7.4+Translation

= __** 7.4  **__**  __Translation__   ** = **7.4.1 Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy. The shape of tRNA and the CCA at the 3’ end are important.**

** Orange book  ****  à  ****  pg. 65 **  **  Green book  ****  à  ****  pg. 125 **  At the 3’ end, the amino acid will bind to the tRNA. Each amino acid has its own tRNA with a unique anticodon. The amino acid is bound to the tRNA in a two step process catalysed by its own tRNA-activating enzyme: As there are 20 different amino acids, there will also be 20 different tRNA-activating enzymes. The different tRNA-activating enzymes will recognize the correct tRNA to bind to the correct amino acid. The structure of each tRNA is unique, allowing only the correct tRNA (with the correct anticodon) to be matched with the correct amino acid. The CCA sequence at the 3’ end of the tRNA molecule forms a covalent bond with the appropriate amino acid (which is determined by the anticodon) by a condensation reaction. The –OH of the –COOH group of the amino acid will react with the –OH of adenine at the 3’ end of the tRNA, forming water and leaving an oxygen to connect the two molecules. During translation, the bond between the tRNA and the amino acid is broken when the amino acid forms a peptide link with the growing polypeptide chain.
 * 1)  The amino acid will react with ATP and become activated. The ATP loses its energy in this process.
 * 1)  The activated amino acid will then bind to the acceptor stem of its own tRNA.

** 7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites. **


 * Orange book ****  à  ****  pg. 65  **
 * Green book ****  à  ****  pg. 126  **

Cells can have thousands of ribosomes, eukaryotic cells even tens of thousands. Ribosomes measure approximately 25 nm in diameter. Ribosomes are found in both prokaryotic and eukaryotic cells but have a slightly different structure in each and are larger in eukaryotic cells than in prokaryotic cells. Ribosome are measured by their size and density in units ‘S’. Prokaryotic ribosomes and ribosomes of chloroplasts nad mitochondria are 70S, whereas eukatyotic ribosomes are 80S. ** Ribosomal RNA (rRNA) **

 · rRNA, together with proteins, form ribosomes, which are the site of mRNA translation and protein synthesis.  · Ribosomes have two subunits, small and large, and are assembled in the nucleolus of the nucleus and exported into the cytoplasm. The smaller subunit is made of one mlecule of rRNA and some proteins, the larger subunit is made of two molecules of rRNA and some proteins. · The smaller subunit has the binding site for mRNA, the larger subunit has the binding sites (known as the A, P and E sites) for tRNA.  · Ribosomes free in the cytoplasm make proteins for use in the cell, while those attached to the RER make proteins for export.

** 7.4.3 State that translation consists of initiation elongation, translocation and termination. **


 * Orange book ****  à  ****  pg. 66-67  **
 * Green book ****  à  ****  pg. 126  **

see powerpoint 'on the bridge'

** 7.4.4 State that translation occurs in a 5’ à 3’ direction. The start codon is at the 5’ end. **


 * Orange book ****  à  ****  pg. 66-67  **
 * Green book ****  à  ****  pg. 126  **

The start codon on the mRNA (AUG) which codes for the amino acid methionine is at the 5’ end of the mRNA. The ribosome will initiate translation here and move along the mRNA in a 5’ à 3’ direction, elongating the polypeptide chain as it goes. This means that translation will take place in a 5’ à 3’ direction and the termination sequence will be towards the 3’ end of the mRNA.

** 7.4.5 Draw and label a diagram showing the structure of a peptide bond between two amino acids **

** Orange book  ****  à  ****  pg. 47 **
 * Green book ****  à  ****  pg. 127  **

In a protein the polypeptide chain may be hundreds of amino acids long. Amino acid polymerisation to form polypeptides is part of protein synthesis. It takes place in ribosomes, and is special because it requires an RNA template. The sequence of amino acids in a polypeptide chain is determined by the sequence of the genetic code in DNA.
 * ﻿ [[image:peptide_bond.png]] **

** 7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons and stop codons. **

** 7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes. **

** green book p148  **

The distrubition of ribosomes depends on the function of the protein they make; if the proteins are to be used inside the cell, the ribosomes tend to be found throughout the cytoplasm; if the protein is to be exported (secretion) or used by lysosomes, the ribosomes are generally associated with the endoplasmic reticulum. These proteins will then enter the lumen of the RER as they are produced and from there move to the Golgi apparatus where they are modified and packaged into vesicles.