A sufficient amount of a small DNA fragment is available for dideoxy sequencing. The fragment to be sequenced contains 20 nucleotides following the site of primer binding: 5'-ATCGCTCGACAGTGACTAGC-[primer site]-3' Dideoxy sequencing is carried out, and the products of the four sequencing reactions are separated by gel electrophoresis. Draw the bands you expect will appear on the gel from each of the sequencing reactions.

Verified step by step guidance

1. Understand the principle of dideoxy sequencing (Sanger sequencing): This method uses dideoxynucleotides (ddNTPs) to terminate DNA synthesis at specific bases. Each reaction contains a DNA template, a primer, DNA polymerase, normal deoxynucleotides (dNTPs), and one type of ddNTP (ddATP, ddTTP, ddCTP, or ddGTP). The ddNTPs lack a 3'-OH group, preventing further elongation of the DNA strand.
2. Identify the sequence of the DNA fragment to be sequenced: The sequence provided is 5'-ATCGCTCGACAGTGACTAGC-[primer site]-3'. The primer binds to the complementary strand, and DNA synthesis proceeds in the 5' to 3' direction, complementary to the template strand.
3. Determine the expected termination points for each ddNTP: For each reaction (ddATP, ddTTP, ddCTP, ddGTP), DNA synthesis will terminate at positions where the corresponding base is incorporated. For example, ddATP will terminate synthesis at positions where 'A' is added, ddTTP at positions where 'T' is added, and so on.
4. Predict the lengths of the terminated fragments: For each reaction, calculate the lengths of the DNA fragments based on the positions of the corresponding bases in the sequence. For example, if 'A' appears at positions 3, 10, and 18, the ddATP reaction will produce fragments of lengths 3, 10, and 18 nucleotides.
5. Visualize the gel electrophoresis results: Arrange the fragments by size for each reaction. Shorter fragments will migrate farther down the gel, while longer fragments will remain closer to the top. Label the lanes for ddATP, ddTTP, ddCTP, and ddGTP, and draw bands corresponding to the fragment lengths determined in the previous step.

Key Concepts

Dideoxy Sequencing

Dideoxy sequencing, also known as Sanger sequencing, is a method for determining the nucleotide sequence of DNA. It involves the incorporation of dideoxynucleotides (ddNTPs) during DNA synthesis, which terminate the elongation of the DNA strand. This results in fragments of varying lengths that can be separated by gel electrophoresis, allowing for the identification of the sequence based on the size of the fragments.

Gel Electrophoresis

Gel electrophoresis is a technique used to separate DNA fragments based on their size. In this process, an electric current is applied to a gel matrix, causing negatively charged DNA fragments to migrate towards the positive electrode. Smaller fragments move faster and travel further through the gel than larger ones, allowing for the visualization of the different lengths of DNA produced in the sequencing reactions.

Primer Binding Site

The primer binding site is a specific sequence of nucleotides on the DNA template where a short single-stranded DNA primer anneals to initiate DNA synthesis. In dideoxy sequencing, the primer is crucial as it provides a starting point for DNA polymerase to extend the new DNA strand. The sequence of the primer determines which part of the DNA fragment will be amplified and sequenced, influencing the resulting bands observed in the gel.