Transcription Translation From Gene to Protein protein synthesis

Transcription/ Translation From Gene to Protein (protein synthesis)

• Transcription Protein is made outside of the nucleus at the ribosome, but DNA can’t leave the nucleus. • Transcription - is the making of m. RNA from DNA. • messenger RNA (Ribo Nucleic Acid) is a copy of DNA that can leave the nucleus.

• • • m. RNA is single stranded m. RNA contains no T’s (thymine) It replaces that T’s with U’s (uracil) It can leave the nucleus. It’s base pairs are read it sets of 3 called codons. • A-T-G • U-A-C C-G-G G-C-C T-T-A A-A-U DNA m. RNA

Transcription in Three stages • The three stages of transcription: – Initiation – starting transcription – Elongation – laying down the m. RNA nucleotides – Termination – finalizing the m. RNA

Promoter • Promoter – the beginning of the DNA sequence to be copied. (RNA polymerase II attaches at the promoter) • TATA Box – the promoter begins with a sequence of DNA that includes the base pairs T and A.

Transcription Factors • Transcription Factors – Proteins that allow the RNA Polymerase II to bind to the promoter.

Transcription Initiation Complex • Transcription initiation complex – the promoter, RNA Polymerase II, and Transcription factors all combined and ready to start transcription.

RNA Polymerase • RNA Polymerase – enzyme that unwinds DNA and lays down RNA nucleotides.

Termination • A termination code caused the m. RNA to be cut free from the DNA. (ex. AAUAAA)** • At this point the m. RNA is pre-m. RNA is needs a few modifications

Alteration of m. RNA • The 5 end receives a modified 5 cap – a modified guanine nucleotide with 3 phosphate groups • The 3 end gets a poly-A tail – several adenine nucleotides in a row (50+) • These modifications have several functions: – They allow m. RNA to leave the nucleus – They protect m. RNA from hydrolytic enzymes – They help ribosomes attach to the 5 end

Eukaryotic RNA Splicing • Finally, the sections of the pre-m. RNA that will not be used to code for the protein are cut out. • introns – section that are cut of the pre-m. RNA • exons –sections that are left in the finalized m. RNA

Fig. 17 -10 5 Exon Intron 3 Pre-m. RNA 5 Cap Poly-A tail 1 30 31 Coding segment m. RNA 5 Cap 1 5 UTR 104 105 146 Introns cut out and exons spliced together Poly-A tail 146 3 UTR

Spliceosomes • Spliceosomes – carry out RNA splicing • Made of ribonucleoproteins (sn. RNPs) that recognize the splice sites

Ribozymes • Ribozymes -are catalytic RNA molecules that function as enzymes and can splice RNA (present in sn. RNP’s) (ribozyme)

Alternative RNA splicing • alternative RNA splicing – choosing different regions of introns or exons from the same prem. RNA sequence • So one gene can code for more than one protein.

t. RNA • • t-RNA – Transfer RNA (single stranded) Each t. RNA has an anti-codon that matches up with a codon on the messenger RNA Each t. RNA also has a particular amino acid attached to it.

Amino Acids • Each t. RNA also has a particular amino acid attached to it. • aminoacyl-t. RNA synthetase – enzyme that attaches amino acids to t. RNA

Ribosomes • Ribosomes match the t. RNA anticodons with m. RNA codons in protein synthesis • The two ribosomal subunits are made of proteins and ribosomal RNA (r. RNA) • small subunit holds m. RNA • Large subunit holds the t. RNA

Building a Polypeptide • A ribosome has three binding sites for t. RNA: – A site- holds the t. RNA that carries the next amino acid to be added to the chain – P site- holds the t. RNA that carries the growing polypeptide chain – E site- is the exit site, where discharged t. RNAs leave the ribosome. ** P A

Fig. 17 -17 3 U A C 5 Met 5 A U G 3 Initiator t. RNA P site Met Large ribosomal subunit GTP GDP E m. RNA 5 Start codon m. RNA binding site 3 Small ribosomal subunit 5 A 3 Translation initiation complex

Fig. 17 -18 -4 Amino end of polypeptide E 3 m. RNA Ribosome ready for next aminoacyl t. RNA P A site 5 GTP GDP E E P A GDP GTP E P A

Polyribosomes • polyribosome (or polysome) -several ribosomes translate a single m. RNA simultaneously, allowing a cell to make many copies of a protein very quickly Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Completing the Protein • signal peptide – a short peptide added to the finished protein. ** • signal-recognition particle (SRP) – attaches to the signal peptide that it may enter the ER.

Predicting Proteins • • Predict the protein DNA = ATG CGG RNA = UAC GCC Prot. = Tyr Ala