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
Translation
- The synthesis of polypeptides using info from the mRNA
- Occurs in the ribosome
- Nucleotide sequence becomes an amino acid sequence
- tRNA is a key player in translating mRNA to an amino acid sequence
Transfer RNA
- tRNA has an anticodon region-complementary and antiparallel to mRNA
- tRNA carries the amino acid that the mRNA codon codes for
- Enzyme aminoacyl-tRNA synthetase responsible for attaching amino acids to tRNA
-when a tRNA carries an amino acid its “charged”
Ribosomes and translation
-Translation occurs at the ribosome
-ribosomes have two subunits: small and large
-Prokaryotic and eukaryotic ribosomal subunits differ in size
The large subunit has three sites
A site: amino acid site, holds the next tRNA carrying an amino acid
P site: polypeptide site
-holds the tRNA carrying the growing polypeptide chain
E site: Exit site
How does the tRNA interact with the mRNA?
anticodon and codon interactions
Translation 3 steps
- Initiation
- Elongation
- Termination
Elongation translation
-The next tRNA comes into the A site
-mRNA moved through ribosome and its codons are read
-each mRNA codon codes for a specific amino acid
When does translation begin? (initiation)
When the small ribosomal subunit binds to the mRNA and a charged tRNA binds to the start codon, AUG, on the mRNA
-tRNA carries methione
-next the large subunit binds
Elongation translation steps
- Codon recognition- appropriate anticodon of the next tRNA goes to the A-site
- Peptide bond formation-Peptide bonds are formed that transfer the polypeptide to the A-site
- Translocation- tRNA in A site moves to the P and then to the E site.
