Solenoid (DNA)
Encyclopedia
The solenoid defines the packing of DNA
as a 30nm fiber of chromatin
and results from the helical winding of at least five nucleosome
strands.
In eukaryotic cells
, 146 bp of DNA
are wrapped approximately 1.76 times around a histone octamer
(each histone consists of 2 H2A, H2B dimers, and H3, H4 tetramer) which together are called a nucleosome. Histone H1
, which is not part of the binding histones, tightens the DNA bound to the eight protein complex. The nucleosomes, which at this point resemble bead
s on a string, are further compacted into a helical shape via the NH2 terminal protein interactions of the octameric histones, called a solenoid.
DNA packed into solenoids, unlike DNA in nucleosome form, is not transcriptionally
active. With more packing, solenoids are able to become increasingly more packed, forming chromosomes. At this point, solenoids coil around each other to form a loop (anywhere from 20 to 80,000 base pairs), followed by a rosette (consisting of six connected loops), then a coil, and at last, two chromatids. The end result is the metaphase
chromosome
. The completely condensed chromatin
has a diameter of up to 600 nm.
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
as a 30nm fiber of chromatin
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
and results from the helical winding of at least five nucleosome
Nucleosome
Nucleosomes are the basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a histone protein core. This structure is often compared to thread wrapped around a spool....
strands.
In eukaryotic cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, 146 bp of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
are wrapped approximately 1.76 times around a histone octamer
Histone octamer
A histone octamer is an octamer of the histones found at the center of a nucleosome core particle. It consists of 2 copies of each of the four core histone proteins . The octamer assembles when a tetramer, containing two copies of both H3 and H4, complexes with two H2A/H2B dimers...
(each histone consists of 2 H2A, H2B dimers, and H3, H4 tetramer) which together are called a nucleosome. Histone H1
Histone H1
Histone H1 is one of the five main histone protein families which are components of chromatin in eukaryotic cells. Though highly conserved, it is nevertheless the most variable histone in sequence across species.- Structure :...
, which is not part of the binding histones, tightens the DNA bound to the eight protein complex. The nucleosomes, which at this point resemble bead
Bead
A bead is a small, decorative object that is usually pierced for threading or stringing. Beads range in size from under to over in diameter. A pair of beads made from Nassarius sea snail shells, approximately 100,000 years old, are thought to be the earliest known examples of jewellery. Beadwork...
s on a string, are further compacted into a helical shape via the NH2 terminal protein interactions of the octameric histones, called a solenoid.
DNA packed into solenoids, unlike DNA in nucleosome form, is not transcriptionally
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
active. With more packing, solenoids are able to become increasingly more packed, forming chromosomes. At this point, solenoids coil around each other to form a loop (anywhere from 20 to 80,000 base pairs), followed by a rosette (consisting of six connected loops), then a coil, and at last, two chromatids. The end result is the metaphase
Metaphase
Metaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
. The completely condensed chromatin
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
has a diameter of up to 600 nm.