There is only one possible sequence of amino acids when deduced from a given nucleotides. But multiple nucleot

NCERT Exemplar · Q51

Q. There is only one possible sequence of amino acids when deduced from a given nucleotides. But multiple nucleotides sequence can be deduced from a single amino acid sequence. Explain this phenomena.

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Concept understanding — DNA Fingerprinting

DNA Fingerprinting: A Beginner's Guide

Imagine you have a unique signature — not your name, not your face, but something written in every cell of your body. That is your DNA. And just like a fingerprint on your fingertip, this DNA signature is different for every person (except identical twins). DNA fingerprinting is the technique that reads that signature.

The Everyday Intuition

Think of a barcode on a product. Every product has a unique pattern of black lines. A scanner reads that pattern to identify exactly which product it is. Your DNA works the same way — it is a long, coiled molecule inside your cells, and certain regions of it have repeating patterns that vary from person to person. DNA fingerprinting scans those repeating patterns and creates a "barcode" unique to you.

What the NCERT Textbook Says

The NCERT Class 12 Biology textbook defines DNA fingerprinting as a technique used to identify the differences in the DNA sequences of individuals. It focuses on Variable Number Tandem Repeats (VNTRs) — short, repeating sequences of DNA that are scattered throughout our genome. The number of repeats at a particular location varies greatly between people. Two unrelated individuals are extremely unlikely to have the same number of repeats at the same locations.

Note

DNA fingerprinting does not read your entire DNA sequence. It only looks at a few specific spots where the repeats vary a lot between people. This makes it fast and practical.

How It Works (Simplified)

The process has a few key steps, but you don't need to memorise every chemical name. Focus on the logic:

Extract DNA from a sample — blood, hair, saliva, or even a tiny skin cell.
Cut the DNA at specific points using special enzymes (called restriction enzymes). This chops the long DNA into fragments of different lengths.
Separate the fragments by size using a technique called gel electrophoresis. Smaller fragments move faster through a gel, so they end up farther from the starting point.
Transfer the fragments onto a nylon membrane (Southern blotting).
Probe the membrane with a radioactive or fluorescent probe that sticks only to the VNTR regions. This reveals a pattern of bands — like a barcode.
Compare the patterns between samples.

Why It Matters

DNA fingerprinting is not just a lab curiosity. It has real-world applications that you will encounter in exams and in life:

Forensic science: Matching crime scene evidence (blood, hair, semen) to a suspect. This is what you see in crime shows — but it is real.
Paternity testing: A child's DNA fingerprint is a mix of the mother's and father's. By comparing bands, you can determine biological parenthood with near-certainty.
Identifying victims: After disasters (earthquakes, plane crashes), DNA fingerprinting helps identify bodies when other methods fail.
Studying evolution: Comparing DNA fingerprints of different species reveals how closely related they are.

Important

DNA fingerprinting is not the same as DNA sequencing. Sequencing reads the entire order of bases (A, T, G, C). Fingerprinting only looks at the pattern of fragment lengths. It is faster and cheaper, but gives less detailed information.

A Common Misunderstanding …

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The genetic code is degenerate — multiple codons can specify the same amino acid — so while a protein's amino acid sequence can be translated back into many possible DNA sequences, the reverse (DNA to protein) is uniquely determined.

This question touches on a fundamental asymmetry in the flow of genetic information — the relationship between nucleic acids and proteins is not a two-way street. To understand why, we need to look closely at how the genetic code works.

The central dogma of molecular biology tells us that DNA is transcribed into mRNA, and mRNA is translated into protein. During translation, the mRNA sequence is read in groups of three nucleotides called codons. Each codon specifies one amino acid. There are 64 possible codons (4³) but only 20 standard amino acids. This means that most amino acids are encoded by more than one codon. For example, the amino acid leucine is specified by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG. This property is called the degeneracy of the genetic code.

Now, consider the two directions of this mapping. When you have a given nucleotide sequence — say, a stretch of mRNA — the ribosome reads it codon by codon, and each codon unambiguously points to one amino acid. There is no ambiguity here: UUU always means phenylalanine, GGU always means glycine, and so on. So from a given DNA or mRNA sequence, the amino acid sequence is fixed and unique.

But reverse the process. Suppose you have a protein with the amino acid sequence — let's say — alanine–glycine–serine. How many different mRNA sequences could code for that? Alanine has four codons (GCU, GCC, GCA, GCG), glycine has four (GGU, GGC, GGA, GGG), and serine has six (UCU, UCC, UCA, UCG, AGU, AGC). Multiply these possibilities: 4 × 4 × 6 = 96 different mRNA sequences that would all produce exactly the same tripeptide. And if you consider the DNA that codes for it (remembering that DNA uses T instead of U), the number of possible sequences is even larger because the coding strand can be either of the two DNA strands.

Note

This degeneracy is not random. Many mutations at the third nucleotide of a codon do not change the amino acid at all, because that position is often the one that varies among a given amino acid's several codons — so the protein's function is preserved. This is why the code is often described as "degenerate but not ambiguous." …

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Q.There is only one possible sequence of amino acids when deduced from a given nucleotides. But multiple nucleotides sequence can be deduced from a single amino acid sequence. Explain this phenomena.

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