Download ChapteR 16 The molecular basis of inheritance

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
• 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