Chapter 4 - DNA and polypeptide synthesis

• Prokaryotic DNA


  • Single circular chromosome in the nucleoid region
  • No membrane around the chromosome
  • Codes for proteins made on ribosomes in the cytoplasm
• Prokaryotic DNA structure


  • Two circles of single-stranded DNA twisted around each other
  • Circular, not a helix
  • Supercoiled and looped around a central protein scaffold

  
• Plasmids


  Small rings of non-chromosomal DNA in prokaryotes that provide selective advantages
• Eukaryotic DNA


  • Located in the membrane-bound nucleus
  • Arranged into a number of separate linear chromosomes
  • Each chromosome is larger and more complex than a prokaryotic chromosome
• The number of chromosomes is not a measure of how complex the organism is

  
• Coding DNA (exons)


  DNA sequences that code for products like proteins or RNA
• Non-coding DNA (introns)


  DNA sequences that do not directly code for products
• Packaging of eukaryotic DNA

  • DNA wound around histone proteins into nucleosomes
  • Nucleosomes condense into chromatin
  • Chromatin further condenses into chromosomes

  
• Non-nuclear DNA in eukaryotes

  DNA in mitochondria and chloroplasts, inherited independently of nuclear DNA
• Mitochondrial DNA (mtDNA)


  • Small circular molecule with 37 genes
  • 13 genes code for proteins in electron transport chain
  • 24 genes code for RNA molecules
• mtDNA has a higher mutation rate than nuclear DNA
• Uses of mtDNA


  • Tracing maternal inheritance
  • Evolutionary relatedness studies
  • Forensic identification
  • Human ancestry studies
• Genome size (human)


  ± 3 billion base pairs
• Number of genes (human)

  ± 20 000
• Types of proteins encoded (human)

  100 000
• Mitochondrial DNA (mtDNA)


  • Exists outside the cell nucleus
  • Has 13 protein-coding genes
  • Has 24 genes coding for RNA
  • Has about 16 500 base pairs
• Mutation rate of mtDNA

  Higher than nuclear DNA
• mtDNA is used to study evolutionary relatedness, construct evolutionary trees, investigate family relatedness and identify people in forensic science
• Prokaryotic DNA


  Circular, double-stranded DNA molecule, supercoiled to form a nucleoid and found in the cytoplasm

  
• Eukaryotic DNA


  Linear double-stranded helix wound around histones to form nucleosomes

  
• Eukaryotic cells have non-nuclear DNA, such as mtDNA in mitochondria in the cytoplasm
• Prokaryotic cells have non-chromosomal DNA in the form of plasmids
• Central dogma of molecular biology

  DNA leads to the formation of RNA, which in turn leads to the synthesis of proteins
• Codon


  Three-letter 'words' that code instructions for each amino acid in a protein chain
• Polypeptide


  A molecule made up of a chain of many amino acids, joined by peptide bonds
• Gene expression


  When the end product of a gene has been made by the cell
• In specialised cells in multicellular organisms, only certain genes are expressed in each cell type
• Polypeptide synthesis


  1. DNA remains in the nucleus

  2. mRNA is created and carries instructions from nucleus to ribosomes in cytoplasm

  3. Ribosomes translate mRNA message into protein
• DNA


  Long chains of nucleotides wound into a double helix, sequence of bases determines the message

  
• RNA


  Single-stranded, has ribose sugar, has uracil instead of thymine
• Types of RNA


  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
• mRNA


  Single-stranded, carries information from DNA in nucleus to ribosomes in cytoplasm
• tRNA


  Twisted into clover leaf shape, has anticodon that attaches to complementary codon on mRNA, carries specific amino acid

  
• rRNA


  Forms structural part of ribosomes
• Transcription


  RNA polymerase binds to promoter, DNA unzips, RNA polymerase assembles complementary mRNA molecule, mRNA moves to cytoplasm
• Translation


  Ribosomes move along mRNA, attach tRNA molecules, amino acids from tRNA are linked to form polypeptide chain, tRNAs move away
• Translation


  1. Ribosomes move along mRNA molecule

  2. Attach tRNA molecules to mRNA by pairing anticodons with codons

  3. Amino acids from tRNA linked to form polypeptide chain

  4. tRNAs move away from mRNA, leaving growing polypeptide chain

  5. tRNAs move back into cytoplasm to pick up another amino acid
• Transcription


  1. DNA unzips

  2. Single strand of mRNA made using non-coding strand of DNA as template

  
• mRNA


  Carries codons (base triplets) that instruct amino acid assembly by ribosomes
• tRNA


  Carries an anticodon of three bases at one end and a specific amino acid at the other end