds in the nucleus in eucaryotes; in the cytoplasm in procaryotes.
The Three Steps of Transcription Initation Elongation Termination
Transcription Initiation Procaryotes RNA polymerase binds to the DNA and is associated with the so called sigma factor. The sigma factor aids in finding the starting point of transcription: the region -10 and -35 basepairs downstream of the promoter. The initation complex opens and the first phosphodiester bond is formed.
Transcription Initiation Eucaryotes Transcription factors mediate binding of the RNA polymerase.
Transcription Initiation RNAP = RNA polymerase
Transcription Elongation One DNA strand is used as the template for transcription (the 3‘–5‘ strand). The RNA polymerase traverses the template strand. It produces an RNA copy that is complementary to the template (T are replaced with U).
Transcription Elongation
Transcription Termination Procaryotes Two different termination strategies: Rho dependent: protein factor Rho destabilizes the interaction between DNA and RNA, releasing the RNA. Rho independet: termination occurs when the transcript forms a G-C rich hairpin loop, followed by a run of Us, which leads to relase of the mRNA from the DNA template.
Transcription Termination Eukaryotes The termination process is less well understood than in procaryotes. It involves cleavage of the new transcript. template independent addition of As at the 3‘ end (poly-adenylation).
Transcription: Termination
Translation The Way From RNA to Protein
? How does the information in mRNA codons get translated into an amino acid sequence and v therefore in polypeptides ?
Through adapter molecules called transfer RNAs tRNAs. The tRNA anticodon base pairs with the codon in the mRNA and carries an amino acid corresponding to that codon.
Transfer RNAs (tRNAs) About 80 nucleotides long RNA with a complex secondary and tertiary structure. Contain non-standard base pairs, stems and loops, and modified bases. Each cell contains different types of tRNAs that can incorporate one of the 20 different amino acids into protein. Some tRNAs can recognize more than one codon.
? What is the correspondence between the mRNAVnucleotides and the amino acids of the protein??
One codon consists of THREE nucleotides Proteins are formed from 20 amino acids in humans. Codons of one nucleotide: A G C U Codons of two nucleotides: AA GA CA UA AG GG CG UG AC GC CC UC AU GU CU UU Can only encode 4 amino acids Can only encode 16 amino acids One codon consists of THREE nucleotides
The 3rd Base Position is Variable The genetic code is nearly universal Exceptions: Yeast Mitochondria Tetrahymena Mycoplasma
The Three Steps of Translation Initiation Elongation Termination
Translation Initiation Translation begins at a START codon: AUG (methionine) The small ribosomal subunit binds to the mRNA. Initiator tRNA (fMet-tRNA) binds and builds H-bonds with its anticodon to the AUG codon on the mRNA (codon-anticodon interaction).
Translation Elongation The large ribosomal subunit binds to the initiation complex. The ribosome has three tRNA binding sites: A-site, P-site, E-site. The incoming tRNA, carrying the amino acid corresponding to the next codon, binds to the A-site.
Translation Elongation A peptide bond is formed between the amino acids of the P-site and A-site tRNAs. After transfer of the amino acid to the growing peptide chain tRNAs leave the ribosome via the E-site (E: exit). These steps are repeated until the ribosome reaches a STOP codon on the mRNA.
Small ribosomal subunit Large ribosomal subunit
Translation Termination A stop codon on the mRNA leads to binding of a release factor. The ribosomal subunits disassemble and are released separately. The completed peptide chain is released.