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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
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
cell in female
cell in male
union of sperm and
egg at fertilization
Stepped Art
Fig. 6-3a, p. 104
 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
 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
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
 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
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
Somatic Cell Nuclear Transfer
 Clone produced by somatic cell nuclear transfer
Digging into Data:
The Hershey Chase Experiments
Fig. 6-12a, p. 113
outside cells
Virus proteins
labeled with 35S
DNA being
injected into
Virus DNA
labeled with 32P
inside cells
Labeled DNA
being injected
into bacterium
Fig. 6-12b-c, p. 113
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