1. Transcription and Translation HL 2014!
Lets do this!!!

2. 7.3. Transcription
Protein Synthesis occurs in two stages: transcription and translation
Transcription
Occurs in the nucleus
Purpose is to create a molecule of messenger RNA

3. 7.3
carried out in a 5’3’ direction (the 5’ end of the free RNA nucleotide is added to the 3’ end of the RNA molecule which is already synthesized) (7.3.1)
DNA is read 3’  5’
No helicase is needed in this process RNA polymerase breaks the H bonds in DNA and repairs them
RNA polymerase binds to parts of the DNA called promoters in order for separation of DNA strands to occur

5. Promoters
Promoter region is piece of DNA that is non-coding but has a very important function
TATA box –
TATAAAA on 3’5’ strand
ATATTTT on 5’  3’ strand
This signals the start of transcription
Transcription proceeds as nucleosidetriphosphates (type of nucleotide) bind to the DNA template and are joined by RNA polymerase in the 5' to 3' direction.

6. 7.3
Transcription ends when RNA polymerase reaches a termination site on the DNA
When it reaches the terminator, the RNA polymerase releases the RNA strand.
The two DNA strands are labeled as “sense” and “anti-sense”

7. The sense strand is the non-template strand and has the same base sequence as the mRNA (with uracil instead of thymine).
The antisense strand is the template strand (strand being transcribed) and has the same base sequence as tRNA

8. In eukaryotes, some “editing” must occur in order to produce a mature strand of mRNA
After RNA has been transcribed enzymes(splicesome) will “cut” out the introns (useless sequences of repetitive nucleotides”junk DNA”)
“glue” the exons (portions that code for protein ”expressed”) back together

10. Viruses In some viruses (i.e. HIV) RNA is the primary nucleic acid
DNA must be transcribed from RNA in order to enter the host cells genome
The enzyme that helps to catalyze this process is called “reverse transcriptase”

11. Reverse transcriptase can be used for biotechnology
for example, it can make DNA from a section of mature mRNA that codes for the gene for human insulin
this DNA section can then be spliced into host DNA (eg E. coli) without the introns
That bacteria will then create human insulin

12. Prokaryotes
Transcription in Prokaryotes is slightly different than transcription in Eukaryotes due to the fact that they have a circular chromosome
The best of example of prokaryotic transcription is the lac operon model:
operons are found only in prokaryotes and are sections of the chromosome that code for a particular gene or set of genes as well as a promoter and an operator region (which will be explained momentarily).

13. These three genes are part of one operon – the lac operon
E. Coli bacteria use three enzymes to break down lactose (milk sugar).
Lac Z
Lac Y
Lac A
These three genes are part of one operon – the lac operon
A regulator gene, located outside of the operon, codes for a repressor protein that binds to the operator region

14. Binding of the repressor to the operator prevents RNA polymerase from binding to the promoter region and thus prevents transcription of the three lac enzyme genes
However, an isomer of lactose will bind to the repressor protein, and change its shape so it doesn't fit with the operator
This allows for the production of the three enzymes
Therefore, if there is no lactose (and therefore no isomers of lactose), the repressor protein will prevent the transcription of the enzymes

16. Translation Translation Takes place on the ribosomes
Free ribosomes (in the cytoplasm) synthesize proteins for use primarily within the cell
ribosomes on the rough endoplasmic reticulum synthesize proteins for secretion or for lysosomes

17. Translation occurs in the 5’-3’ direction
during translation, the ribosome moves along the mRNA towards the 3’ end. The start codon is nearer to the 5’ end than the stop codon.
Translation consists of three stages: initiation, elongation, and termination

18. Ribosomes consist of two subunits -light and heavy (sometimes called the 30s and 50s subunits respectively)
the light piece
composed of one rRNA and 33 different proteins
the heavy piece
composed of three different types of rRNA and 45 different protein molecules

19. Initiation
After transcription is complete, mRNA goes to the cytoplasm and attaches its 5' end to the small subunit of the ribosome
AUG is called the start codon because it initiates the translation process
remember: codon is a base triplet on the mRNA strand

20. mRNA inserts itself in between the two ribosomal subunits
There are three binding sites for tRNA on the ribosome:
A site (for tRNA binding)
P site (for peptide bonding)
E site (exit site)

23. The anticodon on one end of a tRNA molecule is complimentary to a specific codon on the mRNA

24. The codon AUG hydrogen bonds to the anticodon of a tRNA molecule holding the amino acid methionine (called the initiator tRNA).
The initiator tRNA is now in the ‘A’ site
As the ribosomal subunits slide over mRNA the initiator tRNA moves from the ‘A’ site to the ‘P’ site

25. Elongation
Another tRNA (let's call it tRNA "X") carrying a specific amino acid attaches itself to the next codon at the A site of the larger subunit
these two amino acids (methionine and the amino acid on X) now make a peptide bond with each other
The initiator tRNA lets go of methionine, moves to the E site, then leaves the ribosome

27. At the same time, tRNA X moves from the A site to the P site
Another tRNA molecule (tRNA "Y") attaches to the codon in the A site
The amino acid that is attached to tRNA Y makes a peptide bond with the amino acid from tRNA X

29. tRNA X lets go of it’s amino acid, moves to the E site and leaves the ribosome
tRNA Y moves to the P site to make room for the next tRNA to enter the A site
The process repeats itself

30. Termination
The stop codon is one that does not code for an amino acid and that terminates the translation process
the polypeptide is released and the mRNA fragments return to the nucleus
These nucleotides are recycled and used for RNA and DNA synthesis
tRNA also is returned to its free state and attaches to its specific amino acid so as to be ready for the translation process when needed.

32. Polysomes Many ribosomes can translate the same mRNA at the same time
This allows for multiple proteins to be created in a very short time from the same piece of mRNA.

33. tRNA specificity
Each amino acid has a specific tRNA-activating enzyme that helps tRNA to combine with its complimentary mRNA codon.
the enzyme has a 3-part active site that recognizes three things:
a specific amino acid
ATP
A specific tRNA

34. The enzyme attaches the amino acid to the 3' end of the tRNA.
The amino acid attachment site is always the base triple CCA.
It is important to note that each tRNA molecule can attach to one specific amino acid, but an amino acid can have a few tRNA molecules with which is can combine