Transcription and Translation

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Transcription and Translation

• From DNA to RNA to protein

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Overview

• Genes in DNA contain information to make
  proteins.
• The cell makes mRNA copies of genes that are
  needed.
• The mRNA is read at the ribosomes in the rough
  ER.
• Protein is produced.

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Key Players

• mRNA carries the information from a gene in DNA.
• Ribosomes, made of rRNA, consist of subunits and
  carry out an enzyme-like role.
• tRNA carries specific amino acids to the ribosome.

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Transcription
RNA polymerase is the enzyme responsible for
making mRNA copies of genes. DNA unzips at the
site of the gene that is needed.
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Transcription
RNA polymerase matches bases in the sense strand
with RNA bases, building a strand of mRNA that
carries the information encoded in the DNA.
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Transcription
Encoded in DNA is a signal telling RNA polymerase
where to stop. Transcription ends at that point.
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Transcription
The completed mRNA molecule then moves from the
nucleus to the rough ER for translation.
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Translation

• Initiation begins with a tRNA bearing methionine
  (met) attaching to one of the ribosomal units.
  The codon for methionine is a universal start
  codon for reading the mRNA strand.

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Translation

• The ribosomal unit binds to mRNA where the code
  for met is located (AUG). The anticodon (UAC) of
  the tRNA matches the start codon on mRNA (AUG).

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Translation

• The larger ribosomal subunit now binds to the
  smaller unit, forming a ribosomal complex. The
  tRNA binds to the first active site on the
  ribosome. Translation may now begin.

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Translation

• The second codon in mRNA (GUU) matches the
  anticodon of a tRNA carrying the amino acid
  valine (CAA). The second tRNA binds to the second
  active site on the large subunit.

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Translation

• A catalytic site on the larger subunit binds the
  two amino acids together using dehydration
  synthesis, forming a peptide bond between them.

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Translation

• The first tRNA now detaches and goes of to find
  another met in the cytoplasm. The mRNA chain
  shifts over one codon, placing the second codon
  (CAU) over the second active site.

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Translation

• A tRNA with an anticodon (GUA) matching the
  exposed codon (CAU) moves onto the ribosome. This
  tRNA carries histidine (his).

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Translation

• A new peptide bond forms between val and his on
  the catalytic site. The tRNA that carried val
  will detach and find another val in the
  cytoplasm. The mRNA strand will then shift over
  one more codon.

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Translation

• The process continues until the ribosome finds a
  stop codon. The subunits detach from one
  another, the mRNA is released, and the
  polypeptide chain moves down the ER for further
  processing. The initial met is removed and the
  chain is folded into its final shape.

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Summary
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Transcription Animation

• http//learn.genetics.utah.edu/units/basics/transc
  ribe/

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Mutations

• Any mistakes in the DNA code can result in a
  broken (non-functional) protein.
• A mutation affecting only a few somatic cells
  (body cells) might not have any effect, unless
  the mutation turns the cell cancerous.
• A mutation affecting a sex cell can be passed on
  to the offspring.

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Types of Mutations

• Point mutation base substitution that may or may
  not code for a different amino acid.
• Insertion mutation one or more bases is inserted
  into the DNA strand.
• Deletion one or more bases is deleted from the
  DNA strand.

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Effects of mutations

• Original THE FAT CAT SAT
• Point mutation THA FAT CAT SAT
• Insertion THE FTT ATC ATS AT
• Deletion THE FTC ATS AT

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Hemoglobin mutations
DNA mRNA Amino Acid Properties of AA Effect on protein Disease
Original codon 6 CTC GAG Glutamic Acid Hydrophilic Normal None
Mutation 1 CTT GAA Glutamic Acid Hydrophilic Neutral None
Mutation 2 GTC CAG Glutamine Hydrophilic Neutral None
Mutation 3 CAC GUG Valine Hydrophobic Loses water solubility Sickle Cell Anemia