1. Protein Synthesis
Transcription

2. Transcription
Initiation
Elongation
Termination

3. Transcription- Initiation
The enzyme, RNA Polymerase, binds to the segment of DNA that is to be transcribed
Once, RNA Polymerase binds, it opens the double helix and transcription begins
This occurs both in prokaryotes and eukaryotes
DNA transcription is not a random phenomenon
If RNA polymerase randomly transcribed DNA, the cell would not make the correct polypeptide

4. Transcription- Initiation
RNA polymerase binds to the DNA molecule upstream of the gene to be transcribed
Upstream: Region of DNA adjacent to the start of a gene
The upstream region is a sequence on one strand of DNA located adjacent to the start of the gene

5. Upstream Region
This region indicates where the RNA polymerase should start transcribing and which DNA strand should be transcribed
This upstream region is know as the promoter
In most genes, the promoter consists of a characteristic base-pair pattern

6. Promoter
Promoter region in DNA is high in adenine and thymine bases, which serve as a recognition site for RNA Polymerase
A=T share only 2 hydrogen bonds as compared with C and G that share 3 H bonds
It takes less energy to break to 2 bonds instead of 3

7. Transcription-Elongation
Elongation starts as soon as the RNA Polymerase moves to the start of the gene and binds to the promoter
It then opens up the double helix and starts building the single-stranded mRNA in the 5'-3' direction
Unlike the case in DNA replication, the RNA polymerase does not require a primer to start building the complementary strand

8. Transcription- Elongation
Note: The promoter itself does not get transcribed
The process of elongation of the RNA transcript is similar to that of DNA replication
RNA uses only one of the strands of DNA as a template for mRNA synthesis
This chosen DNA strand is called the template strand
The strand not used for transcription is the coding strand

9. Know your strands DNA Replication mRNA Transcription
Done by DNA polymerase
DNA synthesized in the 5'-3' direction
Leading strand: strand synthesized continuously, using the 3'-5' template strand as its guide and is build toward the replication fork
Lagging strand: strand synthesized discontinuously in the opposite direction to replication fork
mRNA Transcription
Done by RNA polymerase
mRNA synthesized in the 5'-3' direction
Template strand: strand of DNA that RNA polymerase uses as a guide to build complementary mRNA
Coding strand: strand of DNA that is not used for transcription and is identical in sequence to mRNA except it contains thymine instead of uracil* (p.242)

10. Know your strands
Both DNA and mRNA are synthesized in the 5'-3' direction

11. Transcription- Termination
mRNA is synthesized until the end of the gene is reached
Terminator sequence: RNA Polymerase recognizes the end of the gene when it comes across this sequence
Then, the newly synthesized mRNA dissociates with the DNA template strand
Transcription stops and RNA Polymerase is free to bind to another promoter region and transcribe another gene

12. mRNA Synthesis

13. Posttranscriptional Modifications:
In eukaryotes, mRNA cannot leave the nucleus directly following transcription
mRNA must be modified to primary transcript
A 5' cap is added to the start of the primary transcript

14. Structure of the 5' cap
7-methyl guanosine forming a modified guanine nucleoside triphosphate
Part of mRNA transcript
5’ end of mRNA transcript

15. 5' cap Added to the start of primary transcript
Function: protects the mRNA from digestion by enzymes such as nucleases and phosphatases as it exits the nucleus and enters cell cytoplasm
Plays a role in initiation of translation

16. Further mRNA Modificatin:
Poly-A tail
Enzyme poly-A polymerase involved in furhter modification of mRNA
Enzyme responsible for adding a string of ~ 200 adenine ribonucleotides to the 3' end of mRNA
This string is known as the poly-A tail
This is done to protect the mRNA from degradation later on

17. Capping and Tailing
Addition of a 5' cap and a poly-A tail to the primary transcript is a process known as capping and tailing

18. Further Modification
DNA of a eukaryotic gene is made of various regions
Exons: Segments of DNA that code for part of a specific protein
Introns: Non-coding regions of a gene
The introns are interspersed among the exons

19. Introns and Exons
Primary transcript contains both introns and exons, i.e, it contains both regions that code and those that do not code for part of the protein
If the introns (non-coding regions) are translated, the protein will not fold properly causing the proteins to be dysfunctional and useless to the cell
Therefore, before the primary transcript leaves the nucleus, the introns are removed

20. Spliceosomes Particles made of RNA and protein
They cut introns from mRNA primary transcript
And join together the remaining coding exon regions
mRNA exits the nucleus, but the introns stay, where they get degraded and recycled

21. mRNA transcript Primary transcript gets capped
Primary transcript gets tailed
The introns get excised
Primary transcript transformed to mRNA transcript
mRNA transcript exits nucleus into the cytoplasm
mRNA transcript ready to be translated by a ribosome into a protein

22. mRNA is not error proof
Unlike DNA replication, there is no quality control enzyme to ensure that the mRNA transcript is correct
Therefore, more errors are made during transcription than translation
Errors are not as detrimental as those occurring during DNA replication, since a single gene is transcribed repeatedly to make hundreds of transcripts