The initial RNA transcript is spliced into mature mRNA Exon Intron Exon Intron Exon DNA Transcription Addition of cap and tail Cap RNA transcript with cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence Nucleus Cytoplasm

Eukaryotic cells modify RNA after transcription Enzymes in the nucleus modify pre-mRNA before the genetic messages are sent to the cytoplasm At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the ______. This 5’ cap helps protect mRNA from hydrolytic enzymes and functions as an “attach here” signal for ribosomes 5’ cap

At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the __________. The poly(A) tail also seems to help with the export of mRNA from the nucleus. It also inhibits hydrolysis and helps ribosome attachment to the mRNA. The mRNA molecule also includes nontranslated leader and trailer segments. poly(A) tail

RNA Splicing Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides. Noncoding segments, introns, lie between coding regions. The final mRNA transcript includes coding regions, exons, that are translated into amino acid sequences, plus the leader and trailer sequences. “intervening sequences” “expressed sequences”

RNA splicing removes introns and joins exons to create an mRNA molecule with a continuous coding sequence RNA splicing appears to have several functions. First, at least some introns contain sequences that control gene activity in some way. Splicing itself may regulate the passage of mRNA from the nucleus to the cytoplasm. One clear benefit of split genes is to enable a one gene to encode for more than one polypeptide. Alternative RNA splicing gives rise to two or more different polypeptides, depending on which segments are treated as exons.

From nucleic acid to protein: Translation

The mechanics of translation: In the cytoplasm, three players come together… The mRNA molecule which carries the information in its codons. Another type of RNA molecule, transfer RNA (______), matches up the right amino acid with the right mRNA codon. The ribosome – composed of two subunits – is the structure which helps the mRNA and tRNA match up properly. It also catalyzes the synthesis of the new peptide (covalent) bonds between the amino acids. The ribosome is made of ________ and ______. tRNA proteins rRNA

tRNA tRNA Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon

How tRNA anti-codons match up with mRNA codons Ser

Let’s look at the genetic code again… What tells translation to start and stop? mRNA Codon Chart

Translation initiation Met Met Large ribosomal subunit Initiator tRNA P site A site Start codon mRNA Small ribosomal subunit 1 2

Translation, cont. Amino acid Polypeptide P site A site Anticodon mRNA Codons 1 Codon recognition

Amino acid Polypeptide P site A site Anticodon mRNA Codons 1 Codon recognition 2 Peptide bond formation

Amino acid Polypeptide P site A site Anticodon mRNA Codons 1 Codon recognition 2 Peptide bond formation New peptide bond 3 Translocation

Amino acid Polypeptide P site A site Anticodon mRNA Codons 1 Codon recognition mRNA movement Stop codon 2 Peptide bond formation New peptide bond 3 Translocation

Protein Synthesis, from Transcription to Translation DNA Transcription 1 mRNA is transcribed from a DNA template. mRNA RNA polymerase Amino acid Translation Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. 2 Enzyme ATP tRNA Anticodon Initiator tRNA Large ribosomal subunit Initiation of polypeptide synthesis 3 The mRNA, the first tRNA, and the ribo- somal sub-units come together. Start Codon Small ribosomal subunit mRNA New peptide bond forming Growing polypeptide 4 Elongation A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. Codons mRNA Polypeptide 5 Termination The ribosome recognizes a stop codon. The poly- peptide is terminated and released. Stop codon

polypeptide synthesis Transcription DNA mRNA is transcribed from a DNA template. 1 mRNA RNA polymerase Amino acid Translation Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. 2 Enzyme ATP tRNA Anticodon Initiator tRNA Large ribosomal subunit Initiation of polypeptide synthesis 3 The mRNA, the first tRNA, and the ribosomal sub-units come together. Start Codon Small ribosomal subunit mRNA

to the polypeptide chain as the mRNA is moved through the ribosome, New peptide bond forming Growing polypeptide 4 Elongation A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. Codons mRNA Polypeptide 5 Termination The ribosome recognizes a stop codon. The polypeptide is terminated and released. Stop codon

Let’s review translation… http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html# (Click on Protein Synthesis Link) http://www.stolaf.edu/people/giannini/biological%20anamations.html http://www.biostudio.com/d_%20Transcription.htm http://www.dnalc.org/resources/3d/15-translation-basic.html (realistic animation, real-time) http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html http://www.biostudio.com/demo_freeman_protein_synthesis.htm