Suppose we have to do the sequencing of 5’ CCTAGTCGATCGTCTCGATCGTCGT 3’ using primer against underlined portion using Sanger’s sequencing.

Steps of DNA sequencing of 
5’ CCTAGTCGATCGTCTCGATCGTCGT 3’ using primer against underlined portion:

DNA Sequence 5’ CCTAGTCGATCGTCTCGATCGTCGT 3’ (The yellow portion is unknown and to know it, we do sequencing. This sequence in yellow is shown for reference)

                           ↓

Add 5’ ACGACGATC 3‘ Primer

                           ↓

5’ CCTAGTCGATCGTCTCGATCGTCGT 3’ DNA sequence

3’ CTAGCAGCA 5’ Primer complementary to a small stretch of DNA.

                           ↓

Start reading from the bottom: 5’GAGACGATCGACTAGG 3’, and to get the original sequence reverse and complement the sequence: 5’CCTAGTCGATCGTCTC 3’.

The obtained sequence in this process: 5’CCTAGTCGATCGTCTC 3’

Basis of chain termination method of sequencing:

Dideoxynucleotides or the ddNTPs are the artificially synthesized modified nucleotides used in the Sanger sequencing technique. The ddNTPs are different from the normal dNTPs as they don’t have the hydroxyl group (-OH) at the 3′-C as well as the 2′-C of the deoxyribose sugar. 

The 3’ OH end of the deoxynucleotides facilitates DNA synthesis by Taq DNA polymerase while due to the lack of -OH at the 3’ end, the polymerase (Klenow segment/fragment in this case) can’t elongate the DNA chain when it encounters the ddNTPs. 

Although the 5′-C can form a phosphodiester bond with the previous nucleotide in the chain, the 3′-C cannot form a bond with the next incoming dNTP. The addition of a ddNTP during DNA replication, therefore, stops synthesis.

Once the analog is incorporated at the growing point of the DNA chain, the 3′ end lacks a hydroxyl group and no longer is a substrate for chain elongation. Thus,
the growing DNA chain is terminated, i.e.
dideoxynucleotides act as chain terminators.

As these ddNTPs are radiolabelled, they can be detected using
radiographic techniques.

PROCESS OF CHAIN –TERMINATION METHOD OF SEQUENCING:

1. A sample of DNA is taken and made single-stranded (used as template strand).

2. These samples of single-stranded DNA are divided into 4 different test tubes.

3. The template is attached to the primer (a short length of DNA, oligonucleotides), which is complementary to the small section of the template

4. The 3’-OH group of primer initiates the DNA synthesis.

5. Each tube contains primer-DNA, Klenow segment (larger fragment of DNAthe polymerase of E. coli having polymerization activity but lacking 5’ → 3’ exonuclease activity), dNTPs, modified nucleotides ie; ddNTPs which is radiolabelled (mostly) or labeled with fluorescent dyes.

Two properties of DNA polymerases are utilized:

(i) their ability to synthesize a complimentary copy of a single-stranded DNA template; and (ii) their ability to use 2’, 3dideoxynucleotides as substrates.

6. Synthesis of DNA in each tube takes place, which contains fragments of DNA of variable length. 

7. The sample (fragment of DNA) from each tube is separated by polyacrylamide gel electrophoresis (PAGE).

8. The sequence of bases in a DNA fragment is determined by the electrophoretic bands (radiolabelled mostly) by autoradiography.

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Rahul Jha

Rahul Jha has graduated in Biotechnology. He will be posting science-related concepts exclusively for you while he is exploring the field of Biotechnology. Let’s learn and share!!
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