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