DNA Structure and Function

Chapter 6

1.1 Impacts/IssuesHere 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

Fig. 6-3a, p. 104

diploid reproductive cell in female

diploid reproductive cell in male

X X X Y

eggs sperm

X X X Y

X Y

X

X

XX XY

XX XY

union of sperm and egg at fertilization Stepped Art

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

Fig. 6-8, p. 108

Stepped Art

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.

3) Each parent strand serves as a template for assembly of a new DNA strand from nucleotides, according to base-pairing rules.

2) As replication starts, the two strands of DNA unwind at many sites along the length of the molecule.

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).

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

Fig. 6-12b-c, p. 113

35S remains outside cellsVirus proteins

labeled with 35S

DNA being injected into bacterium

32P remains inside cells

Virus DNA labeled with 32P

Labeled DNA being injected into bacterium