Choose the correct statements about satellite DNA from the following:
A. During density gradient centrifugation, repetitive DNA is separated from bulk genomic DNA as different peaks.
B. During density gradient centrifugation, the bulk DNA forms the small peaks, which are referred to as satellite DNA.
C. Depending on base composition, length of segment and number of repetitive units, satellite DNA is classified into categories such as micro-satellites and mini-satellites.
D. The VNTR used as a probe by Alec Jeffreys belongs to the class of satellite DNA referred to as micro-satellite.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A and C only
Repetitive DNA is DNA in which a small stretch is repeated many times. When genomic DNA is spun in a density gradient, this repetitive DNA settles apart from the rest. The satellite DNA is the name given to these small extra peaks, and it is the DNA used in DNA fingerprinting.
A. During density gradient centrifugation, repetitive DNA is separated from bulk genomic DNA as different peaks. — True. The repeated stretches have a different density from the rest of the DNA, so they form their own peaks.
B. During density gradient centrifugation, the bulk DNA forms the small peaks, which are referred to as satellite DNA. — False. It is the other way round. The bulk DNA forms the major peak; the small peaks are the satellite DNA.
C. Depending on base composition, length of segment and number of repetitive units, satellite DNA is classified into categories such as micro-satellites and mini-satellites. — True. Base composition here means whether the stretch is A : T rich or G : C rich. These three features sort satellite DNA into classes.
D. The VNTR used as a probe by Alec Jeffreys belongs to the class of satellite DNA referred to as micro-satellite. — False. VNTR (Variable Number of Tandem Repeats) is a mini-satellite, not a micro-satellite. Micro-satellites are the repeats used in the Human Genome Project for genetic and physical maps.
NCERT Extract:
DNA Fingerprinting – “DNA fingerprinting involves identifying differences in some specific regions in DNA sequence called as repetitive DNA, because in these sequences, a small stretch of DNA is repeated many times. These repetitive DNA are separated from bulk genomic DNA as different peaks during density gradient centrifugation. The bulk DNA forms a major peak and the other small peaks are referred to as satellite DNA. Depending on base composition (A : T rich or G:C rich), length of segment, and number of repetitive units, the satellite DNA is classified into many categories, such as micro-satellites, mini-satellites etc.”
DNA Fingerprinting – “… The VNTR belongs to a class of satellite DNA referred to as mini-satellite. A small DNA sequence is arranged tandemly in many copy numbers. The copy number varies from chromosome to chromosome in an individual.”