Chromatin Structure and Organization Quiz

Chromatin Structure and Organization

• Chromatin is composed of DNA wound around histones, which are positively charged proteins containing lysine and arginine residues, and electrostatic interactions between histones and DNA hold them together
• Chromatin can be further condensed into 30 nm fibers, which can be folded upon themselves for even greater condensation
• The structure of 30 nm fibers remained unknown until 2005, and models include the solenoid (helical) structure and the zig-zag aggregation of nucleosomes
• Nonhistone proteins, around 1000 types, play functional roles in gene expression and replication within chromatin
• Chromosomes in interphase nuclei appear to be in the form of 30 nm or 60 nm chromatin fibers, and loops of 30 nm chromatin fibers fold upon themselves to form compact metaphase chromosomes in mitotic cells
• Metaphase chromosomes are organized into large loops attached to a protein scaffold, and higher order chromosome organization involves structural domains composed of chromatin loop domains containing thousands of base pairs
• The attachment between chromatin and nuclear matrix is established at special DNA sequences rich in AT, called MARs or SARs
• The ends of chromatids are called telomeres, which are important in preventing chromosome shortening during replication and are characterized by many repeats of the sequence 5’-GGATT-3’
• Metaphase chromosomes are duplicated structures containing two sister chromatids joined at the centromere, and centromeres are required for chromatid separation late in mitosis
• The kinetochore, a protein structure on chromatids, forms in eukaryotes, assembles on the centromere and links the chromosome to microtubules from the mitotic spindle during mitosis and meiosis
• Telomeres are capped at chromosome ends, and a special enzyme called telomerase synthesizes the very ends of these sequences to keep telomeres from shrinking during DNA replication
• Telomeres are important for stabilizing chromosome structure and preventing chromosome shortening during replication