From NCERT NEET - 2020
In the polynucleotide chain of DNA, a nitrogenous base is linked to the –OH of:
1. 2'C pentose sugar
2. 3'C pentose sugar
3. 5'C pentose sugar
4. 1'C pentose sugar
Ans:4. 1'C pentose sugar
DNA Keyword from NCERT:
- TOOLS OF RECOMBINANT DNA TECHNOLOGY
restriction enzymes, polymerase enzymes, ligases, vectors
and the host organism.
- Restriction Enzyme
In the year 1963, the two enzymes responsible for restricting the growth
of bacteriophage in Escherichia coli were isolated. One of these added
methyl groups to DNA, while the other cut DNA. The later was called
restriction endonuclease.
- The first restriction endonuclease–Hind II, whose functioning
depended on a specific DNA nucleotide sequence was isolated and
characterised five years later. It was found that Hind II always cut DNA
molecules at a particular point by recognising a specific sequence of
six base pairs. This specific base sequence is known as the
recognition sequence for Hind II.
- Traditional hybridisation procedures used in plant and
animal breeding, very often lead to inclusion and multiplication of
undesirable genes along with the desired genes. The techniques of genetic
engineering which include creation of recombinant DNA, use of
gene cloning and gene transfer, overcome this limitation and allows us
to isolate and introduce only one or a set of desirable genes without
introducing undesirable genes into the target organism.
- In a chromosome there is a specific DNA sequence called the
origin of replication, which is responsible for initiating replication.
Therefore, for the multiplication of any alien piece of DNA in an organism
it needs to be a part of a chromosome(s) which has a specific sequence
known as ‘origin of replication’.This can also be called as cloning or
making multiple identical copies of any template DNA.
- The construction of the first recombinant
DNA emerged from the possibility of linking a gene encoding antibiotic
resistance with a native plasmid (autonomously replicating circular
extra-chromosomal DNA) of Salmonella typhimurium. Stanley Cohen and
Herbert Boyer accomplished this in 1972 by isolating the antibiotic
resistance gene by cutting out a piece of DNA from a plasmid which was
responsible for conferring antibiotic resistance.When this DNA is transferred into
Escherichia coli, a bacterium closely related to Salmonella, it could
replicate using the new host’s DNA polymerase enzyme and make multiple
copies. The ability to multiply copies of antibiotic resistance gene in
E. coli was called cloning of antibiotic resistance gene in E. coli.
- Restriction enzymes belong to a larger class of enzymes called
nucleases. These are of two kinds; exonucleases and endonucleases.
Exonucleases remove nucleotides from the ends of the DNA whereas,
endonucleases make cuts at specific positions within the DNA.
Each restriction endonuclease recognises a specific
palindromic nucleotide sequences in the DNA.
When cut by the same restriction enzyme, the resultant DNA fragments
have the same kind of ‘sticky-ends’ and, these can be joined together
(end-to-end) using DNA ligases.
- Separation and isolation of DNA fragments : The cutting of DNA by
restriction endonucleases results in the fragments of DNA. These fragments
can be separated by a technique known as gel electrophoresis. Since
DNA fragments are negatively charged molecules.
Nowadays the most commonly used matrix is agarose
which is a natural polymer extracted from sea weeds.
The separated DNA fragments can be
visualised only after staining the DNA
with a compound known as ethidium
bromide followed by exposure to UV
radiation (you cannot see pure DNA
fragments in the visible light and
without staining). You can see bright
orange coloured bands of DNA in a
ethidium bromide stained gel
exposed to UV light.
The
separated bands of DNA are cut out
from the agarose gel and extracted
from the gel piece. This step is known
as elution.
