Please explain how the genome sequence is assembled.?

Answer 1

DNA sequencing, the process of determining the exact order of the 3 billion chemical building blocks (called bases and abbreviated A, T, C, and G) that make up the DNA of the 24 different human chromosomes, was the greatest technical challenge in the Human Genome Project. Achieving this goal has helped reveal the estimated 20,000-25,000 human genes within our DNA as well as the regions controlling them. The resulting DNA sequence maps are being used by 21st Century scientists to explore human biology and other complex phenomena.

* Chromosomes, which range in size from 50 million to 250 million bases, must first be broken into much shorter pieces (subcloning step).

* Each short piece is used as a template to generate a set of fragments that differ in length from each other by a single base that will be identified in a later step (template preparation and sequencing reaction steps).

See a figure depicting the sequencing reaction in following web link.
http://www.ornl.gov/sci/techresources/Human_Genome/publicat/tko/05b_img.html

* The fragments in a set are separated by gel electrophoresis (separation step).

New fluorescent dyes allow separation of all four fragments in a single lane on the gel.

* The final base at the end of each fragment is identified (base-calling step). This process recreates the original sequence of As, Ts, Cs, and Gs for each short piece generated in the first step.

Automated sequencers analyze the resulting electropherograms, and the output is a four-color chromatogram showing peaks that represent each of the four DNA bases.

After the bases are "read," computers are used to assemble the short sequences (in blocks of about 500 bases each, called the read length) into long continuous stretches that are analyzed for errors, gene-coding regions, and other characteristics.

Finished sequence is submitted to major public sequence databases, such as GenBank.

Answer 2

Well, they basically broke the entire genome into a mess of random fragments, each of which began and ended at a different place. Then they sequenced each of those fragments. Compunters then took the sequences and looked for places where different fragments overlapped.