Protein Biosynthesis Flow of genetic informations from DNA to protein: The genetic information of the cell is stored and transmitted in the nucleotide sequences of DNA
Expression of this genetic informations involves two stages:- • The first stage is transcription to form mRNA that carries specific and precise messages in the form of codons from DNA to the cytoplasmic sites of protein synthesis. • The second stage is translation of the nucleotide sequence of a mRNA (Codons) into an amino acid sequence of a protein. • Each codonconsists of a sequence of 3 nucleotides i.e. it is a triplet code. Collection of these codons makes up the genetic code.
Components of translational process:- • m RNA as a carrier of genetic information. • t RNA as an adapter molecule, which recognizes an amino acid on one end and its corresponding codon on the other end. • Ribosomes as the molecular machine coordinating the interaction between mRNA, tRNA, the enzymes and the protein factors required for protein synthesis.
Genetic Code: • mRNA contain 4 different nucleotides (A, G, C, and U). Each amino acid is specified by one or more codon (nucleotide triplets). Thus, there are 43codons (i.e. 64 code words). • Three codons (UAA, UAG, UGA) do not specify any aminoacid, and function as the stop (termination or non-sense) codons in protein synthesis. The remaining 61 codons code for the 20 amino acids used in protein synthesis. • There is one start codon (initiation codon), AUG, coding for methionine ( Met).
Characters of genetic code: • Genetic code is degenerate i.e. multiple codons can code for the same amino acid except tryptophan and methionine. The 3rd nucleotide in a codon is less important than the other two in determining the specific amino acid to be incorporated (Wobble theory). • Genetic code is unambiguousi.e. each codon specifies no more than one amino acid. • Genetic code is non-overlappingand Commalessi.e. the reading of the genetic code does not involve any overlap of codons, and the message is read in a continuing sequence of nucleotide triplets until a termination sequence is reached. • Genetic code is universali.e. the same code words are used in all organisms (pro- and eukaryotes) with exception in mitochondria.
Protein Biosynthesis: It can be described in 3 phases; initiation, elongation and termination. The protein sequence is synthesized and read from the amino terminus to the carboxy terminus. I. Initiation: For initiation of protein biosynthesis, there must be:- - tRNA - rRNA - mRNA - Eukaryotic initiation factors (eIFs). - GTP, ATP and different amino acids.
tRNA charging It means recognition and attachment of the specific amino acid to the 3` hydroxyl adenosine terminus ( to the sugar) of tRNA in an ester linkage.
Initiation involves 4 steps:- 1.Ribosomal dissociation: The 80 S eukaryotic ribosome is dissociated into 40 S and 60 S subunits. eIF –3 and eIF-1 bind to 40 S subunit thus preventing reassociation between the 2 subunits . 2. Formation of pre initiation complex: The first step in this process involves the binding of GTP by eIF-2. This binary complex then binds to Met – tRNA (a tRNA specifically involved in binding to the initiation codon AUG). This ternary complex binds to the 40 S ribosomal subunit to form preinitiation complex.
3. Formation of 48 S initiation complex: mRNA binds to the preinitiation complex to form the 48 S initiation complex with hydrolysis of ATP to ADP + Pi. This initiation complex scans the mRNA for a suitable initiation codon. 4.Formation of 80 S initiation complex: The binding of 60 S ribosomal subunit to the 48 S initiation complex involves the hydrolysis of the GTP bound to eIF-2. This reaction results in the release of the initiation factors bound to 48 S initiation complex (these factors are recycled) and rapid association of 40 S and 60 S subunits to form the 80 S ribosome. At this point, Met – tRNA is on the peptidyl (P) site of ribosome, ready for the elongation cycle to commence.
II. Elongation: It is a cyclic process involving 3 steps 1. Binding of aminoacyl – tRNA to the A site: In the complete 80 S ribosome formed during initiation, the aminoacyl (A) site is free. The binding of the proper aminoacyltRNA in the A site requires:- • Proper codon recognition. • Activation of aminoacyltRNA by binding of eukaryote elongation factor - 1 (e EF-1) and GTP (ternary complex). When aminoacyltRNA binds to A site, GTP is hydrolysed and e EF-1 is released.
2. Peptide bond formation: The -amino group of the new aminoacyltRNA in the A site attacks the carboxyl group of the peptidyl- tRNA occupying the P site. This reaction is catalyzed by peptidyltransferaseof the 60 S subunit (this is an example of ribozyme activity). The reaction results in attachment of the growing peptide chain to the tRNA in the A site.
3- Translocation: Upon removal of the peptidyl moiety from the tRNA in the P site, the discharged tRNA quickly dissociates from the P site. eEF-2 and GTP are responsible for the translocation of the newly formed peptidyltRNA at the A site into the empty P site. The translocation of the newly formed peptidyltRNA and its corresponding codon into the P site then frees the A site for another cycle of amino acyl – tRNAcodon recognition and elongation. The formation of one peptide bond requires energy resulting from hydrolysis of 4 high energy phosphate bonds:- Charging of tRNA with amino acyl moiety requires hydrolysis of an ATP to an AMP. The entery of amino tRNA into the A site requires one GTP hydrolysis to GDP. The translocation of the newly formed peptidyl – tRNA in the A site into the P site results in hydrolysis of one GTP to GDP.
Diagrammatic representation of the peptide elongation process of protein synthesis the small labeled n-1,n,n+1, etc represent the amino acid
III. Termination: After many cycles of elongation, the non-sense or termination codon of mRNA (UAA, UAG or UGA) appears in the A site. Normally, there is no tRNA with an anticodon capable of recognizing such a termination signal. Releasing factors (eRFS) can recognize the termination signals in the A site. eRF, GTP and peptidyltransferase promote the hydrolysis of the bond between the peptide and the tRNA occupying the P site. Then the 80 S ribosome dissociates into 40 S and 60 S subunits and mRNA, tRNA, eRF, GDP and Pi are released.
Diagrammatic representation of the termination process of protein synthesis
Cell death Dr. hassan el-banna
Introduction: • The normal cell is confined a fairly narrow range of functions and structure and can be able to handel normal physiologic demands. • If the cell expose to severe physiologic stresses or pathologic stimuli it undergoes a number of physiologic and morphologic cellular adaptations.
If the limits of adaptive response to a stimulus are exceeded, a sequence of events follows that is termed cell injury. • Cell injury may be reversible or irreversible. If the stimulus presists the cell reaches a “point of no return “ and suffers irreversible cell injury and ultimately cell death.
There are 2 different mechanisms of cell death which are necrosisand apoptosis. 1- Necrosis= accidental cell death. • It is a pathological process. • It is occurs due to various unfavorable factors: - Hypoxia. - Radiation. - Pathogens. ( e.g. Viruses )
Necrosis characterized by : • Injury and damage to the plasma membrane. • Severe cell swelling and cell rupture. • Denaturation or coagulation of cytoplasmic proteins. • Breakdowen of cell organelles. • Extensive surrounding tissue damage (inflammation).
2- Apoptosis = programmed cell death. • It is a physiological process during which cell can initiate their own death. • It is controlled by several genes during embryonic development. • It occurs in the following conditions: • In normal embryonic development by removal of excess cells that serve no function during morphogenesis and for determination of organ size. Apoptosis also eliminates developmentally defective cells in the immune system.
2.Hormone-dependent involution in the adult, such as endometrial cell breakdowen during the menstrual cycle, regression of lactating memmary gland after weaning and pregnant uterus after delivery and regression of prostate in old age. 3.To maintain the organ size and function in adult. 4.To get rid of cells whose cell cycle is interrupted or its DNA is damaged beyond repair.