PS

• Protein Synthesis (Gene Expression)


  Proteins make up all living materials
• DNA


  • DNA contains genes, sequences of nucleotide bases
  • These Genes code for polypeptides (proteins)
  • Proteins are used to build cells and do much of the work inside cells
• Proteins


  • Made of amino acids linked together by peptide bonds
  • 20 different amino acids exist
• Polypeptides


  Amino acid chains
• Protein Synthesis


  1. DNA is found inside the nucleus

  2. Proteins are made in the cytoplasm of cells by organelles called ribosomes

  3. Ribosomes may be free in the cytosol or attached to the surface of rough ER

  4. DNA's code must be copied and taken to the cytosol

  5. In the cytoplasm, this code must be read so amino acids can be assembled to make polypeptides (proteins)
• RNA


  • DNA is the MASTER PLAN
  • RNA is the BLUEPRINT of the Master Plan
• Three Types of RNA

  • Messenger RNA (mRNA) - copies DNA's code & carries the genetic information to the ribosomes
  • Ribosomal RNA (rRNA) - along with protein, makes up the ribosomes
  • Transfer RNA (tRNA) - transfers amino acids to the ribosomes where proteins are synthesized
• Messenger RNA (mRNA)


  • Long Straight chain of Nucleotides
  • Made in the Nucleus
  • Copies DNA & leaves through nuclear pores
  • Contains the Nitrogen Bases A, G, C, U (no T)
  • Carries the information for a specific protein
  • Made up of 500 to 1000 nucleotides long
  • Sequence of 3 bases called codon
  • AUG - methionine or start codon
  • UAA, UAG, or UGA - stop codons
• Ribosomal RNA (rRNA)


  • rRNA is a single strand 100 to 3000 nucleotides long
  • Globular in shape
  • Made inside the nucleus of a cell
  • Associates with proteins to form ribosomes
  • Site of protein Synthesis
• The Genetic Code


  1. A codon designates an amino acid

  2. An amino acid may have more than one codon

  3. There are 20 amino acids, but 64 possible codons

  4. Some codons tell the ribosome to stop translating
• The Genetic Code


  • AUG codes for Methionine
• The genetic code has duplicates, several codons for each amino acid, which provides mutation insurance
• Transfer RNA (tRNA)


  • Clover-leaf shape
  • Single stranded molecule with attachment site at one end for an amino acid
  • Opposite end has three nucleotide bases called the anticodon
• Codons and Anticodons


  The 3 bases of an anticodon are complementary to the 3 bases of a codon
• Protein Synthesis


  DNA -> mRNA -> tRNA (ribosomes) -> Protein
• Protein Synthesis


  • The production or synthesis of polypeptide chains (proteins)
  • Two phases: Transcription & Translation
  • mRNA must be processed before it leaves the nucleus of eukaryotic cells
• Transcription


  1. The process of copying the sequence of one strand of DNA, the template strand

  2. mRNA copies the template strand

  3. Requires the enzyme RNA Polymerase
• DNA sequence


  • 5'-GCGTATG-3'
• Complementary RNA strand


  • 3'-CGCAUAC-5'
• Transcription


  1. RNA polymerase binds to DNA and separates the DNA strands

  2. RNA Polymerase then uses one strand of DNA as a template to assemble nucleotides into RNA
• Promoters


  • Regions on DNA that show where RNA Polymerase must bind to begin the Transcription of RNA
  • Specific base sequences act as signals to stop, called the termination signal
• mRNA Processing


  1. After the DNA is transcribed into RNA, editing must be done to the nucleotide chain to make the RNA functional

  2. Introns, non-functional segments of DNA are snipped out of the chain

  3. Exons, segments of DNA that code for proteins, are then rejoined by the enzyme ligase

  4. The newly processed mRNA can then leave the nucleus
• In prokaryotes, transcribed mRNA goes straight to the ribosomes in the cytoplasm
• In eukaryotes, freshly transcribed mRNA in the nucleus is about 5000 nucleotides long, but when the same mRNA is used for translation at the ribosome it is only 1000 nucleotides long. The mRNA has been edited.
• Exons


  Parts of the mRNA which are kept for gene expression
• Introns


  Parts of the mRNA which are edited out (by spliceosomes)
• mRNA Transcript


  mRNA leaves the nucleus through its pores and goes to the ribosomes
• Translation


  1. Translation is the process of decoding the mRNA into a polypeptide chain

  2. Ribosomes read mRNA three bases or 1 codon at a time and construct the proteins
• Retroviruses (e.g. HIV) carry RNA as their genetic information and when they invade their host cell they convert their RNA into a DNA copy using reverse transcriptase. This modifies the central dogma: DNA ↔ RNA Protein.
• Ribosomes


  • Made of a large and small subunit
  • Composed of rRNA (40%) and proteins (60%)
  • Have two sites for tRNA attachment
• tRNA


  Transcribed from DNA and folded into clover shape
• Step 1- Initiation


  1. mRNA transcript start codon AUG attaches to the small ribosomal subunit

  2. Small subunit attaches to large ribosomal subunit
• Ribosome Structure


  • P Site
  • A Site
  • Large subunit
  • Small subunit
• Step 2 - Elongation

  1. As ribosome moves, two tRNA with their amino acids move into site A and P of the ribosome

  2. Peptide bonds join the amino acids
• Initiation


  1. mRNA transcript AUG codon attaches to small ribosomal subunit

  2. tRNA with anticodon complementary to codon binds in P site

  3. Second tRNA with amino acid binds in A site
• Elongation


  1. Peptide bond forms between amino acids

  2. Ribosome moves over one codon
• Termination


  1. Stop codon UAA, UAG, or UGA in mRNA binds to release factors

  2. Completed polypeptide chain is released
• Primary structure of a protein

  Sequence of amino acids bonded together by peptide bonds
• mRNA sequence


  • AUG/GGC/UCC/AUC/GGC/GCA/UAA
• DNA is a very stable molecule, good for storing genetic material, but it requires enzymes for replication and gene expression. The information in DNA is required to synthesise enzymes (proteins) but enzymes are required to make DNA function. This paradox raises the question of which came first in the origin of life, DNA or enzymes?