syllabus transcription as the production of mrna from dna. the role of rna polymerase. the splicing...
TRANSCRIPT
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 synthesisGenetic information flows from DNAmRNA protein made
Code for protein is in sequence of bases on DNA
TRANSCRIPTIONCode is copied to pre-mRNA
as complementary bases
Pre-mRNASplicedTo form mRNA
TRANSLATIONmRNA 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-mRNA2. Where in the cell does it occur? Nucleus3. 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 C G C A G A T T A C
A U G C G U C U A A 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 exonexon
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 aprotein – Exonscontain the protein code and introns are non-coding
TRANSCRIPTIONPre- mRNAtranscript in which exons and introns have been transcribed
Functional messenger RNA
POST TRANSCRIPTIONALMODIFICATION
Catalytic RNA molecules that function like enzymes, move along the primary transcript cutting out the introns and splicing the exon sequences together
TRANSLATIONThe final protein productis synthesised during translation