Concept
of Gene.
1) A particular segment of DNA responsible for
expression and inheritance of character is called as gene.
2) Mendel was first to give idea about gene, and
he called them as ‘factors’ or ‘determinar’.
3) Genes are fundamental physical units’ of
heredity and inheritance.
4) Johansen used term ‘gene’ for Mendelian factor.
5) Genes are located in linear fashion on
chromosomes at specific position called as locus.
6) Gene is segment of DNA and chemically made up
of DNA only.
7) Chemicals of which genes are composed of are
collectively termed as genetic material and chromosome are carriers of genetic
material.
8) Inheritance of particular character is due to
presence of specific genes over chromosomes.
9) Genes show presence of linkage groups over a
chromosome.
10) Seymour Benzer stated that gene is unit of
function (Cistrons), unit of mutation (Muton) and unit of recombination
(Recon).
11) Total number of genes in haploid set of
chromosomes is called as genome.
12) Beadle
and Tatum proposed ‘one gene one polypeptide hypotheses.
Characteristics
of genes or genetic material
A)
Chemicals of which genes are composed of are called as genetic Material.
B) Characteristics
of Genetic Material:-
1) It must be ubiquitous i.e. found everywhere.
2) It should be able to replicate.
3) It must show inheritance from one generation to
other.
4) It should store genetic information which must
be expressed at
appropriate time.
5) It should exhibit diversity.
6) It should be able to undergo mutation.
Functions
of genes.
1) Genes are hereditary units and are transmitted
from one generation to other.
2) Genes act mainly by producing enzymes which
regulate metabolic reaction.
3) Product of reaction is expressed as character.
4) Genes control synthesis of proteins which are
either hormones or
enzymes.
Packaging of chromatin material in
eukaryotic nucleus.
1) The graded organization or systematic
arrangement of DNA molecule on the protein core within the nucleus is called as
chromatin packaging.
2)
Chemical composition of chromatin:-
a)
In eukaryotes, the chromatin is chemically composed of nucleoproteins.
b)
The nucleoprotein consist of DNA (nucleic acid) and histones (Proteins)
c)
Histones are the basic proteins because they are formed of basic amino
acids like arginine, lysine. The
molecular weight of histones is lower than that of non-histones proteins.
d)
The histone molecule are made up of five types such as H2A, H2B, H3, H4 and H1.
3) Nucleosome:
a) Under
electron microscope, chromatin appears as beads on string. This heads are called as nucleosome.
b) Each
nucleosome shows presence of four histone types in the form of octamer unit.
This are four pairs of H2A, H2B, and H3 & H4.
c) Around
this octamer, protein are DNA is wound.
d) DNA
molecule is much more elongated structure which extends as a continuous thread
from one nucleosome to other.
e) The
portion of DNA between two successive nucleosomes is called as linker DNA to
which H1 protein or histone is attached.
f) The
linker DNA + nucleosome together form on entire nucleosome.
4) Nucleosome fiber:-
a)
The chromatin thread i.e. string of heads shows. Condensation by coiling
to form a nucleosome fiber.
b) In
the nucleosome fiber, linker DNA cannot be observe.
5)
Solenoid fiber:-
a)
The nucleosome fiber by further coiling and supercoiling forms very short fiber
called as solenoid fiber.
b)
In thus fiber about six nucleosomes are present in each turn of the
coil.
c)
Solenoid fiber is characterized by presence of highly condensed and tightly
packed DNA showing maximum 10,000 olds (domain).
6)
Significance:
a)
Accommodation: - Hereditary material
DNA is tightly packed on the protein core to accommodate it inside the nucleus.
b)
Easy distribution: - During
cell division DNA can be easily distributed among the daughter cells.
7)
DNA thread shows one complete turn and other 3/4th turn.
8)
Linker DNA is also termed as spacer DNA and plays no role in
inheritance.
The organisms like
bacteria and cyanobacteria show prokaryotic DNA.
It remain attached to plasma membrane.
It shows a double stranded helically twisted
polynucleotide structure.
It is associated with non-histone proteins forming
a single circular chromosomes called bacteria chromosome or nucleoid.
It is highly folded into many loops and each loop
in its term is super coiled.
DNA molecule that does not show supercoiling
is called relaxed.
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