Cbse class 12

36. 1. DNA has two strands that are complementry.
2. Deoxyribose sugar. OH group at carbon atm
no 2 in the Ribose sugar is replaced by only
H which makes it less reactive and hence
more stable.
3. presence of thymine in place of uracil also
make DNA more stable

124. Satellite DNA
consists of very large arrays of tandemly repeating, non-coding
DNA. Satellite DNA is the main component of functional
centromeres.
The name "satellite DNA" refers to the phenomenon that
repetitions of a short DNA sequence tend to produce a different
frequency of the bases adenine, cytosine, guanine and
thymine, and thus have a different density from bulk DNA such
that they form a second or 'satellite' band when genomic DNA
is separated on a density gradient.[2]

125. Lengthof Satellite DNA
A repeated pattern can be between 1 base pair long (a mononucleotide repeat) to several
thousand base pairs long,[5] and the total size of a satellite DNA block can be several
megabases without interruption. Long repeat units have been described containing
domains of shorter repeated segments and mononucleotides (1-5 bp), arranged in clusters
of microsatellites, wherein differences among individual copies of the longer repeat units
were clustered.[5] Most satellite DNA is localized to the telomeric or the centromeric region
of the chromosome. The nucleotide sequence of the repeats is fairly well conserved across
species. However, variation in the length of the repeat is common. For example,
minisatellite DNA is a short region (1-5kb) of repeating elements with length >9
nucleotides. Whereas microsatellites in DNA sequences are considered to have a
length of 1-8 nucleotides .[6] The difference in how many of the repeats is present
in the region (length of the region) is the basis for DNA fingerprinting.[citation needed]

126. Restriction fragment length
polymorphism
In molecular biology, restriction fragment length polymorphism (RFLP) is a technique that exploits
variations in homologous DNA sequences, known as polymorphisms, in order to distinguish individuals,
populations, or species or to pinpoint the locations of genes within a sequence.The term may refer to a
polymorphism itself, as detected through the differing locations of restriction enzyme sites, or to a
related laboratory technique by which such differences can be illustrated. In RFLP analysis, a DNA
sample is digested into fragments by one or more restriction enzymes, and the resulting restriction
fragments are then separated by gel electrophoresis according to their size.

128. Seven steps to understanding DNA fingerprinting:
● Extracting the DNA from cells
● Cutting up the DNA using an enzyme
● Separating the DNA fragments on a gel
● Transferring the DNA onto paper
● Adding the radioactive probe
● Setting up the X-ray film
● Yes - we've got the result!

129. Extracting DNA from Cells
To perform DNA fingerprinting, you must first have a DNA sample! In order to procure this, a
sample containing genetic material must be treated with different chemicals. Common
sample types used today include blood and cheek swabs.
These samples must be treated with a series of chemicals to break open cell membranes,
expose the DNA sample, and remove unwanted components – such as lipids and proteins –
until relatively pure DNA emerges.

130. PCR Amplification (Optional)
If the amount of DNA in a sample is small, scientists may wish to perform PCR – Polymerase Chain
Reaction – amplification of the sample.
PCR is an ingenious technology which essentially mimics the process of DNA replication carried
out by cells. Nucleotides and DNA polymerase enzymes are added, along with “primer” pieces of DNA
which will bind to the sample DNA and give the polymerases a starting point.
PCR “cycles” can be repeated until the sample DNA has been copied many times in the lab if necessary.

131. Treatment with Restriction Enzymes
The best markers for use in quick and easy DNA profiling are those which can be reliably identified using
common restriction enzymes, but which vary greatly between individuals.
For this purpose, scientists use repeat sequences – portions of DNA that have the same sequence so they
can be identified by the same restriction enzymes, but which repeat a different number of times in
different people. Types of repeats used in DNA profiling include Variable Number Tandem Repeats
(VNTRs), especially short tandem repeats (STRs), which are also referred to by scientists as
“microsatellites” or “minisatellites.”
Once sufficient DNA has been isolated and amplified, if necessary, it must be cut with restriction enzymes
to isolate the VNTRs. Restriction enzymes are enzymes that attach to specific DNA sequences and create
breaks in the DNA strands.

132. Gel Electrophoresis
Gel electrophoresis is a brilliant technology that separates molecules by size. The “gel” in question is a
material that molecules can pass through, but only at a slow speed.
Just as air resistance slows a big truck more than it does a motorcycle, the resistance offered
by the electrophoresis gel slows large molecules down more than small ones. The effect of the gel
is so precise that scientists can tell exactly how big a molecule is by seeing how far it moves within a given
gel in a set amount of time.
In this case, measuring the size of the DNA fragments from the sample that has been treated with a
restriction enzyme will tell scientists how many copies of each VTNR repeat the sample DNA contains.
It’s called “electrophoresis” because, to make the molecules move through the gel, an electrical current is
applied. Because the sugar-phosphate backbone of the DNA has a negative electrical charge, the electrical

133. Gel Electrophoresis
Gel electrophoresis is a brilliant technology that separates molecules by size. The “gel” in question is a
material that molecules can pass through, but only at a slow speed.
Just as air resistance slows a big truck more than it does a motorcycle, the resistance offered
by the electrophoresis gel slows large molecules down more than small ones. The effect of the
gel is so precise that scientists can tell exactly how big a molecule is by seeing how far it moves within a
given gel in a set amount of time.
In this case, measuring the size of the DNA fragments from the sample that has been treated with a
restriction enzyme will tell scientists how many copies of each VTNR repeat the sample DNA contains.
It’s called “electrophoresis” because, to make the molecules move through the gel, an electrical current
is applied. Because the sugar-phosphate backbone of the DNA has a negative electrical charge, the

134. Treatment with Radioactive Probe
Now that the DNA is fixed onto the blotting paper, it is treated with a special probe chemical that sticks to
the desired DNA fragments. This chemical is radioactive, which means that it will create a visible
record when exposed to X-ray paper.
This method of blotting DNA fragments onto nitrocellulose paper and then treating it with a radioactive
probe was discovered by a scientist name Ed Southern – hence the name “Southern blot.”
Amusingly, the fact that the Southern blot is named after a scientist and not the direction “south” did not
stop scientists from naming similar methods “northern” and “western” blots in honor of the Southern blot.

135. X-Ray Film Exposure
The last step of the process is to turn the information from the DNA fragments into a visible record. This is
done by exposing the blotting paper, with its radioactive DNA bands, to X-ray film.
X-ray film is “developed” by radiation, just like camera film is developed by visible light, resulting in a
visual record of the pattern produced by the person’s DNA “fingerprint.”
To ensure a clear imprint, scientists often leave the X-ray film exposed to the weakly
radioactive Southern blot paper for a day or more.
Once the image has been developed and fixed to prevent further light exposure from changing the image,
this “fingerprint” can be used to determine if two DNA samples are the same or similar!
Quiz

136. Applications of
DNA
fingerprinting

137. ● Physically connect a piece of evidence to
a person or rule out someone as a
suspect.
● Show who your parents, siblings, and
other relatives may be.
● Identify a dead body that’s too old or
damaged to be recognizabl

138. ● Match tissues of organ donors with
those of people who need transplants.
● Identify diseases that are passed down
through your family.
● Help find cures for those diseases,
called hereditary conditions.