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DNA Structure and Function
Chapter 6
1.1 Impacts/Issues
Here Kitty, Kitty, Kitty, Kitty, Kitty
 Making clones (exact genetic copies) of adult
animals is now a common practice in research
and animal husbandry
6.2 Chromosomes
 A eukaryotic chromosome is a molecule of DNA
together with associated proteins
 Chromosome
• Structure made of DNA and associated proteins
• Carries part or all of a cell’s genetic information
Chromosome Structure
 Sister chromatid
• One of two attached members of a duplicated
eukaryotic chromosome
 Centromere
• Constricted region in a eukaryotic chromosome
where sister chromatids are attached
Chromosome Structure
 Proteins organize DNA structurally
• Allow chromosomes to pack tightly
 Histone
• Type of protein that structurally organizes
eukaryotic chromosomes
 Nucleosome
• A length of DNA wound around a spool of histone
proteins
Chromosome Structure
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 23 pairs of chromosomes
 Diploid
• Cells having two of each type of chromosome
characteristic of the species (2n)
Examples of Chromosome Number
Types of Chromosomes
 There are two types of eukaryotic chromosomes:
autosomes and sex 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
Sex Chromosomes:
Sex Determination in Humans
diploid
reproductive
cell in female
diploid
reproductive
cell in male
XX
XY
eggs
sperm
X
X
X
X
Y
X
XX
XY
X
XX
XY
union of sperm and
egg at fertilization
Y
Stepped Art
Fig. 6-3a, p. 104
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
Constructing a Karyotype
6.3 Fame and Glory
 Erwin Chargaff
• Discovered the relationships between DNA bases
 Rosalind Franklin
• Discovered the basic structure of DNA by x-ray
crystallography
 James Watson and Francis Crick
• Built the first accurate model of a DNA molecule
Key Players
 Rosalind Franklin, Maurice Wilkins, James
Watson, and Francis Crick
The Double Helix
 A DNA molecule consists of two strands of
nucleotide monomers running in opposite
directions and coiled into a double helix
 DNA nucleotide
• A five-carbon sugar (deoxyribose)
• Three phosphate groups
• One nitrogen-containing base (adenine, thymine,
guanine, or cytosine)
The Double Helix
 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
The Four DNA Nucleotides
DNA Structure
Patterns of Base Pairing
 The order of bases (DNA sequence) varies
among species and among individuals
• Each species has characteristic DNA sequences
 DNA sequence
• The order of nucleotide bases in a strand of DNA
6.4 DNA Replication and Repair
 A cell replicates its DNA before it divides
 Each strand of the double helix serves as a
template for synthesis of a new, complementary
strand of DNA
 DNA replication results in two double-stranded
DNA molecules identical to the parent
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
 DNA ligase seals any gaps to form a continuous
strand
DNA Replication and Repair
 DNA replication
• Duplication of a cell’s DNA before cell division
 DNA polymerase
• DNA replication enzyme; assembles a new strand
of DNA based on sequence of a DNA template
 DNA ligase
• Enzyme that seals breaks in double-stranded DNA
DNA Replication
1) The two strands of a DNA molecule are
complementary: their nucleotides match up
according to base-pairing rules (G to C, T to A).
2) As replication starts, the two strands of
DNA unwind at many sites along the length
of the molecule.
3) Each parent strand serves as a template
for assembly of a new DNA strand from
nucleotides, according to base-pairing rules.
4) DNA ligase seals any gaps that remain
between bases of the “new” DNA, so a
continuous strand forms. The base sequence
of each half-old, half-new DNA molecule is
identical to that of the parent.
Stepped Art
Fig. 6-8, p. 108
Animation: DNA replication
DNA Replication: The Double Helix
Checking for Mistakes
 DNA repair mechanisms fix damaged DNA
• Proofreading by DNA polymerase corrects most
base-pairing errors
 DNA repair mechanisms
• Any of several processes by which enzymes
repair DNA damage
Mutations
 Uncorrected errors in DNA replication may
become mutations
 Mutation
• A permanent change in DNA sequence
6.5 Cloning Adult Animals
 Reproductive cloning technologies produce an
exact genetic copy of an individual (clone)
 Reproductive cloning
• Technology that produces genetically identical
individuals
Somatic Cell Nuclear Transfer
 Somatic cell nuclear transfer (SCNT)
• Method of reproductive cloning in which nuclear
DNA from an adult somatic cell is transferred into
an unfertilized, enucleated egg
 Therapeutic cloning
• Using SCNT to produce human embryos for
research
Somatic Cell Nuclear Transfer
Clones
 Clone produced by somatic cell nuclear transfer
Digging into Data:
The Hershey Chase Experiments
Fig. 6-12a, p. 113
35S
remains
outside cells
Virus proteins
labeled with 35S
DNA being
injected into
bacterium
Virus DNA
labeled with 32P
32P
remains
inside cells
Labeled DNA
being injected
into bacterium
Fig. 6-12b-c, p. 113
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