Transcription, RNA Processing, & Translation Sections 9.3 – 9.5 Transcription, RNA Processing, & Translation

9.3 RNA Synthesis (Transcription) General Info Produces molecules of RNA One strand of DNA acts as the template for RNA synthesis (C ↔ G; A → U; T → A) Happens in the nucleus. Main enzyme = RNA polymerase, which adds the RNA nucelotides to pair with the DNA Transcription occurs in 3 stages.

9.3 RNA Synthesis (Transcription) 3 Stages 1. Initiation - RNA polymerase binds to the DNA at the promoter region. It is directed to this region by proteins called initiation factors.

9.3 RNA Synthesis (Transcription) 3 Stages 2. Elongation - RNA polymerase moves along the coding strand of DNA away from the promoter, building the RNA strand. - RNA polymerase adds the RNA nucleotides to their complements on the coding strand (A now bonds to U), forming the primary transcript.

9.3 RNA Synthesis (Transcription) 3 Stages 3. Termination - RNA polymerase reaches the terminator region of the DNA & is released from the DNA with the primary transcript. - The primary transcript is considered unfinished, and now needs to undergo processing before a protein can be made.

Transcription Animation

9.4 RNA Processing General Info All 3 types of RNA (mRNA, tRNA, & rRNA) are processed in the nucleus before they leave it. This can involve adding, chemically changing, and/or removing nucleotides.

9.4 RNA Processing All RNA transcripts undergo the following: Enzymes attach a methyl-guanine (mG) cap to the starting end of the RNA molecule.

9.4 RNA Processing All RNA transcripts undergo the following: Other enzymes add 100-200 adenine nucleotides to the other end (called the poly-A tail). These 2 additions help transport RNA out of the nucleus & protect the RNA from enzymes found in the cytoplasm.

9.4 RNA Processing All RNA transcripts undergo the following: All RNA transcripts also undergo splicing. The removal of meaningless segments of RNA, called introns, and fusing together the remaining, meaningful sections, called exons. Requires precise recognition of the site to be cut.

After the 3 major changes, what happens to the RNA if it becomes…. 9.4 RNA Processing After the 3 major changes, what happens to the RNA if it becomes…. mRNA → it is completed. tRNA → the molecule is folded into a cloverleaf shape. rRNA → proteins are added to it to form the ribosome of a cell.

Cap & Tail Animation

9.5 Translation (Protein Synthesis) General Info The sequence of mRNA is translated into the amino acid sequence of a protein. It happens on ribosomes in the cytoplasm. tRNA acts as a carrier molecule that brings specific amino acids to the ribosome. A tRNA anticodon matches with an mRNA codon. (They are complementary) ATP provides the energy to form the bonds. Divided into same 3 stages as Transcription: initiation, elongation, & termination.

9.5 Translation (Protein Synthesis) Initiation Ribosomal subunits bind to the mRNA at the start codon (AUG).

9.5 Translation (Protein Synthesis) Elongation tRNA molecules pick up their specific amino acids & bring them to the ribosome. Each tRNA molecule’s anticodon is complementary to a codon on the mRNA. The amino acid from the previous tRNA is joined to the next amino acid via a peptide bond. This continues & the polypeptide chain grows.

9.5 Translation (Protein Synthesis) Elongation (3 sites) A site: The next site in line for arrival of a tRNA. P site: This is where the newly arriving amino acid will join with the previously arrived amino acid by a peptide bond. E site: After a tRNA has dropped off the amino acid, it will exit & go pick up another amino acid.

9.5 Translation (Protein Synthesis) Termination Elongation continues until a codon is reached for which there is no complementary anticodon (no tRNA). No amino acid arrives & the polypeptide chain ends. This is called a stop codon (UAA, UAG, UGA). Instead of tRNA, a protein called a release factor, knocks the protein factory apart, releasing the ribosome, mRNA, & the polypeptide. The polypeptide is then used to build a functional protein for the organism.

Translation Animation