back into the soil, while they are still young and green.
This practice is used to enrich nitrogen, phosphorus and
other mineral content of the soil.(ii) The steps for green manuring are :
(a) Many leguminous and non leguminous crops are
grown in the field.
(b) When these plants grow to sufficient height, then
they are ploughed back into the soil. The plants must
be still young and green while ploughing back. The
most important advantage of using green manure is
that the complex organic matter present in plants
is decomposed by microorganisms and the nitrogen
which is present in the organic matter is converted
to ammonia, which is either absorbed by the plants
or converted into nitrates.
- (i) Cloning vectors are DNA molecules that can carry a
foreign DNA segment and replicate inside the host cell.
Characteristics of a cloning vector are as follows :
- Origin of replication : Sequence from where
replication starts and any piece of DNA, when
linked to this sequence can be made to replicate
within the host cells. - Selectable marker (antibiotic resistance gene):
It helps in identifying and eliminating non-
transformants and selectively permitting the growth
of transformants. Transformation is a process
through which a piece of DNA is introduced in a
host bacterium. - Cloning sites : In order to link the alien DNA, the
vector needs to have recognition sites for the
commonly used restriction enzymes. Presence of
recognition site allows the particular enzyme to cut
the vector only once.
(ii) In order to force bacteria to take up the plasmid, the
bacterial cells must be made competent to take up DNA.
This is called transformation, which is done by treating
them with specific concentration of a divalent cation,
such as calcium. This increases the efficiency with which
DNA enters the bacterium through pores in cell wall.
Recombinant DNA can then be forced into such cells
by incubating the cells with recombinant DNA on ice,
followed by placing them briefly at 42°C (heat shock) and
then putting them back on ice. This enables the bacteria
to take up the recombinant DNA.
OR
(i) Source of Taq polymerase is Thermus aquaticus, a
bacterium.
Ta q polymerase is a thermostable enzyme. It is
used in polymerase chain reaction (PCR). During the
polymerisation step in PCR, repeated amplification is
achieved by the use of Taq polymerase, which remains
active even at the high temperature induced denaturation
of dsDNA.
(ii) ADA : Adenosine deaminase
It is an enzyme which assists in the functioning of
T-lymphocytes to provide immune response against
invading pathogens.
Deficiency of this enzyme results in severe combined
immuno deficiency (SCID), a hereditary defect in which
patient lacks antibody formation due to impairment
of components of immune system (T-lymphocytes and
B-lymphocytes)
In gene therapy, lymphocytes from the blood of the
patient are grown in culture outside the body. A good
copy of human gene encoding this enzyme (ADA) is
introduced into these cells using a retroviral vector.
These lymphocytes are then transferred into the body
of the patient. The patient requires periodic infusion of
such genetically engineered lymphocytes. If a functional
gene is introduced into the bone marrow cells at early
embryonic stage it would be a permanent cure.
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