Translation Proteins are made by joining amino acids into long chains called polypeptides (proteins). Each polypeptide contains a combination of any or all of the 20 different amino acids.

Translation The properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides. The “language” of mRNA instructions is called the genetic code.

Translation RNA contains four different bases: Adenine = A Uracil = U Cytosine = C Guanine = G

Translation These four letters carry instructions for the 20 different amino acids. This genetic code is read three letters at a time; so each “word” of the coded message is three bases long. Each three-letter “word” in mRNA is known as a codon. A codon consist of three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide.

Translation Example sequence: mRNA: UCGCACGGU mRNA code: UCG-CAC-GGU Amino Acids: Serine – Histidine - Glycine

Translation Because there are four different bases, there are 64 possible three-based codons: 4 x 4 x 4 = 64

Translation Some amino acids can be specified by more than one codon. Example: 6 different codons specify for the amino acid leucine. There is also one codon, AUG, which codes for the amino acid Methionine. This is also the codon that serves as the “start codon” for protein synthesis

Translation There are also three stop codons that do not code for any amino acid. Stop codons act like a period at the end of a sentence

Translation The sequence of nucleotide bases in an mRNA molecule serves as the instructions for the order in which amino acids should be joined together to produce a polypeptide. The structures that assemble the polypeptides from these instructions are called Ribosomes. This process of decoding the mRNA message into a polypeptide chain (protein) is called translation. Translation takes place on Ribosomes.

Step-by-Step Translation Before translation can occur, messenger RNA (mRNA) must first be transcribed from DNA in the nucleus and released into the cytoplasm.

Step-by-Step Translation Translation begins when the mRNA molecule in the cytoplasm attaches to a ribosome (Initiation). As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome and attached to the growing polypeptide chain.

Step-by-Step Translation The ribosome does not know which amino acid to match to each codon. That is the job of the transfer RNA (tRNA).

Step-by-Step Translation Each tRNA has an amino acid attached to one end of the molecule and a region of three unpaired bases on the other

Step-by-Step Translation The three bases on the tRNA molecule, called the anticodon, are complimentary to one of the mRNA codons.

Step-by-Step Translation Like an assembly line worker who attaches one part to another, the ribosome forms a peptide bond between the first and second amino acids (Elongation). At the same time, the ribosome breaks the bond that held the first tRNA molecule to its amino acid and releases the tRNA molecule. The ribosome then moves to the third codon, where a tRNA molecule brings it the amino acid specified by the third codon.

Step-by-Step Translation The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule. When the ribosome reaches a stop codon, it releases the newly formed polypeptide and the mRNA molecule, completing the process of translation (Termination).

The Roles of RNA and DNA You can compare the different roles played by DNA and RNA molecules in directing protein synthesis to the two types of plans used by builders.

The Roles of RNA and DNA A master plan has all the information needed to construct a building. But builders never bring the valuable master plans to the building site, where it might be damaged or lost. Instead they make an invaluable copy of the master plans called blueprints.

The Roles of RNA and DNA The master plans stay safely stored in the office, and the blueprints are taken to the job site. Similarly, the cell uses the vital DNA “master plan” to prepare RNA “blueprints”. The DNA molecule remains safely in the nucleus, while the RNA molecules go to the protein-building sites in the cytoplasm – the ribosomes.

Genes and Proteins Gregor Mendel might have been surprised to learn that most genes contain nothing more than instructions for assembling proteins. He might of asked what proteins could possibly have to do with the color of flowers, the shape of the leaves, a human blood type, or the sex of a newborn baby. The answer is that proteins have everything to do with these things.

Genes and Proteins Remember that many proteins are enzymes, which catalyze and regulate chemical reactions. A gene that codes for an enzyme to produce pigment can control the color of a flower.

Genes and Proteins Another enzyme-specifying gene helps produce a red blood cell surface antigen. This molecule determines your blood type.

Genes and Proteins Genes for certain proteins can regulate the rate and pattern of growth throughout an organism, controlling its size and shape. In short, proteins are the keys to almost everything that living cells do.