IB Biology - 7.3 (Translation)

Translation

Process by which mRNA is decoded and a protein is produced that occurs in three stages: initiation, elongation, and termination

Initiation

- Components needed for the process are assembled - An mRNA molecule binds to the small ribosomal subunit using its 5' end - The small subunit moves along the mRNA until it reaches the start codon, AUG - The appropriate tRNA molecule binds to the codon via its anticodon - The large ribosomal subunit aligns with the tRNA molecule at the P site and binds to the small subunit to form a complex

Ribosome Structure

- Contains protein for stability - Contains rRNA for catalytic activity - Small subunit with an mRNA binding site - Large subunit with three tRNA binding sites; two tRNA molecules can bind at a time - Aminoacyl (A) binding site - Peptidyl (P) site - Exit (E) site

tRNA Activating Enzyme

- Each tRNA molecule is activated after a tRNA activating enzyme binds it with a specific amino acid - Each amino acid is recognized by a specific enzyme, thus there are 20 different tRNA activating enzymes - The enzyme contains a binding site for the specific amino acid, the specific tRNA molecule, and ATP - ATP is needed to create a high energy bond that is used to activate the molecule - The energy is stored in tRNA and used later to link the amino acid to the growing polypeptide chain

Quaternary Sructure of Proteins

- How multiple polypeptide chains along with prosthetic groups may interact through a variety of bonds to form a protein - Prosthetic groups are inorganix compounds involves in protein structure or function

Elongation

- The next codon signals another tRNA molecule to occupy the A site and pair with the codon - A peptide bond is formed between the amino acids on the P site and the A site - The tRNA molecule on the P site is deacylated (no amino acid is attached) and the tRNA molecule on the A site carries the peptide chain - The ribosome moves along the mRNA by one codon position in a 5' to 3' direction - The deacylated tRNA moves to the E site and is released while the next tRNA moves into the P site - The steps of this process are repeated until a stop codon is reached

Primary Sructure of Proteins

- The sequence of amino acids which comprise the polypeptide chain - Controls all subsequent levels of protein organization because it determines the nature between R groups of different amino acids

Secondary Sructure of Proteins

- The way a polypeptide folds in a repeating arrangement, examples including α-helices and β-pleated sheets - A result of the hydrogen bonds between an amine group of an amino acid and a carboxyl group of an amino acid at a different part of the chain - Provides the polypeptide chain with a level of mechanical stability

Tertiary Sructure of Proteins

- The way the polypeptide chain coils and turns to form a complex three-dimensional shape - This may play a role in the function of the protein - A result of interactions between R groups themselves and the surrounding water medium including the following: - Interactions between positively charged R groups with negatively charged R groups - Hydrophobic amino acids will orientate themselves to avoid water while hydrophilic amino acids will orientate themselves to come in contact with water - Polar R groups will form hydrogen bonds with each other - Two cysteine amino acids may form a disulfide bridge between them

Termination

- When the stop codon is reached it does not recruit a tRNA molecule, instead it recruits a release factor that signals for translation to stop - The polypeptide chain is released and the ribosome dissasembles back into its two independent subunits

Polysome

A group of two or more ribosomes translating an mRNA sequence simultaneously. In microscopes they appear as beads on a string; the beads representing the ribosomes and the string representing the mRNA molecule. In eukaryotes polysomes occur in both the cytoplasm and next to the ER In prokaryotes multiple polysomes can be seen associated with one gene while mRNA is still being transcribed

Protein Destinations

Different ribosomes are used to synthesize proteins depending on the destination of the protein. More commonly, translation occurs in the cytosol. Proteins that are meant to be used in the cytoplasm, mitochondria and chloroplasts are synthesized by ribosomes found free in the cytoplasm. Proteins that are meant to be used in the ER, Golgi apparatus, lysosomes, plasma membrane, or outside the cell are synthesized by ribosomes that become bound the rER.

Transcription and Translation in Prokayrotes

In prokaryotes cellular functions are not compartementalized like they are in eukaryotes. In eukaryotes some time passes by between the end of transcription for modifications before translation begins. In prokaryotes translation occurs immediately after transcription

tRNA Structure

Molecules fold into a cloverleaf structure and contain: - Sections that become double-stranded and create loops - A 3'-CCA base sequence that an amino acid can attach to - An anticodon as part of a loop of seven unpaired bases - A loop associated with the binding sites in ribosomes - A loop associated with the tRNA activating enzyme