How Proteins are Made

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How Proteins are Made

• Biology I Chapter 10

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Decoding the Information in DNA

• Traits, such as eye color, are determined by
  proteins that are built according to instructions
  coded in DNA
• Proteins are not built directly from DNA
• RNA is involved

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RNA (ribonucleic acid)

• A nucleic acid
• Made of linked nucleotides

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RNA versus DNA

• Single Stranded
• Sugar Ribose
• Nitrogenous Bases
• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Uracil (U)

• Double Stranded
• Sugar Deoxyribose
• Nitrogenous Bases
• Adenine (A)
• Guanine (G)
• Cytosine (C)
• Thymine (T)

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Transcription

• The process of synthesizing RNA by using one
  strand of DNA molecule as a template
• Prokaryotes
• Occurs in the cytoplasm
• Eukaryotes
• Occurs in the nucleus

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Messenger RNA (mRNA)

• The form of RNA that carries the instructions for
  making a protein from a gene and delivers it to
  the site of translation

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Transcription Step 1

• RNA polymerase binds to the genes promoter
• RNA polymerase An enzyme that adds and links
  complementary RNA molecules during transcription
• Promoter a specific sequence of DNA that acts as
  a start signal for transcription

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Transcription Step 2

• RNA polymerase unwinds and separates the two
  strands of the double helix
• This exposes the DNA nucleotides on each strand

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Transcription Step 3

• RNA polymerase adds and links complementary RNA
  nucleotides
• RNA polymerase moves along the nucleotides of the
  DNA pairing G and C and A and U (not T)

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Codons

• RNA instructions
• A series of 3 nucleotide sequences on the mRNA

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Codons Amino Acids

• The codons make amino acids
• Linked amino acids proteins!
• Genetic Code the 64 possible mRNA codons with
  start and stop signals

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Transcription End

• RNA polymerase eventually reaches a stop signal
  in the DNA
• Stop signal A sequence of bases
• Marks the end of each gene in eukaryotes
• Marks the end of a set of genes in prokaryotes

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Protein Synthesis in Eukaryotes

• The human genome is about 30,000 genes
• Regulatory proteins are used to determine if
  transcription should occur transcription
  factors

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Transcription Factors

• Help arrange RNA polymerases in the correct
  position on the promoter
• Enhancer sequence of DNA that can be bound by a
  transcription factor that can activate
  transcription

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Introns and Exons

• Introns long segments of nucleotides that have
  no coding information
• junk
• Exons the portions of a gene that are translated
  (expressed) in proteins

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Spliceosomes

• After a eukaryotic gene is transcribed, the
  introns in the resulting mRNA are cut out
• Spliceosomes cut out introns and stitch exons
  together

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Translation

• The transfer of genetic material in the form of
  DNA fragments from one cell to another or from
  one organism to another

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Translation

• Eukaryotes and Prokaryotes
• Takes place in the cytoplasm

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Transfer RNA (tRNA)

• Single strands of RNA that help in the synthesis
  of proteins
• Folded into a particular shape with an anticodon
• Anticodon a 3-nucleotide sequence complementary
  to an mRNA codon

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Ribosomal RNA (rRNA)

• RNA molecules that are part of the structure of
  ribosomes
• Ribosomes temporarily hold 1 mRNA and 2 tRNA

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Translation

• The ribosomal subunits, the mRNA, and the tRNA
  carrying methionine (Met) bind together

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Translation

• The tRNA carrying the amino acid specified by the
  codon in the A site arrives
• A peptide bond forms between adjacent amino acids
• The tRNA in the P site detaches and leaves its
  amino acid behind

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Translation

• The tRNA in the A site move to the P site
• The tRNA carrying the amino acid specified by the
  A site arrives
• A peptide bond is formed
• The tRNA in the P site detaches and leaves its
  amino acid behind

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Translation

• The process is repeated until a stop codon is
  reached
• The ribosome complex falls apart
• The newly made protein is released

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Gene Expression

• The manifestation of the genetic material of an
  organism in the form of specific traits
• Also known as protein synthesis

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Protein Synthesis in Prokaryotes

• Prokaryotes have about 2,000 genes
• Operator on-off switch for protein synthesis
• Operon the combined promoter and operator
  regions

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Protein Synthesis in Prokaryotes

• lac operon operon that controls the metabolism
  of lactose
• Repressor protein that binds to an operator and
  blocks RNA polymerase from binding to the
  promoter site
• If lactose is present it binds to the repressor
  and changes the repressors shape
• The repressor falls off the operator and lactose
  can be metabolized

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Protein Synthesis in Prokaryotes

• By producing the enzyme only when the nutrient is
  available, the bacterium saves energy

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Mutations

• A change in the DNA of a gene
• Gene rearrangements
• Transposition or chromosomal rearrangement
• Point mutation
• A single nucleotide changes

• Insertion mutation
• A sizeable length of DNA is inserted into a gene
• Deletion mutation
• Segments of genes are lost, often during meiosis

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Major Types of Mutations