14.3 How Is the Information Content in DNA Transcribed to Produce RNA?

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• Transcription requires
• A DNA template for base pairings
• The four ribonucleoside triphosphates
  (ATP,GTP,CTP,UTP)
• An RNA polymerase
• Salts and pH buffer, if done in a test tube

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• Transcription produces mRNA, tRNA, and rRNA.
• These RNAs are encoded by specific genes.
• Eukaryotes also make several small RNAs,
  including small nuclear RNA (snRNA), microRNA
  (miRNA), and small interfering RNA (siRNA).

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Table 14.1
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• RNA polymerases catalyze synthesis of RNA
• Catalyze addition of nucleotides in a 5'-to-3'
  direction
• Processiveone enzyme-template binding results in
  polymerization of hundreds of RNA bases
• They do not need primers

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Figure 14.3 RNA Polymerase
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• Transcription occurs in three phases
• 1. Initiation
• RNA polymerase binds to a DNA sequence called a
  promoter.
• Promoters tell the enzyme where to start and
  which strand of DNA to transcribe.
• The promoter has an initiation site where
  transcription begins.

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Figure 14.4 DNA Is Transcribed to Form RNA (A)
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Figure 14.4 DNA Is Transcribed to Form RNA (A)
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• Sigma factors and transcription factors are
  proteins that bind to DNA sequences and to RNA
  polymerase.
• They help direct the polymerase onto the
  promoter, and help determine which genes are
  expressed at particular times.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• 2. Elongation
• RNA polymerase unwinds DNA about 10 base pairs at
  a time reads template in 3' to 5' direction.
• The transcript is antiparallel to the DNA
  template strand.
• RNA polymerases do not proofread and correct
  mistakes.

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Figure 14.4 DNA Is Transcribed to Form RNA (B)
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• RNA polymerase uses (ribo)nucleoside
  triphosphates (NTPs) as substrates.
• Two phosphate groups are removed from each
  substrate molecule the energy released is used
  to drive the polymerization reaction.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• 3. Termination
• Specified by a specific DNA sequence.
• Mechanism in eukaryotes is not well understood.
• In bacteria, the transcript forms a loop and
  falls away from the DNA or a helper protein
  binds to the transcript and causes it to detach
  from the DNA.

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Figure 14.4 DNA Is Transcribed to Form RNA (C)
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• The genetic code specifies which amino acids will
  be used to build a protein.
• Codon a sequence of three bases, something like
  a three-letter word.
• Each codon specifies a particular amino acid.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• How was the code deciphered?
• How could 20 code words (amino acids) be
  written with only four letters (the four
  bases)?
• A triplet code seemed likely it could result in
  4 4 4 64 codons.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• Nirenberg and Matthaei used simple artificial
  mRNAs of known composition to identify the
  polypeptide that resulted.
• This led to the identification of the first three
  codons.

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Figure 14.5 Deciphering the Genetic Code
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• Later, scientists used artificial mRNAs only
  three nucleotides long (one codon).
• These would bind to a ribosome and a
  corresponding tRNA carrying an amino acid.
• Thus the codes for all the amino acids were
  determined.

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Figure 14.6 The Genetic Code
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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• There are more codons than amino acids.
• AUG is the start codoninitiation signal for
  translation.
• Stop codonstermination signals, include UAA,
  UAG, and UGA.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• For most amino acids, there is more than one
  codon the genetic code is redundant.
• The genetic code is not ambiguouseach codon
  specifies only one amino acid.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• The genetic code is nearly universal
• The codons that specify amino acids are the same
  in all organisms.
• A few exceptions Within mitochondria and
  chloroplasts, and in one group of protists, there
  are codon differences.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• This common genetic code is also a common
  language for evolution.
• The code is ancient and has remained intact
  throughout evolution.
• The common code also allows genetic
  engineeringhuman genes can be expressed in E.
  coli, for example.

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14.3 How Is the Information Content in DNA
Transcribed to Produce RNA?

• The base sequence of the DNA strand that is
  transcribed is complementary and antiparallel to
  the mRNA codons.
• The non-template DNA strand has the same sequence
  as the mRNA and is called the coding strand.
• By convention, DNA sequences are shown beginning
  with the 5' end of the coding sequence.