From Gene to Protein

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From Gene to Protein

• A.P. Biology

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Typical Gene Structure
Promoter(RNA polymerase binding site)
Regulatory sites
DNA strand
Start transcription
Stop transcription
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• RNA
• Contains ribose instead of deoxyribose
• Single stranded
• Nucleotide uracil replaces thymine

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3 Types of RNA

• Messenger RNA mRNA
• -Carries DNA sequence to the ribosome
• Ribosomal RNA rRNA
• -Component of ribosome
• Transfer RNA tRNA
• -Carries amino acid (anticodon) to the codon
  on mRNA

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Basic Principles of Transcription and Translation

• Transcription
• Is the synthesis of RNA under the direction of
  DNA
• Produces messenger RNA (mRNA)
• Translation
• Is the actual synthesis of a polypeptide, which
  occurs under the direction of mRNA
• Occurs on ribosomes

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• In prokaryotes
• Transcription and translation occur together

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• In eukaryotes
• RNA transcripts are modified before becoming true
  mRNA

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• During transcription
• The gene determines the sequence of bases along
  the length of an mRNA molecule

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• In laboratory experiments
• Genes can be transcribed and translated after
  being transplanted from one species to another

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Molecular Components of Transcription

• RNA synthesis
• Is catalyzed by RNA polymerase, which pries the
  DNA strands apart and hooks together the RNA
  nucleotides
• Follows the same base-pairing rules as DNA,
  except that in RNA, uracil substitutes for thymine

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Synthesis of an RNA Transcript

• The stages of transcription are
• Initiation
• Elongation
• Termination

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RNA Polymerase Binding and Initiation of
Transcription

• Promoters signal the initiation of RNA synthesis
• Transcription factors
• Help eukaryotic RNA polymerase recognize promoter
  sequences
• TATA box

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Transcription Making a Copy of the DNA

1. RNA polymerase separates the DNA strands at a
   promoter region on the DNA (TATA box)
2. RNA polymerase adds nucleotides in sequence to
   mRNA
3. RNA polymerase falls off the DNA at a terminator
   sequence on the DNA

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Unwinding of the DNA Helix by RNA Polymerase
Step 1
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Transcription of mRNA Complementary to DNA
Step 2
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Alteration of mRNA Ends

• Each end of a pre-mRNA molecule is modified
• The 5? end receives a modified nucleotide cap
• The 3? end gets a poly-A tail

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• RNA splicing
• Removes introns and joins exons

• Ribozymes
• Are catalytic RNA molecules that function as
  enzymes and can splice RNA

Introns - Intervening sequence Junk DNA Exons
Expressed sequence
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mRNA Processing in Eukaryotic Cells
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Molecular Components of Translation

• A cell translates an mRNA message into protein
• With the help of transfer RNA (tRNA)

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• Molecules of tRNA are not all identical
• Each carries a specific amino acid on one end
• Each has an anticodon on the other end

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• A tRNA molecule
• Consists of a single RNA strand that is only
  about 80 nucleotides long
• Is roughly L-shaped

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• A specific enzyme called an aminoacyl-tRNA
  synthetase
• Joins each amino acid to the correct tRNA

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• The ribosome
• Is part of the cellular machinery for
  translation, polypeptide synthesis

• Facilitates the specific coupling of tRNA
  anticodons with mRNA codons during protein
  synthesis

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• The ribosome has three binding sites for tRNA
• The P site
• The A site
• The E site

P site (Peptidyl-tRNA binding site)
E site (Exit site)
mRNA binding site
A site (Aminoacyl-tRNA binding site)
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Central Dogma in Biology
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Gene Mutations Substitution, Insertion,
Deletion
Deletion
Insertion
Substitution
Point Mutation
Frameshift Mutation
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Base Pair Substitution

• Missense mutation altered codon codes for a
  different amino acid
• Nonsense mutation altered codon changed to a
  stop codon (no amino acid delivered)

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Chromosomal Mutations
Deletion
Duplication
Inversion
Translocation