1. DNA: The Molecule Of Heredity
Chapter 11
DNA: The Molecule Of Heredity

2. Chapter 11 At a Glance
11.1 How Did Scientists Discover That Genes Are Made of DNA?
11.2 What Is the Structure of DNA?
11.3 How Does DNA Encode Information?
11.4 How Does DNA Replication Ensure Genetic Constancy During Cell Division?
11.5 How Do Mutations Occur?

3. 11.1 How Did Scientists Discover That Genes Are Made of DNA?
By the late 1800s, 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

4. 11.1 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

5. 11.1 How Did Scientists Discover That Genes Are Made of DNA?
Transformed bacteria revealed the link between genes and DNA (continued)
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

6. 11.1 How Did Scientists Discover That Genes Are Made of DNA?
Transformed bacteria revealed the link between genes and DNA (continued)
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

7. Transformation in Bacteria
Bacterial strain(s) injected into mouse
Results
Conclusions
(a)
Mouse remains
healthy
R-strain does
not cause
pneumonia.
Living
R-strain
(b)
Mouse contracts
pneumonia and
dies
S-strain causes
pneumonia.
Living
S-strain
(c)
Mouse remains
healthy
Heat-killed S-
strain does not
cause pneumonia.
Heat-killed
S-strain
(d)
Mouse contracts
pneumonia and
dies
A substance from
heat-killed S-strain
can transform the
harmless R-strain
into a deadly
S-strain.
Mixture of living
R-strain and
heat-killed
S-strain
Fig. 11-1

8. 11.1 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

9. 11.1 How Did Scientists Discover That Genes Are Made of DNA?
The transforming molecule is DNA (continued)
In the 1940s, Oswald Avery, Colin MacLeod, and Maclyn McCarty isolated DNA from S-strain bacteria, mixed it with live R-strain bacteria, and produced live S-strain bacteria
This suggested that the transforming molecule from the S-strain was DNA
To rule out the possibility that a protein contaminant was actually causing the transformation, Avery treated samples with protein-destroying enzymes and still induced transformation
When DNA-destroying enzymes were added to the samples, transformation did not occur

10. 11.1 How Did Scientists Discover That Genes Are Made of DNA?
The transforming molecule is DNA (continued)
Griffith’s experiments can be explained if DNA is the transforming agent
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

11. Molecular Mechanism of Transformation
bacterial
chromosome
DNA fragments are
transported into
the bacterium
A DNA fragment is
incorporated into
the chromosome
Fig. 11-2

12. 11.2 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

13. 11.2 What Is the Structure of DNA?
DNA is composed of four nucleotides
DNA is made of chains of small subunits called nucleotides
Each nucleotide has three components
A phosphate group
A deoxyribose sugar
One of four nitrogen-containing bases
1. Thymine (T)
2. Cytosine (C)
3. Adenine (A)
4. Guanine (G)

14. 11.2 What Is the Structure of DNA?
DNA is composed of four nucleotides (continued)
DNA is a double helix of two nucleotide strands
Hydrogen bonds between complementary bases hold two DNA strands together

15. DNA Nucleotides phosphate base = adenine sugar base =guanine
base = thymine
base = cytosine
base =guanine
Fig. 11-3

16. 11.2 What Is the Structure of DNA?
DNA is composed of four nucleotides (continued)
In the 1940s, biochemist E. Chargaff determined that the amount of A in a DNA molecule equaled the amount of T, and the amount of C equaled the amount of G
This finding was called “Chargaff’s rule”

17. 11.2 What Is the Structure of DNA?
DNA is a double helix
In the 1940s, several other scientists investigated the structure of DNA
Rosalind Franklin and Maurice Wilkins studied the structure of DNA crystals using X-ray diffraction

18. 11.2 What Is the Structure of DNA?
DNA is a double helix (continued)
From X-ray diffraction patterns, they 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

19. X-ray Diffraction Studies of DNA
Fig. 11-4

20. 11.2 What Is the Structure of DNA?
DNA is a double helix (continued)
James Watson and Francis Crick combined the X-ray data with bonding theory to deduce the structure of DNA
DNA is 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

21. 11.2 What Is the Structure of DNA?
James Watson and Francis Crick combined the X-ray data with bonding theory to deduce DNA structure (continued)
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

22. The Discovery of DNA
Fig. E11-3

23. 11.2 What Is the Structure of DNA?
Hydrogen bonds between complementary bases hold two DNA strands together in a double helix
The bases protrude inward toward each other from the sugar-phosphate backbone like rungs on a ladder
Hydrogen bonds hold the base pairs together
The ladder-like structure is twisted into a double helix

24. 11.2 What Is the Structure of DNA?
Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued)
The two strands in a DNA double helix are said to be antiparallel; that is, they are oriented in opposite directions
From one end of the DNA molecule, if one strand starts with the free sugar and ends with the free phosphate, the other stand starts with the free phosphate and ends with the free sugar

25. 11.2 What Is the Structure of DNA?
Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued)
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)
Bases that bond with each other are called complementary base pairs
Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG

26. 11.2 What Is the Structure of DNA?
Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued)
Complementary base pairing explains Chargaff’s rule that for a given molecule of DNA, adenine equals thymine and guanine equals cytosine
Since every adenine, for example, is paired with a thymine, no matter how many adenines are in the DNA molecule, there will be an equal number of thymines

27. 11.2 What Is the Structure of DNA?
Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued)
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
In 1953, James Watson and Francis Crick consolidated all the historical data about DNA into an accurate model of its structure

28. The Watson-Crick Model of DNA Structure
nucleotide
nucleotide
free
phosphate
free
sugar
phosphate
base
(cytosine)
sugar
hydrogen
bonds
free sugar
free
phosphate
(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
Fig. 11-5

29. 11.3 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

30. 11.3 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”)

31. 11.3 How Does DNA Encode Information?
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

32. Replication of DNA is a critical event in a cell’s life
11.4 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

33. 11.4 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 fact of complementary base pairing suggests a model for how DNA replicates
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

34. 11.4 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 (continued)
DNA replication begins with enzymes, called DNA helicases, that pull apart the parental DNA double helix at the hydrogen bonds between the complementary base pairs
A second strand of new DNA is synthesized along each separated strand by DNA polymerases, which pair free nucleotides with their complementary nucleotides on each separated strand

35. 11.4 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 (continued)
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

36. Author Animation: DNA Replication

37. Basic Features of DNA Replication
free nucleotides
The parental DNA
is unwound
New DNA strands
are synthesized with
bases complementary
to the parental astrands
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

38. 11.4 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 (continued)
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

39. 11.4 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 (continued)
The two resulting DNA molecules have one old parental strand and one new strand (semiconservative replication)

40. Semiconservative Replication of DNA
Two identical DNA
double helices, each
with one parental
strand (blue) and
one new strand (red)
One DNA
double helix
DNA replication
Fig. 11-7

41. Accurate replication and proofreading produce almost error-free DNA
11.5 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

42. 11.5 How Do Mutations Occur?
Accurate replication and proofreading produce almost error-free DNA (continued)
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

43. 11.5 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

44. 11.5 How Do Mutations Occur?
Mutations range from changes in single nucleotide pairs to movements of large pieces of chromosomes
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
Insertion mutations occur when one or more new nucleotide pairs are inserted into the DNA double helix
Deletion mutations occur when one or more nucleotide pairs are removed from the double helix

45. Types of mutations (continued)
11.5 How Do Mutations Occur?
Types of mutations (continued)
An inversion occurs when a piece of DNA is cut out of a chromosome, turned around, and re-inserted into the gap
A translocation occurs when a chunk of DNA (often very large) is removed from one chromosome and attached to another

46. Mutations
Fig. 11-8a

47. Mutations
Fig. 11-8b

48. Mutations
Fig. 11-8c

49. Mutations
Fig. 11-8d

50. Mutations
Fig. 11-8e

51. Mutations may have varying effects on function
11.5 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