Protein Synthesis - AHL

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Protein Synthesis - AHL

• Transcription Translation

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Transcription

• Transcription is carried out in a 5 ? 3
  direction.
• The 5 end of the free RNA nucleotide is added to
  the 3 end of the RNA molecule that is already
  synthesized.

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Sense Antisense DNA Strands

• During transcription, the DNA double helix
  unwinds and the two strands separate.
• Only one strand is then transcribed into mRNA.
  This is the antisense strand.
• The sense strand has the same base sequence as
  mRNA, with U instead of T.

Antisense strand
Sense strand
mRNA
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Transcription in Prokaryotes

• Genes are only transcribed if RNA polymerase
  binds to a region of DNA close to the start of
  the gene called the promoter. The promoter is on
  the sense strand.
• RNA polymerase splits the DNA strands.
• The mRNA strand produced continues to grow by the
  addition of free nucleoside triphosphates, again
  due to RNA polymerase.

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Transcription continued.

• Two phosphates are removed as they are linked on,
  converting them into RNA nucleotides.
• The process continues until a region called the
  terminator is reached. This is a base sequence
  on the sense strand that causes RNA polymerase to
  stop transcription.
• RNA polymerase is released after transcribing the
  terminator.
• The DNA is rewound into a double helix by RNA
  polymerase.

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Transcription in Eukaryotes

• In eukaryotic cells, repetitive sequences are
  called INTRONS and coding sequences (genes) are
  called EXONS.
• Both introns and exons are transcribed.
• The introns are then removed (spliced) leaving
  only exons.
• This mature mRNA then travels to the cytoplasm
  for translation.

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Translation

• Translation occurs in a 5 ? 3 direction.
• During translation the ribosome moves along the
  mRNA towards the 3 end. The start codon is
  nearer to the 5 end.
• Translation consists of initiation, elongation,
  translocation termination.

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Ribosomes

• Made of proteins and ribosomal RNA (rRNA)
• Consist of 2 subunits one large, one small.
• There are 3 tRNA binding sites on the surface of
  the ribosome. 2 tRNA molecules can bind at the
  same time.
• There is a mRNA binding site on the surface.

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Ribosomes continued.

• Free ribosomes synthesize proteins for use
  primarily within the cell.
• Membrane bound ribosomes on rough er synthesize
  proteins primarily for secretion or for lysosomes.

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tRNA

• All tRNA molecules have an anticodon.
• All have 3 loops.
• All have the base sequence CCA at the 3
  terminal. This is the site for attachment of the
  amino acid.
• All have sections that become double stranded by
  base pairing.

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tRNA continued

• Each type of tRNA has a distinctive 3D shape and
  chemical properties.
• Each tRNA molecule is recognized by a
  tRNA-activating enzyme that binds a specific
  amino acid to it at the 3 terminal.
• There are 20 different tRNA activating enzymes
  one for each amino acid.
• ATP is used to provide energy for the attachment
  of amino acids.
• A high-energy bond is formed between the amino
  acid and the tRNA which is later broken and used
  to make a peptide linkage during translation.

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Initiation

• A particular codon on mRNA is called the start
  codon.
• tRNA with the anticodon complementary to this
  binds to the small subunit of the ribosome.
• The small subunit carrying the tRNA binds to the
  5 end of the mRNA.
• The small subunit slides along until it reaches
  the start codon.
• The large subunit binds to the small subunit.

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Elongation Translocation

• One of the binding sites for tRNA is vacant. The
  small subunit of the ribosome ensures only the
  tRNA with the complementary anticodon binds to
  it.
• The large subunit slides over the small subunit

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Termination

• The ribosome moves along the mRNA until it
  reaches a stop codon.
• No tRNA has the complementary anticodon.
• The large subunit moves over the small subunit
  releasing the polypeptide from the tRNA.
• The tRNA detaches. The large subunit, the small
  subunit and mRNA all separate.

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Polysomes

• Several ribosomes can translate an mRNA at the
  same time, forming what is called a polysome.
• This makes it possible to produce many
  polypeptides simultaneously from a single mRNA.