The DNA
THE DNA – Length and Characteristics
NCERT Line Focus:
“DNA is a long polymer of deoxyribonucleotides.”
Explanation:
Just as a train is a long chain of many bogies, DNA is a long macromolecule (polymer) made by linking together many smaller units called deoxyribonucleotides (monomers).
Exam Insight:
Note the specific term—deoxyribonucleotides. RNA is made of ribonucleotides. This slight difference in the sugar molecule changes everything!
NCERT Line Focus:
“The length of DNA is usually defined as number of nucleotides (or a pair of nucleotide referred to as base pairs) present in it. This also is the characteristic of an organism.”
Explanation:
How do we measure the length of a DNA molecule? We simply count the number of its building blocks.
• If the DNA is single-stranded, we count the number of nucleotides.
• If the DNA is double-stranded, we count the number of base pairs (bp) (since nucleotides on opposite strands pair up).
Every single species on Earth has a specific, fixed length of DNA. Your DNA length is different from a dog’s, which is different from a bacterium’s.
NCERT Line Focus:
“For example, a bacteriophage known as \bm{\phi \times 174} has 5386 nucleotides, Bacteriophage lambda has 48502 base pairs (bp), Escherichia coli has \bm{4.6 \times 10^6} bp, and haploid content of human DNA is \bm{3.3 \times 10^9} bp.”
Explanation:
NCERT gives four specific examples to prove that DNA length is a characteristic of an organism. This data is pure gold for competitive exams.
Exam Insight (The Biggest Trap):
Notice carefully that \bm{\phi \times 174} has its length given in “nucleotides” (because it has single-stranded DNA), while the others are given in “base pairs” (bp) (because they have double-stranded DNA). Also, for humans, NCERT specifically mentions the haploid content (one set of chromosomes).
