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Chapter 12: DNA
Lab Biology CP
Chapter 12: DNA
• 12.1 Identifying the Substance of Genes
• 12.2 The Structure of DNA
• 12.3 DNA Replication
Identifying the Substance
in Genes
– A section of DNA on a
chromosome in a
– Each chromosome
contains hundreds of
– contains most of your
characteristics: hair
color, skin color, how
things taste to you, etc.
– Contains the instructions
for making a specific
– Determines the order of
amino acids in making
Bacterial Transformation
Frederick Griffith’s Experiment
• Injected mice with 4 different samples of bacteria
• Heat-killed diseasing causing bacteria injected mice
• Live, harmless bacteria injected mice LIVED
• The 2 strains together caused fatal pneumonia
• Concluded: genetic info could be transferred from one
bacterial strain to another
• Transformation: when one type of bacteria (harmless)
changed permanently into another (disease causing
• The transforming factor must be a GENE!!!
Griffith’s Experiment
Oswald Avery’s Experiment
• By observing bacterial
transformation, he
discovered that DNA
stores and transmits
genetic information
from one generation to
the next.
• DNA is responsible for
Avery’s Experiment
Bacterial Viruses
• Bacteriophage: a virus
that infects bacteria
The Hershey-Chase Experiment
• Studied a bacteriophage composed of a DNA
core and a protein coat
• They wanted to determine what part entered
the bacterial cell (the DNA or protein coat)
• Used radioactive markers to label the DNA
and proteins
• Results: The bacteriophages only injected
DNA, not proteins, into the bacterial cells
The Role of DNA
• The DNA that makes up
genes must be capable
– 1.) storing,
– 2.) copying,
– 3.) transmitting, and
the genetic
information in a cell
The Structure of DNA
• Deoxyribonucleic Acid
Structure of DNA
• Double-stranded
• Double-helix
• Side ladders:
– Sugar (deoxyribose) and phosphate group
– Held together by hydrogen bonds
• Steps of ladder:
– 4 nitrogen bases (nucleotides)
• A-T (adenine and thymine)
• C-G (cytosine and guanine)
DNA Scientists
• Erwin Chargaff’s Rule
– Base pairing
– A-T and C-G
• Rosalind Franklin
– Used x-ray diffraction to study DNA and gain
information about it’s structure
• James Watson & Francis Crick (1953):
– Double Helix model from evidence from Franklins xray pattern
DNA Replication
• DNA unzips and new base pairs are formed 
2 new strands of DNA
DNA Replication
DNA must get copied BEFORE a cell can divide
Occurs during late interphase (S phase)
DNA “unzips” into 2 strands
2 new complementary strands are produced
Each new copy has one original strand and
one new strand
• DNA polymerase: An enzyme that joins
individual nucleotides to produce a new
strand of DNA
• Telomeres:
– DNA at the tips of chromosomes
– Very difficult to replicate
• Telomerase: a special enzyme to solve this problem by
adding short, repeated DNA sequences to the
– Often switched off in adult cells
– In cancer, telomerase may be activated to enable
cancer cells to grow rapidly
DNA Replication
in Prokaryotes and Eukaryotes
• Prokaryotes:
– have a singular, circular DNA
– in the cytoplasm
– Begins at a single point
• Eukaryotes:
have 1000x more DNA than prokaryotes
Found in the nucleus
Packed into chromosomes
DNA and proteins packed together: chromatin
Can begin at hundreds of places