Transcription, mRNA processing & Translation (ch16)
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- Transcription is done by RNA polymerase enzyme.
- -Copies only one strand of the DNA double helix to make RNA.
- -This strand is now a template
- -Genes have three parts.
- -Transcribed region
- -Controls the transcription
- -The enzyme (RNA polymerase) that makes RNA binds to the promoter with the help of other proteins known as transcription factors.
- -transcription complex formed.
- -Starts copying gene
What two regions are important in the prokaryotes?
- -Around -10 region
- -Around the -35 region
Numbering of Nucleotides
- Nucleotides downstream get + numbers and upstream get - numbers.
- -Promoters get - numbers.
- Binds to the promoter, and opens the double helix
- -RNA polymerase copies the template strand of DNA
-RNA polymerase reaches terminator, the transcription ends.
- -Its around the -30 region
- -transcription factor protein called TATA binding protein binds to the box.
- -Helps RNA polymerase to bind to the promoter.
mRNA in Eukaryotes
- -Included in the nucleus
- -vefore mRNA comes out of the nucleus, they remove introns and rejoins exons by splicing .
- -makes mRNA shorter than the gene it was copied from.
- -cap is added to the 5' end of the mRNA.
- -protects it from damage
- -directs ribosomes towards the 5' end
poly (A) tail
- -added to the 3' end
- -protects the 3' end from damage.
- -Once mRNA comes to the cytoplasm, ribosomes start translating them.
- -3 steps of translation
Initiation of Translation
- -small subunit of ribosome finds the mRNA, binds to a region close to the first ATG with the help of initiation factor proteins.
- -tRNA arrives to start the AUG codon
- -Binds to the AUG start codon with help of more initiation factors.
- -large subunit of ribosomes binds to this complex
- -Use energy from GTP to help these processes.
Elongation of Translation
- -start moving down the mRNA
- -there are 3 sites for tRNA binding
- -A site, P site, and E site
binds the incoming tRNA that brings the amino acid
The amino acid carried by it makes the peptide bond with the amino acid that is attached to the tRNA at the P site.
- When the ribosomes movies, the tRNAs shift positions.
- -The first tRNA is now pused to the E-site, where it exits the ribosomes.
- -A site opens up, next amino acid binds .
- -process continues until ribosome reaches a stop codon.
Termination of Translation
- -When ribosome reaches the stop codon, there is no tRNA for that.
- -protein called release factor vinds to the A-site.
- -This dissociated the mRNA from the ribosome, the two subunits separate, and the protein chain is released.
- -This terminates the translation
- -More ribosomes bind to the 5' side of the mRNA
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