3. CHROMOSOMES
• The DNA of all eukaryotes is organized into morphologically distinct units
called Chromosomes. Each chromosome contains a single enormous DNA
molecule
• 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.
4. • 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
5.
6. NUCLEOSOME
• 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
7.
8. STRUCTURAL HIERARCHY OF
CHROMOSOME
• 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.
9. • 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.
10.
11. REFERENCE
• Molecular biology -8th edition, David Freifelder
• Cell biology – C. B. Power
• https://courses.lumenlearning.com › ...
• Chromosome Structure | Biology for Majors I – Lumen Learning
