Protein Synthesis 12-3

2 Steps of Protein Synthesis Transcription Translation

Transcription DNA is coded (transcribed) into the RNA “language” The DNA sequence is copied into a complementary RNA sequence G  C C  G T  A A  U

Transcription Transcription requires RNA polymerase, an enzyme a lot like DNA polymerase RNA polymerase binds to DNA, separates the strands, and uses one strand as a template for making mRNA. RNA polymerase “polymerizes” (links together) the nucleotides that make up mRNA

Where to Start? RNA polymerase doesn’t randomly bind to DNA. It will only bind to sites known as promoters—regions of DNA that have specific sequences. Promoters are signals that say “START RNA HERE!” Similar signals also tells transcription to stop.

RNA Editing Some RNA molecules need to be edited before they are ready for making proteins. Sometimes large sequences must be removed so that the RNA can make a functional protein. These intervening sequences are called introns. The remaining portions or expressed sequences are called exons. They are spliced back together to form the final draft of mRNA.

The Genetic Code There are 20 different amino acids found in nature. Proteins are made by joining amino acids into long chains called polypeptides. The genetic code is read 3 letters at a time. Each set of 3 nucleotides is called a codon. Each codon specifies a specific amino acid.

The Genetic Code There are 64 different codons. Some amino acids have more than one codon. The START codon is AUG. It occurs at the beginning of EVERY protein. There are 3 STOP codons—UAA, UAG, and UGA.

Translation mRNA carries the coded message to the ribosome. The message must then be decoded or translated so the ribosome can build the protein. During translation, the ribosome uses information from mRNA to produce proteins. Translation is carried out by tRNA and rRNA.

Translation Each tRNA molecule has an amino acid attached to one end and 3 nucleotide bases at the other end. The 3 nucleotide bases attached to tRNA are called an anticodon because they are complementary to the codon for that amino acid. Example: codon = AAG anti = UUC (AAG codes for Lysine)

Translation tRNA can then give the correct amino acid to the ribosome. The ribosome, which is made of rRNA, attaches the amino acids together to make the polypeptide chain (protein).

DNA, RNA, and Proteins DNA is like the master copy. RNA is like the blueprint the workers use. The workers are ribosomes. Proteins are the finished product. Proteins are the keys to almost EVERYTHING that living cells do!