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What is Genome ?
Genome is the entirety of an organism's hereditary
It is encoded either in DNA or, for many types of virus, in
The genome includes both the genes and the non-coding
sequences of the DNA.
„‟Genome is thus the entire collection of genes and all
other functional and non functional DNA sequences in
an organism in a haploid set of chromosomes. It
includes structural genes, regulatory genes and non
functional nucleotide sequences ‟‟
DNA segments that code for some
specific RNAs or proteins. Encode for mRNAs, tRNAs, snRNAs,
Regulatory sequences- occur
as regulatory elements (initiation sites, promoter sites,operator
Introns and repititive
sequences. Needed for coding, regulation and replication of
DNA. Much more in no than functional sequences.
Genome of all living organisms represents
their hereditary material and is formed of
In Prokaryotic cellsGenomic DNA forms a single circular
chromosome, without basic proteins , lies in the cell
cytoplasm in nucleoid region
In Eukaryotic cellsDNA is associated with basic
proteins(histones), form long chromatin fibers.
Chromatin fibers form a network, enclosed in a
double layered nuclear envelop, condenses into
chromosomes during cell division
comparative genome sizes of organisms
average gene density
1 gene /100,000 bases
1 gene /100,000 bases
1 gene / 9,000 bases
1 gene / 4000 bases
1 gene / 5000 bases
1 gene / 2000 bases
1 gene / 1400 bases
1 gene /1000 bases
‘The nucleus is heart of the cell, which serves as the
main distinguishing feature of the eukaryotic cells. It is an
organelle submerged in its sea of turbulent cytoplasm which
has the genetic information encoding the past history and
future prospects of the cell. Nucleus contains many thread
like coiled structures which remain suspended in the
nucleoplasm which are known as chromatin substance’
Chromatin is the complex combination of DNA and proteins that
makes up chromosomes.
The major proteins involved in chromatin are histone proteins;
although many other chromosomal proteins have prominent roles
The functions of chromatin is to package DNA into smaller
volume to fit in the cell, to strengthen the DNA to allow mitosis and
meiosis and to serve as a mechanism to control gene expression and
ORGANIZATION OF CHROMATIN
In resting non-dividing eukaryotic cells, the genome is in the form
of nucleoprotein-complex- the chromatin.
(randomly dispersed in the nuclear matrix as interwoven network of fine chromatin threads)
The information stored in DNA is organized, replicated and
read with the help of a variety of DNA-binding proteins:
Structural Proteins- Histones(Packing proteins):
main structural proteins found in eukaryotic cells
low molecular weight basic proteins with high proportion of
positively charged amino acids,
bound to DNA along most of its length,
the positive charge helps histones to bind to DNA and play a crucial
role in packing of long DNA molecules.
Functional Proteins- Non- Histones:
associated with gene regulation and other functions of chromatin.
several times to
form the intact
oChemical composition of chromatin
DNA= 20-40 %- most important chemical constituent of chromatin
RNA=05-10 %-associated with chromatin as;
Ribosomal RNA-( rRNA)
Messenger RNA- (mRNA)
Transfer RNA- (tRNA)
PROTEINS=55-60%-associated with chromatin as;
Histones: very basic proteins, constitute about 60% of total protein,
almost 1:1 ratio with DNA.
FIVE TYPES: H1, H2a, H2b, H3 and H4
Non-Histones: They are 20% of total chromatin protein:
50% structural (actin, L & B tubulin and myosin)-contractile proteins,
function during chromosome condensation and in the movement of
chromosomes during mitosis and meiosis.
50% include all enzymes and co-factors –involved in replication,
transcription and regulation of transcription.
Ultrastructure and organization
Folded fiber model
Nucleosome model-( R.D.Kornberg & O. Thomas-1974
NUCLEOSOME MODEL: DNA is tightly bound to histone
proteins which serve to form a repeating array of DNAProtein particles called nucleosomes.
THREE LEVELS OF ORGANIZATION:
DNA wrapping around “Nucleosomes”- The string on beads structure.
A 30 nm condensed chromatin fiber consisting of nucleosome arrays in
their most compact form- The solenoid structure.
Higher levels of packing into metaphase chromosome- The loops, domains
and scaffold structure.
Nucleosomes contain 2 copies
of H2A, H2B, H3 and H4
147 bp of DNA is wrapped
Histone tails emanate from core
contain histone variants
H1 is a linker histone
from Jiang and Pugh, Nature Rev.Genet. 10, 161 (2009)
Hierarchy of Chromatin Organization in the Cell Nucleus:
Nuclear Matrix Associated Chromatin Loops
TYPES OF CHROMATIN
Lightly packed form of chromatin that is rich in gene concentration
takes up light stain and represent most of the chromatin, that disperse after mitosis
Consists of structural genes which replicate and transcribe during G1 and S phase
of the interphase.
Considered genetically active chromatin, since it has a role in their phenotypic
expression of the genes.
DNA is found packed in 3-8 mm fibre.
During metaphase it takes up dark stain.
Tightly packed form of chromatin that takes up deep stain during interphase and
prophase but metaphase takes up light stain.
Chromomeres, centromeric regions, and knobs also take up dark staining, of which
centromeric regions and knobs are the true Heterochromatic. (chromomeres are
transcribed so not true H.C.).
IN the chromosomes all the centromeres fuse to form a long Heterochromatic mass
Heterochromatin consists of highly repetitive DNA sequences. It is late replicating
during the s-phase of the cell and is not transcribed.
•Euchromatin is uncoiled
and active, whereas
remains condensed and
• Eukaryotic Chromosomes Demonstrate Complex
Organization Characterized by Repetitive DNA
• Repetitive DNA sequences are repeated many
times within eukaryotic chromosomes.
• Satellite DNA is highly repetitive and consists
of short repeated sequences.
• are the primary constrictions along
• mediate chromosomal migration during
mitosis and meiosis
• Telomeric DNA sequences consist of short
tandem repeats that contribute to the stability
and integrity of the chromosome.
• Moderately repetitive DNA includes:
– variable number tandem repeats (VNTRs)
• Short interspersed elements (SINES) and long
interspersed elements (LINES) are dispersed
throughout the genome rather than tandemly
repeated, and constitute over 1/3 of the
• These transposable elements are generated
via an RNA intermediate and are referred to as
• The Vast Majority of a Eukaryotic Genome
Does Not Encode Functional Genes
• Only a small portion of the eukaryotic genome
(2%–10%) constitute protein-encoding genes.
• There are also a large number of single-copy
noncoding regions, some of which are
FUNCTION OF CHROMATIN
“The function of the chromatin is to
carry out the genetic information
from one generation to another, by
encoding the past history and future
prospects of the cell. DNA, being the
only permanent component of
chromatin, is the sole genetic material
of eukaryotes. It never leaves the
cell, thus maintaining heredity of the
Chromatin is the complex combination of DNA and proteins that organizes
chromosomes which appear as many thread like coiled and elongated
structures suspended in the nucleoplasm. So the chromatin contains genetic
material instructions to direct cell function.
The first level of packing in Chromatin involves the binding of DNA to
histones into fundamental packing unit particles called nucleosomes.
The second level of packing involves packing of nucleosomes into 30 nm
thick chromatin fibre.
The highest level of packing of chromatin in the chromosome is found at
the metaphase stage of cell division.
There are two distinct types of chromatin- euchromatin
heterochromatin which differ on their staining properties.
In the chromatin, DNA and basic proteins called histones are present in
about equal amounts.
DNA is the permanent component of chromosomes and is the sole genetic
material of eukaryotes.