8 The Molecular Genetics of Gene Expression

Fig. 8.6c Transcription Elongation

Transcription Initiation Promoter = nucleotide sequence bp long—is the initial binding site of RNA polymerase and transcription initiation factors Fig. 8.8

Fig. 8.9 Transcription Termination

5 DNA Prokaryotic Transcipts

6 What’s different about transcription in eukaryotes? Multiple RNA polymerases 5’ capping Splicing, 1 gene/transcript PolyA tail

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

12 Eukaryotic RNA processing events

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

14 RibosomestRNA 2 dimensional 3 dimensional

1) 2) 3) Translation Elongation

1.) Small subunit binds to a ribosome binding site 2.) methionine charged tRNA binds to the P- site on the ribosome 3.) the large subunit tops it off…. This brings you to the first step of elongation Translation Initiation

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.

20 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

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