Protein Synthesis

1
Protein Synthesis

• Transcription and Translation

AP Biology Unit 2
2
Flow of Genetic Information

• All living organisms use DNA to synthesize RNA to
  make proteins
• Same two-step process
• Transcription ? Translation
• Some antibiotics inhibit protein synthesis in
  bacteria.
• Ex. Neomycin (the antibiotic in Neosporin)
  interferes with the process of translation)

3
Genes and Chromosomes

• DNA is organized into chromosomes
• Humans have 46 chromosomes in each cell.
• Genes are coding regions of DNA
• Each gene is the code for how to make a specific
  protein.
• Human chromosomes are made up of
• DNA
• Histone proteins that DNA is wound around

4
Structure of DNA

• The carbons in the 5C sugar each have a number
• Start to the right of the oxygen and go around
  clockwise

5
1
4

• This gives the nucleotide 2 distinct ends
• 5 end (closer to carbon 5)
• 3 end (closer to carbon 3)

2
3
5
Nucleic Acid Structure

• DNA is double stranded
• Hydrogen bonds between bases
• A pairs with T
• C pairs with G

• The strands are antiparallel
• One strand runs 5-3
• The other runs 3-5

Image taken without permission from
http//bcs.whfreeman.com/thelifewire
6
Question

• Why cant the DNA strands be parallel (both
  running 5-3)?
• This wouldnt allow the bases to be near each
  other to hydrogen bond.

7
Transcription

• DNA is transcribed into 3 kinds of RNA
• mRNA messenger RNA (the RNA code used to make
  protein)
• tRNA transfer RNA (participates in translation)
• rRNA ribosomal RNA
• RNA Polymerase is the enzyme that transcribes the
  DNA into RNA

8
Initiation

• How transcription starts
• RNA Polymerase recognizes a promoter sequence on
  the DNA
• RNA Polymerase binds to the promoter
• DNA is unwound to start transcription
• What kind of bonds are being broken to
  unwind/separate the strands of DNA?
• Hydrogen bonds

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Promoter Sequences

• In prokaryotes, RNA Polymerase must find
  these sequences
• 1 is the first base in the RNA

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Eukaryotic Promoter Sequences

• In eukaryotes, the RNA polymerase must find the
  following sequences
• Eukaryotic genes can also have enhancer sequences
  to help RNA polymerase bind
• Well talk about these a little later dont
  worry about them right now ?

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Promoters

• In order for RNA Polymerase to recognize it, the
  promoter sequences
• Must be the correct sequence of bases (small
  changes OK)
• Must be correctly spaced apart
• If these conditions arent met, RNA Polymerase
  cant bind and no transcription occurs.

12
Elongation

• How the RNA strand is built
• RNA Polymerase matches the appropriate
  (complementary) nucleotides to the DNA template
  strand
• Template strand the actual strand RNA
  Polymerase uses to build RNA
• Coding (Nontemplate) strand not used for
  building RNA, but has the same sequence as the
  RNA.

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Building the RNA

• The RNA Polymer grows in a 5-3 direction
• RNA Polymerase only adds new nucleotides on to
  the 3 end.
• Considering this, in what direction must the
  template strand be running?
• 3-5 (since it is building its complement)

14
Question

• In terms of the sequence, how will the RNA differ
  from the sequence of the coding strand?
• Ts are replaced with Us

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Termination

• How transcription of RNA ends
• RNA Polymerase recognizes a termination signal on
  the DNA template
• Usually a long string of As or a series of As
  and Ts
• RNA Polymerase falls off the DNA template
• Stability of mRNA is minutes ? hours (depends on
  type of cell and RNA)

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Question

• How do the specific chemical properties of the
  termination sequence cause termination to occur?
• There are only 2 hydrogen bonds between A and T/U
• With a string of As and Us, there are much
  fewer bonds to hold the DNA template and RNA
  together ? they separate ? transcription ends

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Translation

• Using the mRNA code to create the appropriate
  protein.
• Occurs in the cytoplasm/on the rough ER
• Sequence of 3 nucleotides codes for a particular
  amino acid codon
• 64 different codons

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Question

• Why cant 1 or 2 nucleotides code for an amino
  acid?
• Not enough combinations to code for all 20 amino
  acids
• With 1 nucleotide ? only 4 possibilities
• With 2 nucleotides ? only 4 x 4 16
  possibilities

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The codon table
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tRNA
Amino acid attached here

• tRNA brings the correct amino acid to match with
  the mRNA codon
• Each tRNA holds a specific amino acid and has a
  particular anticodon.
• Aminoacyl tRNA synthetases are enzymes that
  attach the correct amino acids to the tRNA

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Question

• For the anticodon shown in the diagram, what
  would the complementary codon on the mRNA be?
• 5 UUC 3
• Which amino acid is attached to this tRNA?
• Phe

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Ribosomes

• Made up of 2 subunits
• Composed of rRNA and protein
• Not specific to any particular protein can be
  used to translate any RNA into protein
• Workbench for translation holds mRNAs and tRNAs
  in the correct positions to assemble protein.

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Ribosomes

• 3 sites on the ribosome
• A site where tRNA first binds to mRNA
• P site where the amino acid is added on to the
  polypeptide chain
• E site exit site

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Translation

• Begins with the Start codon AUG
• Codes for methionine (Met)
• Not the same thing as 1

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Translation

• Ribosome moves along mRNA in a 5-gt3 direction
  catalyzing the translation of the mRNA into
  protein
• breaks bond between tRNA and amino acid
• creates a new peptide bond to link it to
  polypeptide chain

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Question

• How does the mRNA know if it is correctly matched
  to the tRNA?
• Hydrogen bonding between the bases is correct

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Stopping Translation

• Ribosome is released when a stop codon is reached
• UAA, UAG, UGA stop codons (dont code for any
  tRNA anticodons)
• A release factor binds to the mRNA instead
• Ribosome breaks apart, mRNA and protein are
  released

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Summary of Protein Synthesis

• In Eukaryotes

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Why is this important?

• 1. Changes in the DNA sequence will lead to
  changes in the transcribed RNA.
• 2. This results in a different codon which may
  code for a different amino acid.
• 3. A different amino acid means a different R
  group.
• 4. A different R group may have different
  chemical properties.
• 5. These different chemical properties may lead
  to a different protein structure.
• 6. A different protein structure may affect its
  function!
• 7. See how this is all starting to connect!
  Exciting!!! ?