Steps of Transcription and Translation Flashcards
Three steps of transcription
- Initiation
- Elongation
- Termination
When does transcription begin?
- RNA polymerase molecules attach to a promoter region of DNA
*Promoter regions upstream of the desired gene to transcribe
Eukaryotes transcription
-Promoter region is called TATA box
-Transcription factors help RNA polymerase bind
Prokaryotes transcription
-RNA polymerase can bind directly to the promoter
1st step DNA elongation transcription
RNA polymerase opens the DNA and reads the triplet code of the template strand
*moves in the 3’ to 5’ direction
*mRNA transcript elongates 5’ to 3’
2nd step DNA elongation transcription
*RNA polymerase moves downstream, only opens small sections of DNA at a time
*Growing mRNA strand peels away from the DNA template strand. DNA double helix then reforms
*Single gene can be transcribed simultaneously by several RNA polymerase molecules
-Helps increase the amount of mRNA synthesized
-Increases protein production.
Termination in prokaryotes
-Transcription proceeds through a termination sequence
-Causes termination signal
-RNA polymerase detaches
-mRNA transcript is released and proceeds to translation
-mRNA does not need modifications
modifications for pre-mRNA before translation
- 5’ Cap
- Poly-A-tail
- RNA splicing
5’ cap (GTP)
5’ end of pre-mRNA receives a modified guanine nucleotide “cap”
poly-A-tail
3’ end of the pre-mRNA receives 50-250 adenine nucleotides
Poly-A-tail and 5’ cap function to:
- Help the mature mRNA leave the nucleus
- Help protect the mRNA from degradation
- Help ribosomes attach to the 5’ end of the mRNA when it reaches the cytoplasm
RNA splicing
Sections of the pre-mRNA, called introns, are removed and then exons are joined together
Introns
intervening sequence, don’t code for amino acids
Exons
expressed sections, code for amino acids
Why does splicing occur?
a single gene can code for more than one kind of polypeptide , known as alternative splicing