A ribosome, part of the protein synthesis machinery

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A ribosome, part of the protein synthesis
machinery
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Beadle Tatums Experiment
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Transcription Translation

• Transcription is the synthesis of RNA using DNA
  as a template
• Translation is the synthesis of polypeptides from
  RNA
• Transcription takes place in the nucleus of
  eukaryotic cells
• During transcription, only one strand of the DNA
  is transcribed (the template strand)
• Messenger RNA (mRNA) molecules are produced
  during transcription and carry the genetic
  message of DNA to the ribosomes in the cytoplasm
• The mRNA that is produced is said to be
  complementary to the DNA template
• The instructions for building a polypeptide chain
  are written as a series of three-nucleotide
  groups this is called a triplet code, or codon
• The codons are written in the 5 to 3 direction
  (just like replication)
• The genetic code is redundant, meaning that more
  than one codon codes for each of the 20 amino
  acids

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Overview the roles of transcription and
translation in the flow of genetic information
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The triplet code
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The dictionary of the genetic code
Glu
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A tobacco plant expressing a firefly gene
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Transcription

• Initiation After RNA polymerase binds to the
  promoter (in eukaryotes, this region has multiple
  TATA repeats), the DNA strands unwind, and the
  polymerase initiates RNA synthesis at the start
  point on the template strand.
• Elongation The polymerase moves downstream,
  unwinding the DNA and elongating the RNA
  transcript 5? ? 3?. In the wake of
  transcription, the DNA strand reforms a double
  helix.
• Termination. Eventually, the RNA transcript is
  released, and the polymerase detaches from the
  DNA.

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The stages of transcription initiation,
elongation, and termination (layer 1)
DNA
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The stages of transcription initiation,
elongation, and termination (layer 2)
DNA
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The stages of transcription initiation,
elongation, and termination (layer 3)
DNA
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The stages of transcription initiation,
elongation, and termination (layer 4)
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The initiation of transcription at a eukaryotic
promoter
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Further processing of RNA transcript

• Before the RNA leaves the nucleus, it is further
  modified.
• A modified guanine nucleotide (G with three
  phosphates) is added to the 5? end and is called
  the 5? cap.
• In addition, 50 to 250 adenine nucleotides are
  added to the 3? end, which is called a poly-A
  tail.
• Finally, the RNAis spliced. The
  intervening/non-coding regions called introns,
  are cut out and the remaining coding regions
  called exons are spliced together.

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RNA Processing I Addition of the 5? cap and
poly-A tail
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RNA Processing II RNA splicing
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Correspondence between exons and protein domains
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Translation

• Translation is the RNA-directed synthesis of a
  polypeptide
• It occurs in the cytoplasm at the ribosomes
• mRNA codons are read at the ribosome and the
  corresponding amino acids are placed in the
  appropriate sequence
• tRNA functions in transferring amino acids from a
  pool of amino acids in the cytoplasm to a
  ribosome
• The ribosome accepts the amino acid from the tRNA
  and incorporates the amino acid into the growing
  polypeptide chain
• Each type of tRNA is specific for a particular
  amino acid
• At one end tRNA loosely binds the amino acid, and
  at the other end it has a nucleotide triplet
  called an anticodon
• The anticodon pairs specifically with a
  complementary codon on mRNA
• The mRNA is read codon by codon, and one amino
  acid is added to the growing polypeptide chain
  for each codon read

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The structure of transfer RNA (tRNA)
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Translation the basic concept
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Translation

• Ribosomes are made up of two subunits made up of
  proteins and RNA molecules called ribosomal RNA
  (rRNA)
• Ribosomes have three binding sites for mRNA
• P site growing polypeptide chain
• A site amino acid that will be added next
• E site exit
• Translation, like transcription, has three stages
  initiation, elongation and termination
• Translation video

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Ribosome Structure
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Ribosome Structure
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Initiation Stage of Translation

• A small ribosomal subunit binds to a molecule of
  mRNA. An initiator tRNA, with the anticodon UAC,
  base-pairs with the start codon, AUG. This tRNA
  carries the amino acid methionine (Met).
• The arrival of a large ribosomal subunit
  completes the initiation complex. Proteins called
  initiation factors are required to bring all the
  translation components together. GTP provides the
  energy for the assembly. The initiator tRNA is in
  the P site the A site is available to the tRNA
  bearing the next amino acid.

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The initiation of translation
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Elongation Stage of Translation

• Codon recognition. The anticodon of an incoming
  aminoacyl tRNA base-pairs with the complementary
  mRNA codon in the A site.
• Peptide bond formation. An rRNA molecule of the
  large subunit catalyzes the formation of a
  peptide bond between the new amino acid in the A
  site and the carboxyl end of the growing
  polypeptide in the P site. This step attaches the
  polypeptide to the tRNA in the A site.
• Translocation. The ribosome translocates the tRNA
  in the A site to the P site. The empty tRNA in
  the P site is moved to the E site, where it is
  released. The mRNA moves along with its bound
  tRNAs, bringing the next codon to be translated
  into the A site.

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The elongation cycle of translation
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Termination Stage of Translation

• When a ribosome reaches a stop codon on mRNA
  (UAG, UAA, or UGA), the A site of the ribosome
  accepts a protein called a release factor instead
  of tRNA.
• The release factor hydrolyzes the bond between
  the tRNA in the P site and the last amino acid of
  the polypeptide chain. The polypeptide is thus
  freed from the ribosome.
• The two ribosomal subunits and the other
  components of the assembly dissociate.

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The termination of translation
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Polyribosomes
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The molecular basis of sickle-cell disease a
point mutation
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Base-pair substitution
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Base-pair insertion or deletion
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A summary of transcription and translation in a
eukaryotic cell