CHAPTER 11 : DNA THE MOLECULE OF HEREDITY
Fig. 11-1 Transformation in bacteria*
FREDERICK GRIFFITH'S work focused upon Streptococcus pneumoniae.
TWO STRAINS OF THE BACTERIUM Streptococcus pneumoniae:
R (ROUGH) strain
- did not cause pneumonaie
- does not produce a polysaccharides coat (not encapsulated)
- strain is benign, since it is detected and destroyed by the immune system of the host
S (SMOOTH) strain
- caused death
- produces a polysaccharides coat (encapsulated)
- strain is virulent, since the polysaccharides coat prevents its detection by the host’s immune system
METHOD - injected mice with live, R strain of Streptococcus pneumoniae
RESULT - mice survived
R strain is benign
METHOD - injected mice with live, S strain of Streptococcus pneumoniae
RESULT- mice died of pneumonia
S strain is virulent
METHOD - injected mice with heat-killed S strain of Streptococcus pneumoniae
RESULT - mice survived
- heat-killed S strain cells are benign, polysaccharides coat itself did not cause pneumonia
METHOD - injected mice with a mixture of live R strain cells WITH heat-killed S strain cells
- mice developed pneumonia and died
- the bacteria isolated from the mice were the S strain
- R cells acquired ability to make coats from the dead S cells
- Live R cells were “transformed” to be virulent by a “transforming factor” from the dead S strain cells
Griffith cultured the S cells from the dead mice . produced S daughter cells
Griffith concluded that the newly acquired ability was heritable.
- transforming molecule is DNA; isolated DNA from S-strain bacteria – They treated some samples with protein destroying enzymes and other samples with DNA destroying enzymes
- the protein destroying enzymes did not prevent transformation – but treating samples with DNA destroying enzymes did
- transformation is caused by DNA and not by traces of protein contaminating DNA
- change in phenotype due to the assimilation of external genetic material by a cell.
- Heat denatures protein. Heat did not destroy the transforming ability of the heat-killed S cells.
This suggested that protein is not the genetic material.
Fig. 11-2 Molecular mechanism of transformation
PROTEIN WAS ORIGINALLY THOUGHT TO BE THE GENETIC MATERIAL
- protein macromolecules were considered to possess greater heterogeneity and functional specificity
- less was known concerning nucleic acids (such as DNA)
- the physical and chemical properties of DNA were considered too uniform to account for the numerous inherited traits
We now know that protein is not the heredity material.
DNA IS THE MOLECULE OF HEREDITY.
THE STRUCTURE OF DNA
1953 - James Watson and Francis Crick elucidated the double helix structure of DNA
DNA is a polynucleotide chain.
WHAT ARE THE THREE COMPONENTS OF EACH NUCLEOTIDE?
Fig. 11-3 DNA Nucleotides
Wilkins and Franklin (INFO about DNA)
• DNA is long and thin, diameter of 2 nanometres
• helical twisted like a corkscrew or a spiral staircase
• DNA is a double helix; two strands coil around each other
• DNA consists of repeating subunits
• phosphates are probably on the outside of the helix
DNA Molecule Contains Two Strands
A single strand of DNA is a polymer consisting of many nucleotide subunits
The phosphate group of one nucleotide is bonded to the sugar of the next nucleotide in the strand thus producing a
SUGAR-PHOSPHATE BACKBONE (forms helix) of alternating covalently bonded sugars and phosphates.
Two ends of the strands differ; one end has a free/unbonded sugar and the other has a unbonded phosphate.
HYDROGEN BONDS hold the 2 DNA strands together in a double helix
A complete DNA molecule in living organisms consists of two DNA strands, assembled like a twisted ladder.
The sugar phosphate backbone form the “uprights” of the DNA ladder. The “rungs” are composed of specific pairs of bases
with one member of each pair protruding from the sugar phosphate backbone of each strand. A complete rung consists of
pair of bases held together by hydrogen bonds. The two strands are wound together to form a double helix. The two starnds
are antiparallel to each other. Left hand strand of DNA has a free phosphate group at the top and a free sugar on the bottom; reversed on right end.
Complementary base pairs – Adenine pairs with only Thymine and Guanine only pairs with Cytosine
(Equal to each other)
Adenine and Guanine have two fused rings; Cytosine and Thymine only have a single ring
Figure 11-5 The Watson-Crick model of DNA structure*
HOW DOES DNA ENCODE INFORMATION?
Information is encoded in the order of nucleotides (sequence)
Information depends on the sequence of nucleotides, not their number
WHY IS DNA REPLICATION SO IMPORTANT?
- replication of heredity information is very important since the DNA program assists in the regulation
and control of all biochemical, anatomical, and physiological traits
Fig. 11.6 Basic features of DNA replication
What are the major ENZYMES that are involved in DNA REPLICATION?
DNA HELICASE – pull apart the parental DNA double helix so