Chapter 11 Outline

• 11.1 Large Amounts of DNA Are Packed into a Cell, 286

• 11.2 A Bacterial Chromosome Consists of a Single Circular DNA Molecule, 287

• 11.3 Eukaryotic Chromosomes Are DNA Complexes to Histone Proteins, 288

• 11.4 Eukaryotic DNA Contains Several Classes of Sequence Variation, 295

Chapter 11 Outline

• 11.5 Transposable Elements Are DNA Sequences Capable of Moving, 297

• 11.6 Different Types of Transposable Elements Have Characteristic Structures, 302

• 11.7 Several Hypotheses Have Been Proposed to Explain the Evolutional Significance of Transposable Elements, 308

11.1 Large Amounts of DNA Are Packed into a Cell

• Supercoiling

• Positive supercoiling

• Negative supercoiling

• Topoisomerase: the enzyme responsible for adding and removing turns in the coil

Concept Check 2

A DNA molecule 300 bp long has 20 complete rotations. This DNA molecule is:

a. positively supercoiled.

b. negatively supercoiled.

c. relaxed.

Concept Check 2

A DNA molecule 300 bp long has 20 complete rotations. This DNA molecule is:

a. positively supercoiled.

b. negatively supercoiled.

c. relaxed.

11.3 Eukaryotic Chromosomes Are DNA Complexes to Histone Proteins

Chromatin Structure

• Euchromatin• Regions of DNA that are transcribed

• Heterochromatin• Regions of DNA that remain compacted at all times

• Histone proteins• Positively charged proteins that bind to negatively

charged DNA to aid in the compaction

Chromatin Structure

• Nucleosome– DNA wrapped around Histones

• Chromatosome– Nucleosome plus H1 “connector”

• Linker DNA– DNA between chromatosomes

High-order chromatin structure

Extra coiling and compaction of chromatin

Changes in Chromatin Structure

•Compaction has to be undone to allow transcription

•This can be seen in extreme circumstances as DNA puffs

•Can be determined enzymatically by Dnase sensitivity profiles

•Highly compacted DNA is protected

Changes in Chromatin Structure

• Polytene chromosome

• Chromosomal puffs

Changes in Chromatin Structure

• Centromere structure• Specific repetitive sequences

Bound by special proteins

• Telomere sequences• Protect ends of chromosomes• Repetitive sequences (CCCTAA)• Shorten each replication• Tuck into protective structure

Concept Check 3

Neutralizing their positive charges would have which effect on the histone proteins?

a. They would bind the DNA tighter.

b. They would separate from the DNA.

c. They would no longer be attracted to each other.

d. They would cause supercoiling of the DNA.

Concept Check 3

Neutralizing their positive charges would have which effect on the histone proteins?

a. They would bind the DNA tighter.

b. They would separate from the DNA.

c. They would no longer be attracted to each other.

d. They would cause supercoiling of the DNA.

11.4 Eukaryotic DNA Contains Several Classes of Sequence Variation

• Denaturation

• Melting temperature

• Renaturation

Types of DNA Sequences in Eukaryotes

• Unique sequence DNA

• Gene family: similar but not identical copies of unique DNA sequences that arose through duplication of an existing gene

Types of DNA sequences in Eukaryotes

• Repetitive DNA

• Moderately repetitive DNA: 150 ~ 300 bp long

• Tandem repeat sequences• Interspersed repeat sequences

• Short interspersed elements: SINEs: Alu element• Long interspersed elements: LINEs

Types of DNA Sequences in Eukaryotes

• Repetitive DNA

• Highly repetitive DNA: less than 10 bp long

• Microsatellite DNA

11.5 Transposable Elements Are DNA Sequences Capable of Moving

•Either exit current location and move to new location or duplicate self into new location.

•Results in insertions in DNA sequences

General characteristics of transposable elements

• Flanking direct repeats

• Terminal inverted repeats

The Mechanisms of Transposition

• Replicative transposition

• Nonreplicative transposition

• Transposition through an RNA intermediate

The Mutagenic Effects of Transposition

•Transposon can disrupt gene by insertion•Loss of expression of protein•Can also affect normal regulation

The Regulation of Transposition

• Limiting the production of the transposase enzyme

• High levels of transposition in new cells• Decreases as the number of transposons

increases• Reaches steady state

• Some regulation of transcription of transposase, but most regulation is at translation

Transposable elements in bacteria

• Insertion sequences• Minimal sequences with inverted repeats

• Composite transposons• Have extra genes along with inverted repeats

• Noncomposite transposons• No inverted repeats but still produce direct repeats

Transposable elements in bacteria

• Ty elements in yeast

• Ac Ds elements in maize

• Transposable elements in humans• LINEs or SINEs

11.7 Several Hypotheses Have Been Proposed to Explain the Evolutional Significance of

Transposable Elements

• Cellular function hypothesis

• Genetic variation hypothesis

• Selfish-DNA hypothesis