8 The Molecular Genetics of Gene Expression Transcription

8 The Molecular Genetics of Gene Expression

Transcription Elongation Fig. 8. 6 c 2

Transcription Initiation • Promoter = nucleotide sequence 20 -200 bp long—is the initial binding site of RNA polymerase and transcription initiation factors Fig. 8. 8 3

Transcription Termination Fig. 8. 9 4

Prokaryotic Transcipts DNA 5

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

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 - t. RNA genes and the 5 S component of r. RNA 7

5’ Cap Following initiation a 7 - methylguanosine is added to the 5’-end of the primary transcript = cap 8

Splicing • RNA splicing occurs in nuclear particles known as spliceosomes • The specificity of splicing comes from the five small sn. RNP—RNAs denoted U 1, U 2, U 4, U 5, and U 6, which contain sequences complementary to the splice junctions 9

Splicing 10

Adding a Poly. A tail 11

Eukaryotic RNA processing events 12

Translation • The translation system consists of five major components: § Messenger RNA: m. RNA 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: t. RNA is a small adaptor molecule that translates codons into amino acid § Aminoacyl-t. RNA synthetases: set of molecules catalyzes the attachment of a particular amino acid to its corresponding t. RNA molecule § Initiation, elongation, and termination factors 13

Ribosomes t. RNA 2 dimensional 3 dimensional 14

Translation Elongation 1) 2) 3) 15

Translation Initiation 1. ) Small subunit binds to a ribosome binding site 2. ) methionine charged t. RNA binds to the Psite on the ribosome 3. ) the large subunit tops it off…. This brings you to the first step of elongation 16

Translation Termination 17

Translation • The m. RNA 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 • • 18

Translation • The m. RNA 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 m. RNA often encode the different proteins of a metabolic pathway. 19

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 20

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 21

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 22

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