DNA structure, replication, protein synthesis

• DNA is the molecule that transfers hereditary information from one cell to the next.
• DNA


  Deoxyribonucleic acid

  
• Structure of DNA


  • Tightly coiled double helix
  • Found in the nucleus in long strands called chromatin
  • Coils into chromosomes when cells divide

  
• Gene


  A specific portion of the DNA code that has genetic information

  
• Structure of DNA


  • Made up of smaller units called nucleotides
  • Each nucleotide contains a phosphate group, a sugar molecule (deoxyribose), and a nitrogen base
  • Found in pairs along the double helix

  
• Nitrogen bases in DNA

  • Adenine (A)
  • Thymine (T)
  • Cytosine (C)
  • Guanine (G)

  
• Adenine will only pair with Thymine


  Cytosine will only pair with Guanine

  
• The order of these pairs along the double helix is the genetic code – these form your genes.

  
• Role of DNA


  • Controls all protein synthesis in the cell
  • Proteins produced are expressed as the phenotype
• Proteins are used in the cell for

  • Transport of substances across the cell membrane
  • Reactions in the cell
  • Structures in the cell
• The order of the base pairs determines the genes of every individual.
• Different cells have different roles because they construct different proteins.
• ALL body cells have identical DNA.
• Double helix


  The shape of the DNA molecule

  
• Building blocks of DNA

  • Nucleotides

  
• Nucleotide composition


  • One phosphate molecule
  • A five-sided sugar molecule (deoxyribose sugar)
  • One nitrogen base

  
• Ladder model of DNA

  • Side rails are alternating phosphate and sugar molecules
  • Rungs are paired nitrogen base molecules held together by hydrogen bonds

  
• Types of DNA


  • Nuclear DNA (nDNA)
  • Mitochondrial DNA (mtDNA)

  
• Mitochondrial DNA


  • Passed only from mother to child
  • Indicates that mitochondria were once free-living organisms
• A chromosome consists of segments of DNA known as genes.

  
• Genes contain the instructions for the construction of a particular protein, or RNA.
• It is estimated that there are about 20,000–25,000 genes in the human genome.
• Introns and exons


  • Exons are sections of coding DNA
  • Introns are sections of non-coding DNA
• Replication of DNA


  The base pairs (A–T and C–G) ensure that new strands of DNA are identical to the original strand

  
• RNA is a nucleic acid that uses a slightly different sugar than DNA and the base uracil (U) in place of thymine (T).
• Types of RNA


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


  The bases in the mRNA that match the triplet
• Anticodon


  Three bases in tRNA that match the codon
• The sequence of bases in a gene is a code instructing the cell how to construct a particular protein.
• The sequence of bases is read in groups of three called codons.
• Each codon represents one of the 20 different amino acids.
• DNA has the unusual property of being able to produce an exact copy of itself – this process is called DNA replication.
• DNA replication


  1. Unwind DNA

  2. Helicase enzyme catalyzes unwinding

  3. Stabilized by single-stranded binding proteins

  4. Primase synthesizes RNA chain

  5. Origins of replication

  
• Replication Forks


  • Y-shaped regions of replicating DNA molecules where new strands are growing
• Replication Bubbles


  • Hundreds of replicating bubbles
  • Single replication fork (bacteria- helicase)
• Strand Separation


  • Gyrase enzyme relieves stress on the DNA molecule

  
• This occurs after cell division when the amount of DNA is halved.
• Replication: 1st step


  1. Unwind DNA

  2. Helicase enzyme catalyzes unwinding

  3. Stabilized by single-stranded binding proteins

  4. Primase synthesizes RNA chain

  
• Helicase enzyme


  Catalyzes the unwinding and separation (breaking H-bonds) of the parental double helix

  
• Single-stranded binding proteins (SSB)


  Attach and help keep the separated strands apart