1. SBI 4U: Metablic Processes
TRANSCRIPTION
Copying of the DNA code for a protein into RNA
4 Steps:
Initiation
Elongation
Termination
MODIFICATION
Section 1.3

2. SBI 4U: Metablic Processes
INITIATION
RNA polymerase binds to the DNA molecules UPSTREAM from the gene to be transcribed.
Known as a promoter region
Usually high in A and T
This is because there are only 2 H bonds b/w them, therefore it costs less energy for RNA polymerase to open up the DNA helix here
RNA polymerase can only bind at this site to get started, it does not recognize any other sequence.
Section 1.3

3. SBI 4U: Metablic Processes
ELONGATION
Once bound to the DNA, RNA polymerase opens the DNA and starts building mRNA in the direction of 5' to 3'.
No primer is needed to start building.
The promoter does not get transcribed.
RNA polymerase uses one strand of DNA as its template for mRNA synthesis (TEMPLATE STRAND)
CODING STRAND: the strand not used for the template
The mRNA will be complementary to the template strand and identical (except for Us in place of Ts) to the coding strand.
Section 1.3

4. SBI 4U: Metablic Processes
TERMINATION
RNA polymerase reaches a terminator sequence
mRNA dissociates from the DNA, called primary transcript
RNA polymerase is free to bind with another promoter
In order to prevent the mRNA from being degraded as it exits the nucleus, a few modifications need to be made to the primary transcript.
Section 1.3

5. POSTTRANSCRIPTIONAL MODIFICATION
SBI 4U: Metablic Processes
POSTTRANSCRIPTIONAL MODIFICATION
CAPPING: A 5' cap is added to the start (a 7-methyl guanosine to form a modified guanine nucleoside triphosphate). The cap plays a role in protection and also in the initiation of translation.
TAILING: a string of ~200 adenosine nucleotides are added to the 3' end by poly-A polymerase to create a poly-A tail.
In eukaryotic DNA further modifications are made.
In our DNA we have coding regions (EXONS) and non-coding regions (INTRONS).
The introns sit between the exons, so a primary transcript will have parts that don't code for protein.
Introns need to be removed before translation or the new protein will not fold properly.
Spliceosomes (RNA and proteins) remove the introns and join the exons together.
Section 1.3

6. SBI 4U: Metablic Processes
Section 1.3

7. SBI 4U: Metablic Processes
QUALITY CONTROL
Once the modifications have been made the primary transcript is known as the mRNA transcript.
It can now leave the nucleus.
*Unlike DNA replication, there is no proofreading enzyme, therefore more errors are made in transcription than replication.
Not a big problem, since hundreds of transcripts are being made (vs. one copy of DNA) and therefore enough protein will be synthesized.
Section 1.3