Protein Synthesis Transcription, RNA processing & Translation

An Overview Overview web video

An Overview

Background Evidence (2.6.6) George Beadle and Edward Tatum formulated the 1 gene - 1 enzyme hypothesis. – By growing mutant fungi in different media, they were able to conclude that genes held instructions for the production of essential enzymes in the metabolic pathways of Neurospora fungi.

–Over time it became obvious that all enzymes are proteins, but not all proteins are enzymes. So, the 1 gene - 1 enzyme hypothesis was revised to the 1 gene - 1 protein hypothesis. –Finally, scientists concluded that many functional proteins are actually made up of more than one polypeptide chain. So, the 1 gene - 1 protein hypothesis was revised to the 1 gene - 1 polypeptide hypothesis. –Each gene on a DNA molecule codes for one polypeptide (many amino acids) chain, which then becomes a protein, or part of a protein.

Final revisions We now know that with alternative RNA processing, some genes can code for more than one polypeptide, depending on which introns are removed. We also have knowledge of tRNA molecules and rRNA in ribosomes. SO, a gene is a region of DNA whose final product is either a polypeptide or an RNA molecule.

Transcription Transcription is the production of a mRNA molecule from the DNA code. RNA nucleotides are added in the 5’ to 3’ direction each one base pairing with the complimentary nucleotide on the DNA strand (6.3.1).

DNA and RNA DNA –Double Stranded –Contains Deoxyribose as its 5-carbon sugar (pentose). –Contains the four bases Adenine, Thymine, Cytosine, Guanine RNA –Single stranded –Contains Ribose as its 5-carbon sugar –Contains the four bases Adenine, URACIL, Cytosine, Guanine Uracil replaces thymine, and hydrogen bonds with adenine.

6.3.4 DNA is composed of two complimentary strands, running in opposite directions. When discussing transcription, these are referred to as the “sense” and “anti-sense” strands. The Sense strand is the one with the actual information, but the anti-sense strand is the one that is transcribed. The result is that the mRNA transcript has the same nucleotide sequence as the sense strand of DNA, with Uracil instead of Thymine.

6.3.3 Transcription begins when RNA polymerase binds to the promoter region of the anti-sense DNA strand and unwinds the DNA. RNA polymerase then moves down the DNA, adding nucleoside triphosphates in the 5’ to 3’ direction. Animation 17B.2 and 17B.3Animation 17B.2

Try it... Sense Strand ATG TTT CGC GTC TTG TACAAA GCG CAGAAC Anti-sense strand (spaces are not there ;-))

6.3.3 Elongation of the mRNA transcript continues until the terminator region is reached. The terminator signals the end of transcription, and causes the completed mRNA transcript to be released. Animation 17B.4

6.3.3 Each new RNA nucleotide comes in the form of a nucleoside triphosphate, consisting of a nitrogenous base, a ribose sugar, and three phosphate groups. RNA polymerase uses the energy released by the removal of the two terminal 5’ phosphate groups to catalyze the polymerization. Animation 17B.5

RNA Processing (6.3.5) The completed mRNA transcript needs to be modified before the actual synthesis of the polypeptide can proceed. There are many details to this process, but all we need to know is that non-coding regions of the mRNA called “Introns” (short for interfering regions) are removed, and the remaining “Exons” (expressed regions) are linked together. Animation 17C

RNA Processing (6.3.5)

Translation Translation is the conversion of the nucleotide sequence on the mature mRNA into a polypeptide. It occurs in the cytoplasm, and is carried out by ribosomes. During translation, the mRNA molecule is read, tRNA molecules deliver amino acids, and rRNA in the ribosome catalyzes the elongation of the polypeptide.

The Universal Genetic Code The genetic code is the same in all living cells. The mRNA is “read” in three letter “words” called codons. Each codon calls for a specific amino acid to be added to the polypeptide change.

Initiation of Translation Translation begins when the small ribosomal sub- unit binds to the 5’ end of the mRNA. Then the initiator tRNA with the amino acid methionine binds to the start codon. Finally, the large ribosomal sub-unit binds with the initiator tRNA in the P-site, and the A-site is ready for the next tRNA. Animation 17D.3

Elongation of Translation In elongation of translation, a new tRNA with an anti-codon matching the codon in the A-site, brings the next amino acid. Part of the ribosome catalyzes the formation of a new peptide bond. The ribosome translocates = the tRNA from the A- site moves to the P-site with the growing polypeptide chain. The A-site is now ready for the next tRNA with its associated amino acid. Animation 17D.4

Termination of Translation Elongation continues until termination. There are three stop codons that do not call for an amino acid but instead signal the end of translation. When a stop codon enters the A-site, a protein release factor binds to the A-site, causing the polypeptide to be released, and the ribosomal sub- units to dissociate. Animation 17D.5

Loose Ends

tRNA activation Each tRNA has an activating enzyme (aka = aminoacyl tRNA synthetases) that attaches the correct amino acid. Since the genetic code is universal, these enzymes are the same in all living things.

Free Ribosomes and Polysomes Proteins destined for use inside the cell are synthesized by free ribosomes. Many ribosomes can simultaneously translate a single mRNA. That’s called a polysome.

Bound Ribosomes Proteins bound for export, or for packaging into lysosomes, are translated by ribosomes bound to the rough endoplasmic reticulum.