RNA

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Chapter 8

• RNA
• Protein Synthesis
• Mutations

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RNA Structure Function
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Structure of RNA

• RNA differs from DNA in three ways
• an RNA molecule consists of a single-strand of
  nucleotides
• RNA has ribose (5-C sugar) rather than
  deoxyribose
• RNA has the pyrimidine uracil instead of thymine

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Types of RNA
Function
mRNA transmits information from DNA and serves as a template for protein synthesis
tRNA brings amino acids to ribosomes for protein synthesis
rRNA rRNA and proteins make up ribosomes
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3 Types of RNA
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Protein Synthesis

• Has 2 stages
• Transcription and Translation

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Protein Synthesis Part 1 Transcription

• Transcription is the first part of protein
  synthesis.
• What does it mean to transcribe?
• Step 1 An enzyme called RNA polymerase attaches
  to DNA and unzips it.
• Step 2 RNA polymerase then uses one strand of
  DNA as a template from which nucleotides are
  assembled into a strand of mRNA.

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Adenine (DNA and RNA) Cystosine (DNA and
RNA) Guanine(DNA and RNA) Thymine (DNA
only) Uracil (RNA only)
RNApolymerase
DNA
RNA
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Protein Synthesis Part I Transcription

• The nucleotide sequence of DNA is converted into
  an RNA nucleotide sequence.
• Lets say one strand of a DNA molecule had this
  sequence
• AGCCTACGTAAG
• What would the mRNA sequence produced be?

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Protein Synthesis Part 1 Transcription

• DNA AGCCTACGTAAG

mRNA UCGGAUGCAUUC
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Review of Proteins

• Proteins are made by joining amino acids together
  into a long chain. This chain is a protein.
• The functions and properties of a protein are
  determined by the order in which 20 different
  types of amino acids are joined.
• The language of mRNA is called the genetic
  code.

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The Genetic Code

• RNA has 4 bases adenine, uracil, cytosine, and
  guanine.
• The language of RNA is written in only 4
  letters AUGC
• The code is read three letters at a time
• Each word is 3 letters long
• Each word of 3 nucleotides is called a codon

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The Genetic Code

• The 4 different bases can be arranged into 64
  possible codons (4X4X464), but make only 20
  different amino acids.
• This is because some amino acids correspond to
  several different codons.
• Example

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Genetic Code

• Practice
• For what amino acid does the codon CGG code?
• For what amino acid does the codon AUG code?
• For what amino acid does the codon UGA code?

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Codons With Special Roles
Start Codon
Stop Codons
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Protein Synthesis Part 2 Translation

• The 2nd stage of protein synthesis is
  translation.
• What does it mean to translate?
• The sequence of nucleotides in mRNA serves as
  instructions for the order in which amino acids
  should be joined together in a protein. Think of
  mRNA as the recipe for a protein.

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Protein Synthesis Part 2 Translation

• Transcription occurs in the nucleus.
• In translation, the mRNA travels out of the
  nucleus, and moves through the cytoplasm and
  attaches to a ribosome. So, translation occurs
  at the ribosomes.

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Protein Synthesis Part 2 Translation

• Steps of translation
• 1. mRNA attaches to a ribosome
• 2. As each codon of the mRNA molecule moves
  through the ribosome, the proper amino acid is
  brought to the ribosome by tRNA molecules.
• 3. Each tRNA molecule has 3 unpaired bases on
  it. These bases are called anticodons.

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Protein SynthesisPart 2 Translation

• 4. The amino acids are strung together like a
  chain (remember, a protein is a polymer) by the
  ribosome, forming a long protein molecule.
• 5. The tRNAs are released from the amino acids.
• 6. The protein chain grows longer and longer
  until the ribosome reaches a stop codon on the
  mRNA molecule. Then the protein is released from
  the ribosome.

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Practice
DNA
mRNA
tRNA
Poly- peptide
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Mutations

• Gene Mutation produce changes in a single gene
• Chromosomal Mutations produce changes in an
  entire chromosome

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Gene Mutations

• A gene mutation is a change in a DNA sequence.
• Gene mutations that involve changes in only one
  or just a few nucleotides are called point
  mutations b/c they occur at a single point in the
  DNA sequence.
• There are 3 types of point mutations
  substitutions, deletions, and insertions.

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Gene Mutations

• Substitution Mutations
• One base is changed to another
• Usually affect no more than a single amino acid,
  but could still have an effect on a protein.

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Gene Mutations

• Insertion Mutations
• One extra base is inserted into the DNA sequence.
• Deletion Mutations
• One base is left out of the DNA sequence.

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Genetic Mutations

• Since the genetic code is read 3 letters at a
  time, the message is shifted for every codon that
  follows. Hence, insertions result in frameshift
  mutations. They have a dramatic effect on the
  amino acid sequence, and thus, the protein.
• Questionduring what event do these mutations
  usually occur?

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Gene Mutations
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Chromosomal Mutations

• Chromosomal mutations involve changes in the
  number or structure of chromosomes.
• We discussed many of these last chapter.
  Examples are Down syndrome, Turner syndrome,
  Fragile-X syndrome, Prader-Willi syndrome, and
  Klinefelters syndrome.

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Chromosomal Mutations