-Structure of DNA

-Steps of replication

-Difference between replication, transcription, & translation

-How DNA is packaged into a chromosome

ChapteR 16The 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

16.3Chromosome = 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