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Transcription and Translation

Transcription and Translation

Author: Jessica Libby
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Transcription begins when RNA polymerase separates the DNA strands at the promoter (DNA sequence) and begins added RNA nucleotides in the 5' to 3' direction. It finishes it job when it reaches the terminator (DNA sequence).


The first stage of transcription is initiation. Transcription factors help RNA polymerase II bind to the promoter in eurkaryotes (not needed in prokaryotes). Elongation is the next step and this is where RNA nucleotides are added as the DNA is read. Termination is last and this when a terminator portion of DNA is read. The mRNA is released. Eukaryotes add a 5' cap and poly-A tail to the mRNA before it is complete. The 5' cap allows the mRNA to attach to the ribosome. It is a guanine and two phosphate groups: GTP. The poly-A tail helps stabalize the mRNA as it leaves the nucleus. It is added to the 3' end and is made up of close to 200 adenine nucleotides. The mRNA in eurkayotes is also spliced; this removes introns, which are parts of the DNA that do not code for proteins, and leaves only exons. 



Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print., W.W. Norton Company, . Close Up of Transcription. 2006. Photograph. n.p. Web. 5 Jan 2014. .


Translation occurs at the ribosome where mRNA is read and tRNA carries in the corresponding amino acids to form a protein. There are three binding sites on the ribosome for the tRNA. The P site hold the tRNA carrying the protein chain, the A site holds the tRNA carrying the next amino acid and the E site is where the tRNA exits.



The first stage of translation is initiation. The start codon on mRNA (AUG) binds to a small subunit on the ribosome, the tRNA anticodon UAC binds to the codon. This carries the amino acid methionine. After this a large subunit attaches and the anticodon carrying the methionine moves to the P site and a new anticodon attaches to the A site with the next amino acid.


The second stage is elongation. During this anticodons continue to attach to the codons and rRNA forms peptide bonds between the amino acids located on the P and A sites. The tRNA on the A site oves to the Pstie and the tRNA on the P site (now empty) leaves through the E site. 


The last stage is termination. This is when a stop codon is read and a release factor binds to it. This ends translation and the protein is released form the ribosome. 

Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print., Pearson Education, . Ribosomal Structure. 2012. Photograph. n.p. Web. 5 Jan 2014. ., Pearson Education, . Translation. 2012. Photograph. n.p. Web. 5 Jan 2014. .