Protein Synthesis (Gene Expression)

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Protein Synthesis (Gene Expression)
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Review

• Nucleotide sequence in DNA is used to make
  proteins that are the key regulators of cell
  functions.
• Proteins?polymers of amino acids
• The sequence of nucleotides in DNA contains
  information for assembling the string of amino
  acids that make up a single protein.

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RNA-Ribonucleic Acid
                          

• Differences between DNA and RNA
• RNA is single-stranded (it looks like ½ of a
  zipper.
• The sugar in RNA is ribose
• RNA contains URACIL instead of thymine. Uracil
  pairs with adenine.

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RNA-Ribonucleic Acid

• Role of RNA
• Worker for protein synthesis while DNA is the
  Commander-in-Chief
• RNA takes the instructions on how a protein
  should be built and then assembles the protein,
  amino acid by amino acid.

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RNA-Ribonucleic Acid

• Types of RNA
• Messenger RNA (mRNA)-Bring information from the
  DNA in the nucleus to the cytoplasm
• Ribosomal RNA (rRNA)-Subunit of ribosomes which
  clamps onto the mRNA and use its information to
  assemble the amino acids in the correct sequence.
• Transfer RNA (tRNA)-Supplier of amino acids to
  the ribosome

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Protein Production and the Genetic Code

• Transcription
• Locationnucleus
• The information found in a gene in DNA is
  transcribed into an mRNA molecule
• Steps
• RNA polymerase binds to the genes promoter or
  start signal
• RNA polymerase unwinds and separates the two
  strands of DNA
• RNA polymerase adds the complementary RNA
  nucleotides as it reads the gene (UA GC)
• Transcription proceeds until the RNA polymerase
  reaches a stop signal. At this point, it
  detaches and the mRNA molecule detaches from the
  DNA strand. The DNA strand will twist back up
  and the bonds will be restored.

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Transcription
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Protein Synthesis Transcription
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Protein Production and the Genetic Code

• The nucleotide sequence transcribed from DNA to a
  strand of mRNA acts as a genetic message.
• This message is written in a language that uses
  nitrogen bases as its alphabet.
• The language of proteins uses an alphabet of
  amino acids.
• A code is needed to convert the language of mRNA
  into the language of proteins.

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Protein Production and the Genetic Code

• B. continued
• There are 20 different amino acids, but only 4
  types of N bases in mRNA. How can these bases
  form a code for proteins?

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Protein Production and the Genetic Code

• A group of three nucleotides codes for one amino
  acid. Each set of three N bases that codes for
  an amino acid is called a codon.
• The order of nitrogen bases in the mRNA will
  determine the type and order of amino acids in a
  protein
• 64 combinations are possible when a sequence of 3
  bases is used. Thus, 64 different mRNA codons
  are in the genetic code.

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Protein Synthesis Translation
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A Codon
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The Genetic Code
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Protein Production and the Genetic Code

• Some codons do not code for amino acids they
  provide instructions for assembling the protein.
• UAA is a stop codon indicating that protein
  production should stop at that point.
• AUG is a start codon as well as being the codon
  for the amino acid methionine.

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Protein Production and the Genetic Code

• As you can see from the genetic code chart, more
  than one codon can code for the same amino acid.
  However, for any one codon, there can only be one
  amino acid.
• The genetic code is nearly universal-the same
  codon can code for the same amino acid in many
  different organisms

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Protein Synthesis Translation
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Protein Production and the Genetic Code

• C. Translation
• Locationcytoplasm
• In eukaryotic cells, mRNA leaves the nucleus
  through an opening in the nuclear membrane and
  travels to the cytoplasm.
• When the strands of RNA arrive, ribosomes attach
  to them like clothespins clamped onto a
  clothesline.
• The process of converting the information in a
  sequence of amino acids that make up a protein is
  known as translation

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Eukaryotic Translation
Ribosome
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Protein Production and the Genetic Code

• Role of Transfer RNA (tRNA)
• For proteins to be built, the 20 different amino
  acids dissolved in the cytoplasm must be brought
  to the ribosomes -gt this is the job of tRNA.
• Each tRNA molecule attaches to only one type of
  amino acid
• Correct translation of the mRNA message depends
  on upon the joining of each mRNA codon with the
  correct tRNA molecule
• On the opposite side of the tRNA molecule from
  the amino acid attachment site, there is a
  sequence of 3 nucleotides that are the complement
  of the nucleotides in the codon. These 3
  nucleotides are called an anticodon because
  they bond to the codon on the mRNA by the process
  of base pairing

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Protein Production and the Genetic Code

• Translating the mRNA code
• As translation begins, a tRNA molecule brings the
  first amino acid to the mRNA strand that is
  attached to the ribosome.
• The anticodon forms a temporary bond with the
  codon of the mRNA strand. This places the amino
  acid in the correct position for forming a bond
  with the next amino acid.

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Protein Production and the Genetic Code

• c. The ribosome slides down the mRNA chain to the
  next codon and a new tRNA molecule brings another
  amino acid.
• d. The amino acids form peptide bonds, the first
  tRNA releases its amino acid and detaches from
  the mRNA. This tRNA molecule is now free to pick
  up and deliver another molecule of its specific
  amino acid to a ribosome.

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Translation
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Protein Production and the Genetic Code

1. A chain of amino acids (polypeptide chain)
   continues to form
2. When a stop codon is reached, translation ends,
   and the amino acid strand is released from the
   ribosome.
3. The amino acid chains then twist and curl into
   complex three-dimensional shapes and become
   proteins.

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Forming the Polypeptide Chain
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which functions to