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Chapter 12- DNA
BIG IDEA: What is the
structure of DNA, and
how does it function in
genetic inheritance?
12.1 Identifying the Substance of
Genes
-Scientists were looking for the molecule that
controls heredity.
-Frederick Griffith wanted to know how
bacteria made people sick.
Isolated “R” and “S” bacteria strains
S- smooth, disease carrying
R- rough, harmless
Griffith cntd’
Performed 4 experiments on mice
S strain → Mouse dies
R strain → Mouse lives
Heat-killed S strain → Mouse lives
Mixture of Heat-killed S and R → Mouse dies.
Discovered that the bacteria in the last experiment had
transformed into the S strain.
Somehow the heat killed S strain passed their
disease causing abilities to the live R strain.
Griffith concluded that the transforming factor
must be a gene
Oswald Avery and a team of scientists repeated Griffith’s
experiments
To find what molecule caused transformations they
treated the mixtures w/ enzymes that killed proteins,
lipids, carbohydrates, RNA, and then DNA.
-Occured in all except one w/ DNA killed
Avery and his team discovered that DNA stores and
transmits genetic info. from generation to generation
Hershey-Chase experiment
-Wanted to confirm that DNA was the genetic molecule, not
protein.
Tagged viruses w/ radioactive isotopes to make them glow.
-S isotope made just protein coat on viruses glow, P isotope
only made DNA glow.
Virus w/ DNA tag glowed.
Hershey and Chase confirmed that DNA is the genetic
material in all living things
DNA stores, copies, and transmits genetic
info. in a cell.
Stores info. needed by every living cell
Before cell division, genetic info. must be copied
After cell division the daughter cell needs a
complete copy of genetic info.
12.2 Structure of DNA
DNA is a nucleic acid made of nucleotides
joined by covalent bonds.
Each “Nucleotide”is like ½ the zipper;
containing 5-C-sugar, Phosphate group, and
nitrogenous base
Nitrogenous bases: Adenine (A),
Thymine (T), Cytosine (C), and
Guanine (G)
% of A=T, and % of C=G
%A+%T+%C+%G= 100%
-This is Chargaff’s Rule
-look @ pg. 335
Rosalind Franklin studied the structure of DNA through x ray
diffraction.
- Clues from Franklin’s work enabled Watson and Crick
to build a model of DNA
Double-helix explains Chargaff’s rule.
DNA strands are “Anti-parallel”, meaning they
run opposite directions. They are held together
by H bonds.
12.3 DNA Replication
Replication is the process by which DNA
copies itself before cell division.
-Strands separate, and each old strand serves
as a template for the new strand to form.
Old strand= ATTCGATGGTA
New strand=
Old strand= CATGATTACA
New strand=
*Remember base
pairs: C-G, A-T
Replication fork is the site where the 2 strands
of DNA separate to undergo replication
The enzyme DNA polymerase joins the
individual nucleotides to produce a new
strand of DNA. It also proofreads new strand.
Telomeres are DNA at the tips of chromosomes. They are
difficult to replicate and they prevent genes from being
lost or damaged during replication
Comparing replication
Prokaryotic
❖ 1 starting point
❖ proceeds in 2
directions.
Eukaryotic
❖ Dozens- hundreds of
starting points
❖ Proceeds in both
directions at each
point