Thursday 8th March COPY LO: To be able to describe polypeptide synthesis including transcription and splicing Starter Explain the difference between transcription and translation
BATS Describe and explain how pre-messenger is RNA produced from DNA in the process called transcription Describe and explain how pre-messenger RNA is modified to form messenger RNA
Why does mrna have less nucleotides than the DNA section it is complementary to? The RNA that is made from the DNA coding strand is called pre-mrna To understand this we have to start with Transcription - there is another step between transcription and the formation of mrna
pre-mrna is processed before it becomes the mrna that finally goes to the ribosome. We will look at this in detail
Transcription - these details are all in your textbooks 1. The enzyme DNA helicase acts on a specific region of the DNA molecule to break the hydrogen bonds between the bases, causing the two strands to separate and expose the nucleotide bases in that region.
Transcription - these details are all in your textbooks 2. The enzyme RNA polymerase moves along one of the two DNA strands, known as the template strand, causing the nucleotides on this strand to join with the individual complementary nucleotides from the pool in the nucleus
Transcription - these details are all in your textbooks 3. So an exposed guanine base on the DNA is linked to the cytosine base of a free nucleotide etc. Remember adenine links to uracil rather than thymine
Transcription - these details are all in your textbooks 4. As the RNA polymerase adds the nucleotides one at a time to build a strand of premrna, the DNA strands rejoin behind it. Only about 12 base pairs of the DNA are exposed at a time.
Transcription - these details are all in your textbooks 5. When RNA polymerase reaches a stop triplet code on the DNA - it detaches itself and the production of pre-mrna is complete
Splicing of pre-mrna - what does splicing mean? DNA is made up of sections called exons and introns Exons code for proteins Introns - do not - think of them as intervals between genes.
If the introns were left in they would integer with the synthesis of a polypeptide In eukaryotic cells - these nonfunctioning introns are removed from the pre-mrna The functioning exons are then joined together in a process called splicing.
The exons can be re-joined in lots of different ways This means that a single section of DNA (gene) can code for up to a dozen different proteins depending on the order in which the exons are recombined.
What might affect the splicing of Mutations Certain disorders such as Alzheimer s disease are the result of splicing failures that lead to non-functional polypeptides being made. Where does splicing take place? pre-mrna?
Now do the following: 1. Answer summary questions on page 225 of text book. 2. Worksheet Protein Synthesis
Homework I will be taking this in next lesson - I want to see a real step-up in effort/research/attempts to find out specific information. 1. Read Pages 226-227 of textbook 2. Make detailed notes on the stages of Polypeptide synthesis - you MUST be able to name specific enzymes and molecules in your answers. 3. Answer Summary questions on Page 227 4. Do all applications on Pages 228-229
http://www.youtube.com/watch?v=ztpkv7wc3yu http://www.youtube.com/watch?v=njxobgkpeao&feature=fvsr http://www.youtube.com/watch?v=41_ne5ms2ls http://www.youtube.com/watch?v=bpl4dgvgnck Splicing http://www.youtube.com/watch?v=4x8ek15r8yy Splicing http://www.youtube.com/watch?v=fvuawbgw_pq - Splicing http://www.youtube.com/watch?v=qmbtqppu6jy - Splicing
Other useful Sources http://www.biologymad.com/master.html?http://www.biologymad.com/a2biology.htm The best A Level Biology Website References Glenn Toole, Susan Toole (2008). A2 Biology. Cheltenham: Nelson