Joey Levy

AP Bio

In transcription DNA is transcribed into Messenger RNA (mRNA). In translation the mRNA is translated into a polypeptide chain with the aid of Ribosomal RNA (rRNA) and Transfer RNA (tRNA). This polypeptide chain is then folded into a unique protein used in a particular cell function.

Transcription occurs in the cell nucleus where DNA is housed. Think of DNA as instructions to build hardware (proteins), unfortunately, these instructions are in another language and incomprehensible to the workers that will eventually assemble the hardware. This is where mRNA will come into the picture - to provide new instructions that will be used by the workers. In transcription, DNA is unzipped and the enzyme RNA polymerase runs along the template strand of the DNA. The template strand of DNA can be identified by finding the nucleotide sequence T A C (If the strand is written backwards it may look like C A T). This identifies that strand as the template and the other strand, the information strand, will not be used in this transcription. As the RNA polymerase runs along the DNA template strand it will add the complementary RNA nucleotides to the DNA nucleotides. This means that G will be paired with C, and visa versa, and A (DNA) will be paired with U (RNA - rather than T in DNA replication) and T (DNA) paired with A (RNA). When the single helix mRNA strand is complete it will separate from the DNA and the DNA will re-zip into the double helix. The completed mRNA strand has groups of three nucleotides known as codons. These groups of three will code for a particular amino acid in translation.

Translation occurs when the mRNA strand moves out of the nucleus and into the cytoplasm. At this point mRNA, rRNA and tRNA all come together. The rRNA consists of two parts, the large ribosomal unit and the small ribosomal unit. On the large ribosomal unit are two sites- the A site and the P site. These will be the sites of polypeptide synthesis and elongation. The rRNA is like the factory of translation and if rRNA is the factory than tRNA is the worker. The tRNA molecules have an amino acid attachment site and it also carries an anticodon. The anticodon is the complementary nucleotide sequence to a given codon. The tRNA will pick up the appropriate amino acid in the cytoplasm that is coded for by the mRNA codon that its anticodon matches. It is like a lock and key process.

Differences between transcription and translation: Translation uses the mRNA together with rRNA and tRNA to make the protein. While transcription is the when mRNA takes the DNA instructions to build that protein. Similarities between transcription and translation: Both processes use mRNA. Needs more info!!

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1) 5' Capping: Capping of the pre-mRNA involves the addition of 7-methylguanosine (m7G) to the 5' end. The cap protects the 5' end of the primary RNA transcript from attack by ribonucleases that have specificity to the 3'5' phosphodiester bonds.

2) 3' Processing: The pre-mRNA processing at the 3' end of the RNA molecule involves cleavage of its 3' end and then the addition of about 200 adenine residues to form a poly(A) tail. As the poly (A) tails is synthesized, it binds multiple copies of poly (A) binding protein, which protects the 3'end from ribonuclease digestion.

3) Splicing: RNA splicing is the process by which introns, regions of RNA that do not code for protein, are removed from the pre-mRNA and the remaining exons connected to re-form a