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Eucaryotic Chromosomes DNA is Enclosed in a Cell Nucleus Packaged into a Set of Chromosomes Chromosome sequence elements Chromosome organization Chromosome structure Chromosome dynamics In Eucaryotes, DNA is Enclosed in a Cell Nucleus Packaged into a Set of Chromosomes
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Eucaryotic Chromosomes • DNA is Enclosed in a Cell Nucleus • Packaged into a Set of Chromosomes • Chromosome sequence elements • Chromosome organization • Chromosome structure • Chromosome dynamics
FISH • Identify DNA sequences to be amplified • Attach fluorescent tags to complementary DNA to make a probe • PCR with tagged primers • Break purified DNA, denature, reanneal, extend with DNA polymerase and fluorescent substrates • Denature target DNA “in situ” • In place, cell or tissue preparation on microscope slide • Hybridize • Soak immobile target DNA in solution of tagged probes under denaturing conditions • Slowly cool to allow double strands to form • Wash off unhybridized probe
Chromosome sequence elements • Centromeres (1) • Ensure proper chromosome segregation • Telomeres (2) • Mark and protect the ends of chromosomes • Replication origins (tens to hundreds) • Provide sites for the initiation of DNA synthesis • Genes (thousands) • Repeated sequences (many thousands)
Comparisons Between the DNAs of Related Organisms Distinguish Conserved and Nonconserved Regions of DNA Sequence
Chromosomes Exist in Different States Throughout the Life of a Cell
Nucleosomes Are the Basic Unit of Eucaryotic Chromosome Structure
The Structure of the Nucleosome Core Particle Reveals How DNA Is Packaged
The Positioning of Nucleosomes on DNA Is Determined by Both DNA Flexibility and Other DNA-bound Proteins
The 60 nm Chromatin Fiber is dependent on the linker histone H1
ATP-driven Chromatin Remodeling Machines Change Nucleosome Structure
Covalent Modification of the Histone Tails Can Profoundly Affect Chromatin
THE GLOBAL STRUCTURE OF CHROMOSOMES • Chromatin is arranged in domains • Large loops of DNA attached to common scaffold • Heterochromatin is compact and inaccessible for transcription • Chromatin structure is important for the mitosis, recombination and global control of gene expression
Lampbrush Chromosomes Contain Loops of Decondensed Chromatin
Drosophila Polytene Chromosomes Are Arranged in Alternating Bands and Interbands
Heterochromatin Is Highly Organized and Usually Resistant to Gene Expression
Variegated phenotypes arise from clonal inheritance of chromatin structure
The Ends of Chromosomes Have a Special Form of Heterochromatin
Heterochromatin May Provide a Defense Mechanism Against Mobile DNA Elements • Repeated DNA sequences often are clusters of mobile elements • Different elements can replicate through either RNA or DNA intermediates • Packaging repeats in heterochromatin may keep them from spreading • RNA interference can play a role in silencing and heterchromatin mainenance
Mitotic Chromosomes Are Formed from Chromatin in Its Most Condensed State
Individual Chromosomes Occupy Discrete Territories in an Interphase Nucleus
Molecular genetic processes may take place in localized “factories”