Protein Synthesis

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					                           Protein Synthesis

Function of DNA

  • The bases in the DNA molecule act as a code
  • Each triplet of bases code for one amino acid
  • Definitions: codon – a triplet of bases
                 Amino acids – basic building blocks of proteins
  • DNA cannot leave the nucleus of the cell – how does the DNA code
    get from the nucleus to the ribosomes in the cytoplasm where proteins
    are made?
  • The code of DNA is transferred to RNA (ribonucleic acid)
  • RNA “carries” the message outside the nucleus to the ribosomes
    where the proteins are made
  • Differences between DNA and RNA
              ! RNA is single stranded
              ! Ribose sugar
              ! Uracil replaces thymine
              ! Travels out of nucleus
              ! Three types of RNA

  Three types of RNA
  • Ribosomal RNA (r-RNA)
          Combines with protein to make ribosomes

  • Messenger RNA (m-RNA)
         Carries instructions for protein synthesis from DNA to

  • Transfer RNA (t-RNA)
          - A short molecule with three exposed nitrogen bases which are
          complementary to the codons on the mRNA. These three
          exposed bases are called anticodons.

           - The function of t-RNA is to carry the amino acids from the
           cytoplasm to the mRNA for assembly in the proper sequence.
Protein Synthesis – the process

Each triplet or codon on the DNA and then on the RNA codes for a
             particular amino acid
There are twenty different amino acids which make up all the different
             proteins in our body
For most amino acids, there are several codons - see table page 637
Once the amino acids have been brought to the ribosome in the correct
             sequence according to the mRNA, each one is bonded to the
             next with a peptide bond. This bond is formed by removing a
             molecule of water from two amino acids in a process called
             dehydration. Many amino acids bonded together are often
             called a polypeptide molecule.
Along with the codons for the amino acids, there are also codons to start the
             protein (initiator codons) and to stop the protein (terminator
Any errors in the original DNA, may cause errors in the mRNA and
             ultimately in the sequence of amino acids which are formed.
             The change of one amino acid in a protein can cause the protein
             to be faulty and not work properly. These mistakes are called
             gene mutations and will be discussed later in the course in
             more detail.

Steps in Protein Synthesis
            1. Transcription of m-RNA from DNA
            2. Translation of m-RNA into amino acid sequence

   1. Transcription of RNA:
        a. An enzyme called RNA polymerase cause the DNA molecule
           to be opened up and read.
        b. RNA nucleotides match up with one side of the DNA only and
           form a single stranded molecule complementary to the DNA
        c. Once the RNA molecule is formed it is then released from the
           DNA molecule which then returns to its original structure of a
           double helix
        d. All three types of RNA are made using DNA as pattern
        e. The m-RNA is a relatively long molecule (1 000 to 10 000
      f. The t-RNA molecule is a relatively short molecule (75-85
         nucleotides) and is folded like a clover leaf). One end of the t-
         RNA molecule attaches to a particular amino acid and the other
         end has a group of unbonded bases called the anticodon.

         Eg. Amino acid asparagine
              DNA code –         TTG
              m- RNA codon – AAC
              t-RNA anticodon – UUG

2. Translation of mRNA into proteins
     a. The mRNA attaches to the ribosome
     b. The initiator site is read (AUG)
     c. The t-RNA molecules transport amino acids to the ribosome
        according to the sequence of bases on the mRNA
     d. The protein molecule is built with one amino acid after another
        being added to the polypeptide chain. This process is called
     e. At the end of the m-RNA strand is a terminator codon which
        ends the process
     f. The protein molecule is now released from the ribosome and is
        transported wherever it is needed.

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