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

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 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 10nm fiber

2) The string of nucleosomes folds to form a 30nm fiber 3) Further folding of the 30nm 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