1. Chapter 11
DNA: The Molecule Of Heredity

2. How Did Scientists Discover That Genes Are Made of DNA?
Transformed bacteria revealed the link between genes and DNA
In the 1920s, Frederick Griffith worked with two strains of Streptococcus pneumoniae bacteria
S-strain caused pneumonia when injected into mice, killing them
R-strain did not cause pneumonia when injected
When the S-strain was killed and injected into mice, it did not cause disease
By the late 1800s early 1900s, scientists had a limited knowledge of genes
Heritable information is carried in discrete units called genes
Genes are parts of structures called chromosomes
Chromosomes are made of deoxyribonucleic acid (DNA) and protein
They did not know what genes were made of
He was trying to make a vaccine to prevent bacterial pneumonia.

3. Transformation in Bacteria
Fig. 11-1
Mouse remains
healthy
Mouse contracts
pneumonia and
dies
Living
R-strain
S-strain
Heat-killed
Mixture of living
R-strain and
heat-killed
(a)
(b)
(c)
(d)
Bacterial strain(s) injected into mouse
Results
Conclusions
R-strain does
not cause
pneumonia.
S-strain causes
Heat-killed S-
strain does not
cause pneumonia.
A substance from
heat-killed S-strain
can transform the
harmless R-strain
into a deadly
S-strain.
Griffith made a sample of heat-killed S-strain and mixed it with the R-strain
Injection of the combination into mice caused pneumonia and death
This was unexpected, since neither element alone caused the disease
The results of Griffith’s experiments led to several new deductions about the genetic material
Some substance in the heat-killed S-strain changed the living, harmless R-strain bacteria into the deadly S-strain, a process Griffith called transformation
The substance that caused this transformation might be the long-sought molecule of heredity

4. How Did Scientists Discover That Genes Are Made of DNA?
The transforming molecule is DNA
In 1933, J. L. Alloway showed that transformation occurred just as readily in culture dishes
This showed that the mouse in Griffith’s experiments was not involved in the transformation
protein-destroying enzymes still induced transformation
When DNA-destroying enzymes were added to the samples, transformation did not occur

5. Molecular Mechanism of Transformation
bacterial
chromosome
DNA fragments are
transported into
the bacterium
Heating S-strain cells killed them but did not completely destroy their DNA
When killed S-strain bacteria were mixed with living R-strain bacteria, fragments of DNA from the dead S-strain cells became incorporated into the chromosome of the R-strain bacteria
If these fragments of DNA contained the genes needed to cause disease, an R-strain cell would be transformed into an S-strain cell
Thus, Avery, MacLeod, and McCarty concluded that genes are made of DNA
A DNA fragment is
incorporated into
the chromosome
Fig. 11-2

6. What Is the Structure of DNA?
The secrets of DNA function and, therefore, of heredity itself are found in the three-dimensional structure of the DNA molecule
DNA is a polymer of nucleotides
Each nucleotide has three components
A phosphate group
A deoxyribose sugar
One nitrogenous base
1. Thymine (T)
2. Cytosine (C)
3. Adenine (A)
4. Guanine (G)
sugar
phosphate
base = adenine

7. What Is the Structure of DNA?
DNA is a double helix
From X-ray diffraction patterns, Wilkins and Franklin deduced several qualities of DNA
It is long and thin, and has a uniform diameter of 2 nanometers
It is helical; that is, twisted like a corkscrew and consisting of repeating subunits

8. What Is the Structure of DNA?
James Watson and Francis Crick combined the X-ray data with bonding theory to deduce DNA structure
Double helix - made of two strands of nucleotides
Within each DNA strand, the phosphate group of one nucleotide bonds to the sugar of the next nucleotide in the same strand
The deoxyribose and phosphate portions make up the sugar-phosphate backbone
Covalently bonded together

9. What Is the Structure of DNA?
James Watson and Francis Crick combined the X-ray data with bonding theory to deduce DNA structure
The nucleotide bases protrude from this backbone
All the nucleotides within a single DNA strand are oriented in the same direction and thus have an unbonded sugar at one end and an unbonded phosphate at the other end

10. The Watson-Crick Model of DNA Structure
free
phosphate
hydrogen
bonds
sugar
nucleotide
base
(cytosine)
free sugar
(a) Hydrogen bonds hold complementary
basepairs together in DNA
(b) Two DNA strands form a
double helix
(c) Four turns of a
DNA double helix
Two DNA strands form a double helix
Hydrogen bonds hold complementary base pairs together in DNA
The two strands are antiparallel; that is, they are oriented in opposite directions
one strand starts with a free sugar and ends with a free phosphate, the other stand starts with a free phosphate and ends with a free sugar
Fig. 11-5

11. What Is the Structure of DNA?
Hydrogen bonds between complementary bases hold two DNA strands together in a double helix
Because of their structures and the way they face each other, adenine (A) bonds only with thymine (T) and guanine (G) bonds only with cytosine (C)
Law of complementary base pairs
Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG
Adenine and guanine are large molecules; thymine and cytosine are relatively smaller
Because base pairing always places a large molecule with a small one, the diameter of the double helix remains constant
Complementary base pairing explains Chargaff’s rule that for a given molecule of DNA, adenine equals thymine and guanine equals cytosine

12. Review Describe a nucleotide.
What makes a nucleotide different from the next?
Describe the three-dimensional structure of DNA.

13. How Does DNA Encode Information?
How can a molecule with only four simple parts be the carrier of genetic information?
The key lies in the sequence, not the number, of subunits
Within a DNA strand, the four types of bases can be arranged in any linear order, and this sequence is what encodes genetic information

14. How Does DNA Encode Information?
The genetic code is analogous to languages, where small sets of letters combine in various ways to make up many different words
English has 26 letters
Hawaiian has 12 letters
The binary language of computers uses only two “letters” (0 and 1, or “on” and “off”)
The sequence of only four nucleotides can produce many different combinations
A 10-nucleotide sequence can code for more than 1 million different combinations of the four bases
Must be in the right order to make sense.

15. How Does DNA Replication Ensure Genetic Constancy During Cell Division?
Replication of DNA is a critical event in a cell’s life
All cells come from pre-existing cells
Cells reproduce by dividing in half
Each of two daughter cells gets an exact copy of the parent cell’s genetic information
Duplication of the parent cell DNA is called DNA replication
mitosis

16. How Does DNA Replication Ensure Genetic Constancy During Cell Division?
DNA replication produces two DNA double helices, each with one original strand and one new strand
The ingredients for DNA replication are threefold:
The parental DNA strands
Free nucleotides
A variety of enzymes that unwind the parental DNA double helix and synthesize new DNA strands
When replication is complete, each parental strand and the daughter strand that was copied from it by base pairing wind together into a new DNA double helix, thus creating two copies of the original double helix

17. How Does DNA Replication Ensure Genetic Constancy During Cell Division?
DNA replication produces two DNA double helices, each with one original strand and one new strand
Base pairing is the foundation of DNA replication
An adenine on one strand pairs with a thymine on the other strand; a cytosine pairs with guanine
If a parental strand reads T-A-G, for example, the new strand reads A-T-C
The two resulting DNA molecules have one old parental strand and one new strand (semiconservative replication)
The parental and two new strands should all be identical to each other.

18. Semiconservative Replication of DNA
free nucleotides
The parental DNA
is unwound by DNA
Helicase
New DNA strands
are synthesized with
bases complementary
to the parental strands
using DNA polymerase
Each new double helix is composed
of one parental strand (blue) and one
new strand (red)
Parental DNA
double helix 1 2 4 3
Fig. 11-6

19. How Do Mutations Occur?
Accurate replication and proofreading produce almost error-free DNA
Infrequent changes in the sequence of bases in DNA result in defective genes called mutations
Mutations range from changes in single nucleotides to movements of large pieces of chromosomes
Mutations may have varying effects on function

20. How Do Mutations Occur? Mutations may have varying effects on function
Mutations are often harmful, and an organism inheriting them may quickly die
Some mutations may have no functional effect
Some mutations may be beneficial and provide an advantage to the organism in certain environments
These advantageous mutations may be favored by natural selection and are the basis for evolution of life on Earth

21. 11.5 How Do Mutations Occur?
Accurate replication and proofreading produce almost error-free DNA
During replication, DNA polymerase mismatches nucleotides once every 1,000 to 100,000 base pairs, yet completed DNA strands contain only about one mistake in every 100 million to 1 billion base pairs
In humans, this amounts to less than one error per chromosome per replication
This reduction in errors is accomplished by DNA repair enzymes, which “proofread” each new daughter strand and replace mismatched nucleotides
Proofreading occurs both during and after replication
1. …because of hydrogen bonds making replication highly accurate.

22. How Do Mutations Occur? Mistakes do happen
DNA is altered or damaged in a number of ways
Mistakes are made during normal DNA replication
Certain chemicals (some components of cigarette smoke, for example) increase DNA errors during and after replication
Ultraviolet radiation or X-rays also contribute to incorrect base pairing
2. Free radicals, some mold toxins
Most changes in DNA are repaired by enzymes

23. Mutations
Fig. 11-8a
Point mutations—also called nucleotide substitutions—involve changes to individual nucleotides in the DNA sequence
One type of point mutation occurs when a repair enzyme finds a mismatch but mistakenly cuts out the correct base and puts in the complement of the erroneous base

24. Mutations
Insertion mutations occur when one or more new nucleotide pairs are inserted into the DNA double helix
Fig. 11-8b

25. Mutations
Deletion mutations occur when one or more nucleotide pairs are removed from the double helix
Fig. 11-8c

26. Mutations
An inversion occurs when a piece of DNA is cut out of a chromosome, turned around, and re-inserted into the gap
Fig. 11-8d

27. Mutations
Fig. 11-8e
A translocation occurs when a chunk of DNA (often very large) is removed from one chromosome and attached to another

28. Review How does DNA encode hereditary information?
Why is DNA replication considered “semi-conservative”?
How do mutations occur?
Why are mutations rare?
Why are mutations usually harmful?