Syllabus Transcription as the production of mRNA from DNA. The role of RNA polymerase. The splicing of pre-mRNA to form mRNA in eukaryotic cells.

Polypeptide synthesis – transcription and splicing Aqa p.224-5

Objectives How is pre-messenger RNA produced from DNA in the process called transcription? How is pre-messenger RNA modified to form messenger RNA?

Protein synthesis Genetic information flows from DNA  mRNA  protein made Code for protein is in sequence of bases on DNA TRANSCRIPTION Code is copied to pre-mRNA as complementary bases Pre-mRNA Spliced To form mRNA TRANSLATION mRNA used as a template on a ribosome to add amino acids in the correct order Protein !!

Bakery analogy What is the recipe book? What are the ingredients? What is the cake? How is DNA replication likened in the analogy? Limitations of this analogy…

Transcription of DNA

Transcription slide from sser

Transcription 1.What is made in this process? 2.Where in the cell does it occur? 3.What does DNA helicase do? 4.What does RNA polymerase do? 5.How much of the DNA is unzipped at any one time? 6.How does RNA polymerase know when to stop?

Transcription 1.What is made in this process? Pre-mRNA 2.Where in the cell does it occur? Nucleus 3.What does DNA helicase do? Break H bonds uncurls and unzips DNA (breaks H bonds between complementary base pairs) 4.What does RNA polymerase do? Moves along the template (sense/coding) strand joining complementary ribonucleotides by condensation putting in covalent bonds to make the sugar phosphate backbone 5.How much of the DNA is unzipped at any one time? 12 base pairs 6.When RNA polymerase meets a stop codon it detaches

Transcription and codons During transcription, the mRNA is built up by complementary base pairing, using the DNA as a template. The DNA’s base triplets are converted into complementary mRNA codons. What are the codons in the mRNA transcribed from this sequence of DNA base triplets? DNA mRNA T A CG C AG A TT A C A U GC G UC G UC U AA U G

Processing of mRNA A molecule called a spliceosome removes the introns, producing mature mRNA that contains only exons. Before splicing, mRNA is known as pre-mRNA. DNA contains some regions that do not code for proteins. These are known as introns. To produce functional proteins these introns need to be spliced out of the mRNA, leaving only the regions that code for proteins, called exons. intron exons spliceosome intron exon mature mRNA pre-mRNA

Splicing of pre-mRNA What is an exon? What is an intron? Which type of cells need introns removing? Splicing = rejoining exons How does mRNA then leave the nucleus?

Splicing of pre-mRNA What is an exon? Coding DNA What is an intron? Non coding DNA needs removing from pre-mRNA Which type of cells need introns removing? eukaryotic Splicing = rejoining exons How does mRNA then leave the nucleus? Through nuclear pore

Split gene for a protein – Exons contain the protein code and introns are non-coding TRANSCRIPTION Pre- mRNA transcript in which exons and introns have been transcribed Functional messenger RNA POST TRANSCRIPTIONAL MODIFICATION Catalytic RNA molecules that function like enzymes, move along the primary transcript cutting out the introns and splicing the exon sequences together TRANSLATION The final protein product is synthesised during translation