A.  Steps of Translation
     1.  Initiation
          a)  mRNA binds to the small ribosomal subunit
          b)  mRNA "AUG" codon base pairs with tRNA anticodon "UAC" to bring the first amino acid Methionine, which starts the polypeptide
          c)  The large ribosomal subunit then binds to the complex after Methionine is in position.

               
     2.  Peptide Chain Formation
          a)  Two tRNA Ribosomal Sites
                1)  A site - location where tRNA attaches with the amino acid (A for amino acid attachment)
                2)  P site - location where tRNA attaches while holding the growing polypeptide chain (P for polypeptide)
           
           b)  Steps in Peptide Chain Formation
                1)  AUG is located at the P site where tRNA brought Methionine during initiation (It is only during initiation that the amino acid enters at the P site.  At all other times, it will enter at the A site.)
                2)  Another tRNA (whose anticodon is complementary to the second mRNA codon) attaches to the A site.
                3)  Enzymes join the two amino acids present at the ribosomal sites by forming a peptide bond.
                4)  Translocation - ribosome shifts further along the mRNA strand causing the second tRNA to now be located in the P site
           
                5)  Steps #2 - #4 continue over and over again until termination
                6)  Bacteria can add 20 amino acids every second
                7)  Speed is essential because RNases will degrade mRNA after approximately two minutes

               
     3.  Termination
           a.  This phase of protein synthesis occurs when the ribosome reaches the STOP codon on the mRNA strand.
           b.  Steps of Termination:
                1)  Ribosome encounters STOP codon
                2)  mRNA leaves the ribosome
                3)  Newly formed protein leaves the ribosome
                4)  Ribosomal subunits disassemble

** For practice with DNA Replication, RNA Transcription and Protein Synthesis, click on the link below.

B.  Prokaryotic Protein Synthesis
      1.  Overview of Process
            a.  Only certain proteins are made at any point in time.
            b.  Genes are "switched" on and off by other proteins.
            c.  Some genes are "on" all the time; whereas others are only turned "on" when needed.

      2.  Jacob and Manod - 1961 - conducted an experiment 
            a.  Grew E. Coli on glucose medium
            b.  Switched the bacteria to a lactose medium and found that the E. Coli stopped growing until it was able to produce the proteins needed to break down lactose
            c.  Showed regulation of protein synthesis in prokaryotes

      3.  Gene Regulatory Proteins - proteins that switch genes on or off by binding to DNA, thereby starting or stopping transcription
            a.  Can physically block the binding of RNA Polymerase the the Promoter region of DNA ---> Turns transcription OFF
            b.  Can help RNA Polymerase bind ---> Turns transcription ON
            c.  Gene Regulatory Proteins are very specific - they only work with certain proteins

      4.  Inducers - proteins that act as signals to turn on the production of other proteins
            a.  Lactose was the inducer in Jacob and Manod's experiment
            b.  Inducers help increase the efficiency of translation

C.  Eukaryotic Protein Synthesis
      1.  Overview of Process
            a.  When certain genes are switched on or off, they tend to stay in that "position" for a long amount of time.
            b.  A muscle cell will stay a muscle cell, and so on.

      2.  Enhancers - segment of DNA that is a binding site for gene regulatory proteins; however, it is not physically close to the Promoter region

      3.  Developmental Changes - genetically identical cells of an embryo differentiate (change) into many different types found in the adult form of the organism
            a.  Developmental changes occur as different genes are switched on and off
            b.  Metamorphosis - an abrupt change in an animal's anatomy and physiology as it develops from an immature to a mature form
                1)  Beneficial because it reduces competition between young and old organisms of the same species (Tadpole vs. Frog)
                2)  Thyroxin - hormone found in amphibians which must be present for metamorphosis to occur
                3)  Juvenile Hormone - hormone found in insects that prevents metamorphosis from occurring

D.  Cancer
       1.  Tumor - a clump of cells that grow and divide abnormally
             a.  Benign - harmless
             b.  Malignant - harmful because this type of tumor invades and destroy normal, healthy tissues

       2.  Metastasis - spreading of cancerous cells to other areas of the body; the more time a cancer is left undetected, the more metastasis can occur

       3.  Cancer-forming Process
            a.  Mutation occurs making the previously healthy cell abormal
            b.  Mutation is passed on to the cell's descendants through mitosis or meiosis
            c.  Additional mutations occur and are passed on 
            d.  One of these additional mutations can eventually form a cancerous tumor
            e.  Risk Factor - characteristic that places an individual at a higher risk for developing a disease
                1)  Risk factors differ depending on the type of cancer
                2)  Smoking - Lung, Throat, Mouth Cancers
                3)  Alcohol - Liver Cancer
                4)  Overexposure to Sun - Skin Cancer
             
       4.  Prevention Tips - applicable to ALL cancers
            a.  Eat a healthy diet
            b.  Exercise
            c.  Do NOT drink alcohol, smoke, or do drugs

** For more information on a healthy diet, click on the link below.

                               

       5.  Mutated Genes - genes commonly found in cancerous cells
            a.  Tumor Suppressor Genes - genes that normally code for proteins that prevent cell division; however, when mutated, cell division occurs more often than it should
            b.  Oncogenes - genes with the potential to cause cancer because they produce proteins which stimulate cell division; when mutated, cell division occurs more often than it should

       6.  Viruses - can contribute to causing cancer through inducing mutations
            a.  Leukemia - cancer of the white blood cells caused by a retrovirus (a virus that uses RNA to make DNA)
            b.  Liver Cancer - Hepatits B virus causes this cancer approximately 20-30 years after initial infection

       7.  Incidence in U.S.
            a.  Number 2 cause of death in U.S.
            b.  Responsible for 20% of all deaths in U.S.
            c.  Reasons why cancer incidence is increasing:
                 1)  People are living longer
                 2)  More carcinogens (cancer-causing agents) are in the environment (drugs, pollution, etc.)

       8.  Treatments
            a.  Radiation - the use of x-rays to reduce or eliminate tumor
            b.  Chemotherapy - the use of chemicals to reduce or eliminate tumor
            c.  Surgery - physical removal of tumor
            d.  Genetic Engineering and Gene Therapy - the newest treatment options to cure the cancer at the level of the DNA and gene

** For more information on cancer, click on the link below.

                               

To test your knowledge about Protein Synthesis, click on the Protein Questions Link at the top of this page.  After you answer the questions, be sure to check your responses by clicking on the Protein Answers Link.

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