MSC 2ND SEM
DEPARTMENT OF BIOTECHNOLOGY,
• Histones and chromatin
• Structural hierarchy of chromosome
• The DNA of all eukaryotes is organized into morphologically distinct units
called Chromosomes. Each chromosome contains a single enormous DNA
• For example – DNA molecule in a single chromosome of fruit fly Drosophilla
DNA has a molecular weight greater than 10^10 and length of 1.2 cm
• (In human 23 pairs of chromosome can be found.)
• As we know that DNA cannot spontaneously fold to form such compact
structure bcoz the molecule would b strained enormously.
• Instead, DNA is made compact by hierarchy of different types of foldings,
each of which is mediated by one or more protein molecules.
• DNA molecule in a eukaryotic chromosome is bound to very basic
proteins called histones.The complex comprising DNA and histones
is called Chromatin .
• There are five major classes of histones –H1, H2A, H2B, H3, H4.
• Histones have an unusual amino acids composition in that they are
extremely rich in positively charged amino acids lysine and arginine.
• (H1 – lysine rich , H3 and H4 are arginine rich )
• The positive charge of the histones is one of the major features of
the molecule enabling them to bind to the negatively charged
phosphates of the DNA.
HISTONES AND CHROMATIN
• Nucleosome is the basic structural unit of DNA packaging in
eukaryotes. The structure of a nucleosome consists of a
segment of DNA wound around eight histone proteins and
resembles thread wrapped around a spool. The nucleosome is
the fundamental subunit of chromatin.
• They serve as a scaffold for formation of higher order
chromatin structure as well as for a layer of regulatory control
of gene expression
STRUCTURAL HIERARCHY OF
• In the first level of compaction, short stretches of the DNA double
helix wrap around a core of eight histone proteins at regular intervals
along the entire length of the chromosome
• The DNA-histone complex is called chromatin. The individual
beadlike, histone DNA complex is called a nucleosome, and DNA
connecting the nucleosomes is called linker DNA.
• A DNA molecule in this form is about seven times shorter than the
double helix without the histones, and the beads are about 10 nm in
diameter, in contrast with the 2-nm diameter of a DNA double helix.
• The next level of compaction occurs as the nucleosomes and the
linker DNA between them are coiled into a 30-nm chromatin fiber.
This coiling further shortens the chromosome so that it is now about
50 times shorter than the extended form.
• In the third level of packing, a variety of fibrous proteins is used to
pack the chromatin.
• These fibrous proteins also ensure that each chromosome in a non-
dividing cell occupies a particular area of the nucleus that does not
overlap with that of any other chromosome.
• Molecular biology -8th edition, David Freifelder
• Cell biology – C. B. Power
• https://courses.lumenlearning.com › ...
• Chromosome Structure | Biology for Majors I – Lumen Learning
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