Biology 1-1c Protein Synthesis

1-1c: Protein Synthesis Proteins are a polymer made up of a chain of monomers called amino acids There are 20 different types of amino acids

Where are proteins found? Structure of cells Cell membranes and cellular organelles Muscle filaments (myosin, etc.) Hair (keratin) and hair colour Enzymes Antibodies and hormones Fingernails (keratin) Skin (collagen) Cartilage Ligaments Eye cornea Etc.

1-1c: Protein Synthesis Basically, the production of proteins is the way that the genetic information contained in your DNA becomes expressed in your cells. For example, your DNA contains a gene that codes for the protein responsible for eye colour. When your cells produce that protein, it builds the cells in your eye in the appropriate way and the appropriate colour!

So how do we go from a code in DNA (i.e. a gene) to a protein?

1-1c: Protein Synthesis We’ll break this down, step-by-step…

How can 4 nucleotide bases code for 20 amino acids??? 1-1c: Protein Synthesis An amino acid is a building block of proteins, just as a nucleotide is a building block of DNA. However, unlike the 4 types of nucleotides there are 20 types of amino acids How can 4 nucleotide bases code for 20 amino acids???

1-1c: Protein Synthesis The nucleotides are read in a triplet code. It takes 3 nucleotides to code for 1 amino acid All the different possible triplet combinations of the 4 nucleotides allows enough variety to code for the 20 amino acids

1-1c: Protein Synthesis Also encoded within the DNA are triplet codes that indicate where the instructions for the protein start and stop. So now that we know how the instructions work, let’s figure out how our cells read them and produce a protein!

Transcription & Translation Creating a protein requires a few steps We will break protein synthesis into two main steps: Transcription & Translation

Where does this all occur? Our DNA is found in the nucleus of our cells so the first step of the process, transcription, occurs within the nucleus. The organelles that actually build the proteins, the ribosomes, are found in the cytoplasm of our cells. The second step, translation, occurs outside of the nucleus in the cytoplasm of our cells. OUR DNA CAN NEVER LEAVE THE NUCLEUS, SO HOW DOES THE MESSAGE GET TO THE RIBOSOME?

Transcription – the basics First of all the two sides of the DNA strand need to unzip so the code can be read We know that DNA is very important to our cells, so we can’t risk damaging it. So our cells copy the DNA code into an mRNA (messenger RNA) segment that is small enough to leave the nucleus through a nuclear pore. The main differences between RNA and DNA are the sugar used (ribose versus deoxyribose) and the fact that RNA uses a nucleotide called Uracil (U) in place of Thymine (T)

Transcription – the details An enzyme called RNA polymerase binds to the promoter sequence on DNA This enzyme causes the DNA strand to begin to uncoil As it uncoils, nucleotides floating in the nucleus use the now single strand of DNA as a template, creating a strand of mRNA When the RNA polymerase reaches the terminator sequence on the DNA strand, it releases the mRNA strand.

1-1c: Protein Synthesis The completed mRNA strand now leaves the nucleus through a nuclear pore and encounters a ribosome in the cytoplasm of the cell

Translation – the basics Once in the cytoplasm, the mRNA is attached to by a ribosome The ribosome “reads” the triplet code in the mRNA, and creates the appropriate chain of amino acids. Once it reaches the end of the code, it releases the mRNA and the completed protein.

Translation – the details Once in the cytoplasm, the mRNA is attached to by a ribosome The ribosome “reads” the triplet code in the mRNA. Each set of 3 nucleotides is known as a codon The “start codon” indicates where the ribosome should begin reading the code. It is always the sequence AUG and it codes for the amino acid methionine

Translation – the details tRNA (transfer RNA) is a molecule that picks up amino acids that are circulating within the cytoplasm and shuttles them to the mRNA on the ribosome One end of the tRNA is attached to an amino acid, the other end contains an anticodon that is the opposite sequence to the codon on the mRNA.

Starting at the start codon the ribosome moves along the mRNA strand, selecting tRNA molecules with the appropriate anticodons to the mRNAs codons The tRNA will place its amino acid onto the growing amino acid chain and the ribosome will fuse the amino acids together. When the stop codon is reached (either UAA, UAG or UGA) it signals the ribosome to release both the mRNA strand and the completed amino acid chain

1-1c: Protein Synthesis The completed polypeptide chain will be the new protein!