Download Chapter 11: The Eukaryotic Chromosome: An Organelle for

Chapter 11: The Eukaryotic Chromosome: An Organelle for Packaging
and Managing DNA
Outline
I. The components of eukaryotic chromosomes: DNA, histones, and nonhistone proteins
A. Each chromosome packages a single long molecule of DNA
B. The protein components of eukaryotic chromosomes: histones and nonhistone proteins
1. Histone proteins abound in the chromatin of all eukaryotic cells
2. Nonhistone proteins are a heterogeneous group named by default
II. Chromosome structure: variable DNA-protein interactions create reversible levels of
compaction
A. The nucleosome: the fundamental unit of chromosomal packaging arises from DNA’s
association with histones
B. Models of higher-level packaging seek to explain the extreme compaction of
chromosomes at mitosis
1. Formation of the 300Å fiber through supercoiling
2. The radial loop-scaffold model seeks to explain compaction of the 300Å fiber
C. A closer look at karyotypes: fully compacted metaphase chromosomes have unique,
reproducible banding patterns that identify them
1. Banding patterns are highly reproducible, although no one knows for sure what they
represent
2. Researchers can use bands to locate genes
3. Geneticists can use banding patterns to analyze chromosomal differences between
species
4. Banding pattern differences between individuals can reveal the cause of genetic
disease
III. Specialized chromosomal elements ensure accurate replication and segregation of
chromosomes
A. The accurate duplication of chromosome structure depends on origins of replication
and telomeres
1. There are many origins of replication
2. Telomeres preserve the integrity of linear chromosomes
3. Proteins are an essential part of reproducing chromatin structure
B. The segregation of condensed chromosomes depends on centromeres
1. Centromeres appear as constrictions in the chromosomes
2. Centromeres have two functions that ensure proper segregation
3. Using centromere function to analyze centromere structure
C. Comprehensive example: yeast artificial chromosomes confirm which structural
elements are essential for replication and segregation
IV. How chromosomal packaging influences gene activity
A. Controlled decompaction precedes gene expression
1. Boundary elements delimit areas of decompaction
2. Nucleosomes in the decompacted area unwind to allow the initiation of transcription
3. By influencing gene expression, chromatin packaging affects tissue differentiation
B. Extreme condensation silences expression
1. Position effect variegation in Drosophila: moving a gene near heterochromatin
prevents its expression
2. Barr bodies are another example of the correlation between heterochromatin and a
decrease in genetic activity
C. Unusual chromosome structures clarify the correlation between chromosome
packaging and gene function
1. Polytene chromosomes magnify patterns of compaction and tie them to gene
expression
2.Decondensed chromatin in the nucleolus of interphase cells contains rRNA genes
actively undergoing transcription
Essential Concepts

Each chromosome consists of one long molecule of DNA compacted by histone and
non-histone proteins. The 5 types of histones: H1, H2A, H2B, H3, H4 are essential to
the establishment of generalized chromosome structure. There are more than 1,000
kinds of nonhistone proteins: Some are structural; others are active in replication and
segregation; but most help determine the time and place of gene expression.

DNA-protein interactions create reversible levels of compaction.
a. The naked DNA wraps around core histones to form nucleosomes, which are
secured by H1.
b. Models of higher level compaction suggest that some sort of super-coiling
condenses the nucleosomal fiber to a wider fiber. Non-histone proteins then anchor
this fiber to form loops and higher levels of compaction (perhaps in the form of
rosettes and bunches of rosettes). In metaphase chromosomes, higher levels of folding
compact DNA 10,000-fold.
c. In fully compacted metaphase chromosomes, the centromere and telomeres become
visible under the microscope. Giemsa staining of metaphase chromosomes reveals
highly reproducible banding patterns that researchers can use to locate genes, analyze
chromosomal differences between species, and diagnose some genetic diseases.

Specialized elements are required for normal chromosome function.
a. Origins of replication are sites accessible for the binding of proteins that initiate
replication. In eukaryotic chromosomes, many origins of replication assure timely
DNA replication.
b. Telomeres, composed of repetitive base sequences, protect the ends of
chromosomes, assuring their integrity. The enzyme telomerase helps reconstruct the
complete telomere with each cell division.
c. Centromeres, which appear as constrictions in metaphase chromosomes, ensure
proper segregation by holding sister chromatids together and by elaborating
kinetochores that properly attach sister chromatids to spindle fibers for mitosis and
meiosis.

Chromosomal packaging influences gene activity.
a. Conditions that decondense selected areas of chromatin precede and facilitate gene
expression. Puffs in Drosophila polytene chromosomes and the nucleoli in most
interphase cells contain decondensed chromatin that is highly transcribed. Boundary
elements called insulators delimit these areas of decompaction. Nucleosomes in the
decompacted areas unwind to allow the initiation of transcription.
b. Extreme condensation silences gene expression; extremely condensed
chromosomal areas appear as darkly staining heterochromatin under the microscope.
Position effect variegation in Drosophila and Barr bodies in mammals are examples
of the correlation between heterochromatin and a loss of gene activity.