CHAPTER 16 THE MOLECULAR BASIS OF INHERITANCE Structure

CHAPTER 16 THE MOLECULAR BASIS OF INHERITANCE -Structure of DNA - Steps of replication - Difference between replication, transcription, & translation - How DNA is packaged into a chromosome

16. 1 – DNA IS THE GENETIC MATERIAL • Upon learning that chromosomes carried genes – science started to work toward determining whether DNA or Protein was the genetic material • 1952, Hershey & Chase answered the question: • Utilized bacteriophages (viruses that infect bacteria) • Made of only DNA & protein • Their results showed that only the DNA entered bacteria infected by the virus & protein did not

• DNA structure • Watson & Crick • Formulated the first structure of DNA • Aided by Rosalind Franklin • Used X-ray crystallography • Visual molecules 3 -D

• Watson & Crick’s discovery • 1) DNA is a double helix • The backbone is made of sugar & phosphate • Rungs are nitrogenous bases • 2) The Nitrogenous Bases • Adenine (A), Thymine (T), Cytosine (C), & Guanine (G) • 3) Run from 5’ to 3’ (reading position) to 3’ to 5’

16. 2 – MANY PROTEINS WORK TOGETHER IN DNA REPLICATION & REPAIR • Replication = DNA to DNA • Transcription = DNA to RNA • Translation = RNA to Protein • Replication • Making of DNA from existing DNA strand • Semiconservative process • At the end of replication, one old strand & one new strand

6 MAJOR POINTS OF REPLICATION • 1) Replication of DNA begins at sites called the ORIGINS OF REPLICATION • 2) Initiation proteins bind to the origin of replication & separate the two strands • Forms a replication bubble • DNA replication then proceeds in both directions along the DNA strand until the molecule is copied • 3) A group of enzymes called DNA POLYMERASES catalyzes the elongation of new DNA at the replication fork

• 4) DNA polymerase adds nucleotides to the growing chain one by one • Works in a 5’ to 3’ direction • Matches A to T & C to G • 5) DNA replication occurs continuously along the 5’ to 3’ end (LEADING STRAND) • The strand that runs 3’ to 5’ is copied in series of segments called the LAGGING STRAND

• 6) The lagging strand is synthesized in separate pieces called OKAZAKI FRAGMENTS • Sealed by DNA ligase • Forms a continuous DNA strand

ACCURACY CHECKS • 1) The specificity of base pairing • 2) Mismatch repair • Special repair enzymes fix incorrectly paired nucleotides • 3) Nucleotide excision repair • Incorrectly placed nucleotides are excised or removed by enzymes called NUCLEASES • The gap left over is filled with correct nucleotides

SHORTENING OF DNA OVER TIME • DNA polymerase • Can only add nucleotides to the 3’ end of a molecule • Prevents completion of the 5’ • Every time that it is replicated for mitosis, a small portion of the chromosome is removed • To avoid losing terminal genes, the linear ends are capped with telomeres • Short repetitive nucleotide sequences that do not carry genes

16. 3 CHROMOSOME = A DNA MOLECULE PACKED TOGETHER WITH PROTEINS • Bacterial Chromosome v. Eukaryotic Chromosomes • Bacterial = one double-stranded, circular DNA molecule associated with a small amount of protein • Eukaryotic = linear DNA molecules associated with large amounts of protein

• In Eukaryotes, DNA & proteins are packed together as CHROMATIN • Shows 4 levels of packaging: • 1) DNA wrapped around histones (proteins) • Resembles beads on a string & are called nucleosomes • Basic unit • 10 nm fiber

• 2) The string of nucleosomes folds to form a 30 nm fiber • 3) Further folding of the 30 nm fibers result in LOOPED DOMAINS • 4) As the looped domains fold, a METAPHASE CHROMOSOME is formed

• As DNA becomes highly packaged, it becomes less accessible to transcription enzymes • This reduces gene expression • In interphase cells, most chromatin is in the high extended form (EUCHROMATIN) & is available for transcription • Some chromatin remains more condensed (HETEROCHROMATIN) & is inaccessible to transcription enzymes