Transcription: Making mRNA Flashcards
The code on DNA must be…
Transcribed into mRNA before mRNA can be translated into protein
mRNA Synthesis, 4 Major Steps
- Initiation
- Elongation
- Termination
- mRNA modification
Where does transcription occur in eukaryotes?
In the nucleus
Initiation, general explanation
- Transcription begins at a “Promoter”
What is a “Promoter”?
A sequence of DNA that marks the beginning of transcription
Prokaryotes, Promoter
- RNA binds to a regular Promoter where RNA polymerase attaches and initiates transcription
- No TATA box
Eukaryotes, Promoter
- TATA box marks the beginning of Transcription as part of the Promoter
Step 1 of Eukaryotic Initiation
Transcription factors bind to DNA at the TATA box
Step 2 of Eukaryotic Initiation
RNA Polymerase II binds to the TATA box
Step 3 of Eukaryotic Initiation
- More transcription factors are added
- Creates a transcription initiation complex
Step 4 of Eukaryotic Initiation
Transcription begins
What is a factor?
Small assisting proteins which we don’t need to name
Elongation, simple definition
Lengthening RNA
Elongation Step1
DNA is unwound to form a transcription bubble
Elongation Step 2
RNA polymerase synthesizes mRNA in the 5’ to 3’ direction on the DNA template
Termination Step 1
RNA Polymerase II reaches a transcription termination site from the stop codon
Termination Step 2
Transcription stops
Termination Step 3
mRNA dissociates from the template
Termination Step 4
DNA rewinds
Termination Step 5
mRNA is released by polymerase
mRNA Modification Step 1
- 5’ CAP is placed on the 5’ end of mRNA
Functions of 5’ CAP
- Protects mRNA from nucleases and phosphates
- Attachment signal for ribosomes
mRNA Modification Step 2
Poly-A Tail is placed on 3’ end of mRNA
Functions of Poly-A Tail
- Protects 3’ end from degradation
- Helps ribosomal attachment
- Helps with exporting mRNA out of nucleus
mRNA modification Step 3
Introns are spliced out
What are introns?
Non-coding sequences that are transcribed but not translated
What are exons?
Sequences of mRNA that are important and are translated into protein
How do you know what areas are to be spliced out?
They are marked off by “GU-AG”
What are splicing factors?
Act as markers so a splicing zone can be created
What is a spliceosome?
It pinches and cuts out the introns
What are the two exceptional circumstances in splicing?
- Incorrect Splicing
- Alternate Splicing
Incorrect Splicing
- Splicing may be inaccurate
- Creates mRNA that is not translated or wrong segments are translated
- Could lead to defective proteins
Alternate Splicing
- Deliberate
- By varying areas of splicing more than one possible protein can be made from one gene
Example of Incorrect Splicing
In humans: Beta-Thalassemia
- Causes errors in hemoglobin
