1) The document summarizes a chapter about DNA structure and function from a biology textbook. It describes key discoveries in DNA research including Chargaff's rules, Rosalind Franklin's x-ray images, and Watson and Crick's double helix model. 2) It explains DNA replication as semi-conservative and how DNA polymerase uses each strand as a template to assemble new strands from nucleotides. Errors can lead to mutations if uncorrected. 3) It discusses cloning of adult animals using somatic cell nuclear transfer and describes the process used to create Dolly the sheep, the first mammal cloned from an adult cell.

1. BIOLOGY: Today and TomorrowBIOLOGY: Today and Tomorrow, 4e, 4e
starrstarr eversevers starrstarr
Chapter 6
DNA Structure and Function

2. 6.1 A Hero Dog’s Golden Clones
 Trakr, the search dog that located the final survivor of the
9/11 attacks, later died of toxic exposure
 Trakr’s DNA lives on in his genetic copies (clones)
 To make a clone from an adult cell, researchers must
reprogram its DNA to function like the DNA of an egg
 Cloning mammals is unpredictable; few implanted embryos
result in a live birth, and many have serious health problems

3. James Symington and Trakr at Ground Zero,
September 2001

4. 6.2 Chromosomes
 Each DNA molecule consists of two strands twisted into a
double helix
 In cells, DNA molecules and their associated proteins are
organized into chromosomes
 DNA wraps around “spools” of proteins called histones that
allow chromosomes to pack tightly

5. Chromosome Structure
 When the cell prepares to divide, it duplicates all of its
chromosomes
 Sister chromatid
 One of two attached members of a duplicated eukaryotic
chromosome
 Centromere
 Constricted region in a eukaryotic chromosome where
sister chromatids are attached

6. A Duplicated Chromosome
centromere
one chromatid
its sister
chromatid
a chromosome
(unduplicated)
a chromosome
(duplicated)

7. Chromosome Structure
A) Proteins structurally organize DNA into chromosomes.

8. Chromosome and Uncondensed DNA
B) A human chromosome.
C) DNA, the substance,
extracted from human cells.

9. Chromosome Number
 A eukaryotic cell’s DNA is divided into a characteristic number
of chromosomes
 Chromosome number
 Sum of all chromosomes in a cell of a given type
 A human body cell has 46 chromosomes
 Diploid
 Cells having two of each type of chromosome
characteristic of the species (2n)

10. Two Types of Eukaryotic Chromosomes
 Autosomes
 Paired chromosomes with the same length, shape,
centromere location, and genes
 Any chromosome other than a sex chromosome
 Sex chromosomes
 Members of a pair of chromosomes that differ between
males and females
 In humans, XY and XX

11. Karyotype
 Karyotyping reveals characteristics of an individual’s
chromosomes
 Karyotype
 Image of an individual’s complement of chromosomes
arranged by size, length, shape, and centromere location

12. Human Female Karyotype
A) Karyotype of a female human, with identical sex chromosomes (XX).

13. Bird Female Karyotype
B) Karyotype of a female chicken, with nonidentical sex chromosomes (ZW).

14. ANIMATED FIGURE: Chromosome
structural organization

15. ANIMATION: Karyotype preparation

16. 6.3 Fame, Glory, and DNA Structure
 A DNA molecule consists of two strands of nucleotide
monomers running in opposite directions and coiled into a
double helix
 DNA nucleotide
 One five-carbon sugar (deoxyribose)
 Three phosphate groups
 One nitrogen-containing base (adenine, thymine, guanine,
or cytosine)

17. adenine (A)
deoxyadenosine
triphosphate
three
phosphate
groups
base
Four nucleotides in DNA

18. thymine (T)
deoxythymidine
triphosphate
Four nucleotides in DNA

19. cytosine (C)
deoxycytidine
triphosphate
Four nucleotides in DNA

20. guanine (G)
deoxyguanosine
triphosphate
Four nucleotides in DNA

21. Discovering DNA Structure
 Erwin Chargaff
 Discovered the relationships between DNA bases
 Rosalind Franklin
 Discovered structure of DNA by x-ray crystallography
 Maurice Wilkins
 Experimental evidence of DNA structure
 James Watson and Francis Crick
 Built the first accurate model of a DNA molecule

22. Chargaff’s Rules
 Two double-helix strands are held together by hydrogen
bonds between nucleotide bases
 Chargaff’s rules
 Bases of the two DNA strands in a double helix pair in a
consistent way: A – T and C – G
 Proportions of A and G vary among species

23. Rosalind Franklin’s x-ray diffraction image of
DNA

24. C) Maurice Wilkins.
Watson, Crick, and Wilkins
B) Watson (left) and Crick (right) with
their model of DNA.

25. The Double Helix
 Watson and Crick proposed that DNA consists of two strands
of nucleotides, running in opposite directions, coiled into a
double helix
 Hydrogen bonds between bases hold the two strands together
 Only two kinds of base pairings form: A to T, and G to C
(Chargaff’s rule)

26. DNA Structure
 Covalent bonds
between sugar and
phosphate form the
backbone of each chain
sugar–phosphate backbone
hydrogen bonds link
internally positioned
nucleotide bases
A) Structure of DNA, as illustrated by a
composite of three different models. The two
sugar–phosphate backbones coil in a helix
around internally positioned bases.

27. DNA’s Base-Pair Sequence
 The order of nucleotide bases in a DNA strand (DNA
sequence) is genetic information
 The order of bases varies among species and among
individuals
 The two strands of a DNA molecule are complementary

28. Complementary Base Pairs

29. ANIMATED FIGURE: DNA close up

30. ANIMATED FIGURE: Subunits of DNA

31. 6.4 DNA Replication and Repair
 DNA replication
 Process by which a cell copies its DNA before it divides
 Each strand of the double helix serves as a template for
synthesis of a new, complementary strand of DNA
 Results in two double-stranded DNA molecules identical to
the parent
 Semiconservative replication
 One strand of each molecule is parental (old), and the
other is new

32. DNA Replication and Repair
 During DNA replication, the double-helix unwinds
 DNA polymerase uses each strand as a template to assemble
new, complementary strands of DNA from free nucleotides
 Each type of DNA polymerase requires a primer in order to
initiate DNA synthesis
 DNA ligase seals any gaps to form a continuous strand

33. DNA Replication and Repair
 DNA polymerase
 DNA replication enzyme; assembles a new strand of DNA
based on sequence of a DNA template
 Primer
 Short, single strand of DNA that base-pairs with a targeted
DNA sequence
 DNA ligase
 Enzyme that seals breaks in double-stranded DNA

34. DNA Replication
enzymes
primer
DNA
polymerase
DNA ligase

35. How Mutations Arise
 Proofreading by DNA polymerase corrects most base-pairing
errors during DNA replication
 Uncorrected errors in DNA replication may become mutations
 Mutation
 A permanent change in DNA sequence

36. How Mutations Arise
 Ionizing radiation (x-rays, most UV light, and gamma rays)
can knock electrons out of atoms, breaking DNA
 Non ionizing radiation (UV light 320–380 nm) forms nucleotide
dimers that kink DNA and increase mutation rate
 Chemicals in tobacco smoke transfer methyl groups (CH3) to
nucleotide bases in DNA, causing mispairs during replication
 Breakdown products of many environmental pollutants bind
irreversibly to DNA, causing replication errors

37. Effects of Ionizing Radiation
A) Major breaks (red arrows) in chromosomes of a human white blood cell after exposure to
ionizing radiation. Pieces of broken chromosomes often become lost during DNA replication.

38. thymine
dimer
B) Thymine dimer. This type of DNA damage is caused by exposure to UV
light between 230 and 380 nanometers in wavelength.
Effects of Thymine Dimers

39. ANIMATED FIGURE: DNA replication details

40. ANIMATED FIGURE: Duplication

41. ANIMATED FIGURE: Translocation

42. 6.5 Cloning Adult Animals
 Reproductive cloning
 Technology that produces an exact genetic copy of an
individual (clone)
 Somatic cell nuclear transfer (SCNT)
 Nuclear DNA from an adult somatic cell is transferred into
an unfertilized, enucleated egg
 Common in livestock breeding

43. A) A cow egg is held in place by suction through a hollow glass tube called a
micropipette. DNA is identified by a purple stain.
Somatic Cell Nuclear Transfer

44. B) Another micropipette punctures the egg and sucks out the DNA. All that remains inside
the egg’s plasma membrane is cytoplasm.
Somatic Cell Nuclear Transfer

45. C) A new micropipette prepares to enter the egg at the puncture site. The pipette
contains a cell grown from the skin of a donor animal.
Somatic Cell Nuclear Transfer

46. D) The micropipette enters the egg and delivers the skin cell to a region between the
cytoplasm and the plasma membrane.
Somatic Cell Nuclear Transfer

47. E) After the pipette is withdrawn, the donor’s skin cell is visible next to the cytoplasm of the
egg. The transfer is now complete.
Somatic Cell Nuclear Transfer

48. F) An electric current causes the foreign cell to fuse with and empty its nucleus into the
cytoplasm of the egg. The egg begins to divide, and an embryo forms.
Somatic Cell Nuclear Transfer

49. Clone produced by SCNT
A) Liz the championship Holstein cow (right) with her clone.

50. ANIMATED FIGURE: How Dolly was created

51. 6.6 A Hero Dog’s Golden Clones (revisited)
B) Trakr’s clones with James Symington. Today, the clones are search and rescue dogs for
Team Trakr Foundation, an international humanitarian organization that Symington
established.

52. Digging into Data:
The Hershey Chase Experiments
Virus coat proteins
labeled with 35
S
35
S remains
outside
cells
DNA being
injected into
bacterium
A) In one experiment, bacteria were infected with virus particles that had been labeled with
a radioisotope of sulfur (35S). The sulfur had labeled only viral proteins. The viruses were
dislodged from the bacteria by whirling the mixture in a kitchen blender. Most of the
radioactive sulfur was detected in the viruses, not in the bacterial cells. Thus, the viruses
had not injected protein into the bacteria.

53. Hershey Chase Experiments
Virus DNA
labeled with
32
P
Labeled DNA
being injected
into bacterium
32
P remains
inside cells
B) In another experiment, bacteria were infected with virus particles that had been labeled
with a radioisotope of phosphorus (32P). The phosphorus had labeled only viral DNA. When
the viruses were dislodged from the bacteria, the radioactive phosphorus was detected
mainly inside the bacterial cells. Thus, the viruses had injected DNA into the cells—evidence
that DNA is the genetic material of this virus.

54. Hershey Chase Experiments
C) Detail of Alfred Hershey and Martha Chase’s publication describing their experiments with
bacteriophage. “Infected bacteria” refers to the percentage of bacteria that survived the blender. The
micrograph on the right shows three bacteriophage particles injecting DNA into an E. coli bacterium.