PROTEIN SYNTHESIS

PROTEINS Proteins are the basic building materials of a cell, though the basic structure of proteins is linear, they are usually folded and folded again into complex structures. Different proteins perform different functions. Like DNA, proteins are polymers, that is, complex molecules made up of simple subunits.

PROTEINS THE PROTEIN CONTENT OF THE CELL A ‘typical' mammalian cell, for example a liver hepatocyte, is thought to contain 10 000 – 20 000 different proteins, representing approximately 0.5 ng of protein or 18–20% of the total cell weight. The copy numbers of individual proteins vary: < 20 000 molecules per cell for the rarest types. 100 million copies for the commonest ones. N.B. >50 000 per cell is considered to be relatively abundant.

PROTEIN SYNTHESIS Protein synthesis inside the cells is a complicated biochemical process, begins with transcription, the separation of a DNA molecule into two strands, a section of one strand acts as a template to produce a new strand called messenger RNA. The mRNA leaves the cell nucleus and attaches to the ribosomes,

Ribosome

RIBOSOMES Specialized cellular structures that are the sites of protein synthesis. Amino acids are carried to the ribosomes by transfer RNA (tRNA) (translation); the amino acids are linked together in a particular sequence, dictated by the mRNA, to form a protein. Activation of amino acids is carried out by a two step process catalyzed by aminoacyl-tRNA synthetases and requires energy in the form of ATP.

PROTEIN SYNTHESIS Translation proceeds in an ordered process. 1- Accurate and efficient initiation occurs. 2- Chain elongation. 3- Accurate and efficient termination.

Translation

Termination

PROTEIN SYNTHESIS

Direction of translation

PROTEOME The proteome is the final product of genome expression and constitute all the proteins present in a cell at a particular time, It is considered as the central link between the genome and the cell.

Protein Synthesis Inhibitor: Many of the antibiotics utilized for the treatment of bacterial infections as well as certain toxins function through the inhibition of translation. Inhibition can be affected at all stages of translation from initiation to elongation to termination.

Protein Synthesis Inhibitor: Several Antibiotic and Toxin inhibitors of Translation: Chloramphenicol: inhibits prokaryotic peptidyl transferase Cycloheximide: inhibits eukaryotic peptidyl transferase Diptheria toxin catalyzes ADP-ribosylation of and inactivation of eEF-2 Erythromycin: inhibits prokaryotic translocation through the ribosome large subunit

Protein Synthesis Inhibitor: Fusidic acid: similar to erythromycin only by preventing EF-G from dissociating from the large subunit Neomycin: similar in activity to streptomycin Puromycin: resembles an aminoacyl-tRNA, interferes with peptide transfer resulting in premature termination in both prokaryotes and eukaryotes Ricin: found in castor beans, catalyzes cleavage of the eukaryotic large subunit Rrna Streptomycin: inhibits prokaryotic peptide chain initiation, also induces mRNA misreading Tetracycline: inhibits prokaryotic aminoacyl-tRNA binding to the ribosome small subunit

THE GENETIC CODE Genetic code is degenerate, unambiguous, none overlapping The genetic code consists of 64 triplets of nucleotides. These triplets are called codons. Genetic code is required to account for all 20 amino acids found in proteins. A two-letter code would have only 42 = 16 codons, which is not enough to account for all 20 amino acids, whereas a three-letter code would give 43 = 64 codons. The 64 codons fall into groups, the members of each group coding for the same amino acid.

THE GENETIC CODE Degeneracy all amino acid are coded by two, three, four or six codons except tryptophan and methionine have just a single codon each. The code also has four punctuation codons, which indicate the points within an mRNA where translation of the nucleotide sequence should start and finish. The initiation codon is usually 5′-AUG-3′, which also specifies methionine (so most newly synthesized polypeptides start with methionine), with a few mRNAs other codons such as 5′-GUG-3′ and 5′-UUG-3′ are used. The three termination codons are 5′-UAG-3′, 5′-UAA-3′ and 5′-UGA-3′; these are sometimes called amber, opal and ochre, respectively.

THE GENETIC CODE The code is not uambiguous because a given codon designates only one amino acid. One codon, AUG serves two related functions: It signals the start of translation. It codes for the incorporation of the amino acid methionine (Met) into the growing polypeptide chain. The genetic code can be expressed as either RNA codons or DNA codons:

THE GENETIC CODE RNA codons: Occur in messenger RNA (mRNA) and are the codons that are actually read during the synthesis of polypeptides. But each mRNA molecule acquires its sequence of nucleotides by transcription from the corresponding gene.

THE GENETIC CODE The DNA Codons: (genes at the level of DNA): These are the codons as they are read on the sense (5' to 3') strand of DNA. Except that the nucleotide thymidine (T) is found in place of uridine (U), they read the same as RNA codons. However, mRNA is actually synthesized using the antisense strand of DNA (3' to 5') as the template.

REFRENCES&FURTHER READING GenetiCCode:en.wikipedia.org/wiki/Genetic_code Genetic Code: www.accessexcellence.org/AB/GG/genetic.html http:/www.med.uottawa.ca/patho/devel/index.html Protein Synthesis: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Codons.html#rna_codons

REFRENCES&FURTHER READING http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookPROTSYn.html

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