Download 10/02 Chromatin and Chromosome structure

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.
b.
c.
d.
They would bind the DNA tighter.
They would separate from the DNA.
They would no longer be attracted to each other.
They would cause supercoiling of the DNA.
Concept Check 3
Neutralizing their positive charges would have
which effect on the histone proteins?
a.
b.
c.
d.
They would bind the DNA tighter.
They would separate from the DNA.
They would no longer be attracted to each other.
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