24 Questions
What is the basic structural unit of chromatin?
Nucleosome
What is the function of histones in chromatin?
Package the DNA in an orderly manner in the cell nucleus
What is the diameter of a chromatin fiber?
10 nm
How many major types of histones are there?
5
What is the total extended length of DNA in a human cell?
2 meters
What is the first level of folding of chromatin that involves coiling of DNA around a protein core?
Nucleosome
What is the function of linker DNA in chromatin structure?
It binds to linker histones between the nucleosomes
What is the appearance of 30 nm chromatin fibers under an electron microscope?
Beaded
What is the composition of a nucleosome core particle?
Histone octamer and wrapped DNA
Who first described the basic structural unit of chromatin (nucleosome)?
Roger Kornberg
What is the length of DNA wrapped around a nucleosome core particle?
147 bp
What is the role of H1 histones in chromatin structure?
Binding to linker DNA between the nucleosomes
What are histones composed of?
Positively charged proteins containing lysine and arginine residues
How can chromatin be further condensed beyond the DNA-histone complex?
By folding 30 nm fibers upon themselves
What is the function of nonhistone proteins in chromatin?
They are involved in gene expression and replication
What is the structure of metaphase chromosomes in mitotic cells?
Loops of 30 nm chromatin fibers
What is the function of telomeres?
To prevent chromosome shortening during replication
What is the role of the kinetochore in mitosis and meiosis?
To link the chromosome to microtubules from the mitotic spindle
What is the attachment between chromatin and nuclear matrix established at?
Special DNA sequences rich in AT (MARs or SARs)
What is the structure of 30 nm fibers in chromatin?
Solenoid (helical) structure
What is the function of the protein scaffold in metaphase chromosomes?
To organize large loops of chromatin
What is the structure of telomeres?
Many repeats of the sequence 5’-GGATT-3’
What is the role of centromeres in metaphase chromosomes?
To join sister chromatids
What is the function of telomerase?
To synthesize the very ends of telomeres
Study Notes
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
Test your knowledge of chromatin structure and organization with this quiz. Explore the role of histones, nonhistone proteins, chromosome organization, telomeres, centromeres, and kinetochores in DNA packaging and cell division.
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