Frequently Asked Questions About Human Genome
Sequencing Project
What is a genome?
A genome is a complete deoxyribonucleic acid (DNA) of an organism, a compound
containing genetic instructions needed to develop and direct each biological activity.
A DNA molecule consists of two twisting, paired strands. Each strand consists of four
chemical units called nucleotide bases. The bases are adenine (A), thymine (T),
guanine (G) and cytosine (C). The basis of the opposite strand is specific; A is always
paired with T, and C is always paired with G.
The human genome contains approximately 3 billion of these base pairs, which are
located in 23 pairs of chromosomes in the nucleus of our cells. Each chromosome
contains hundreds to thousands of genes with instructions for making proteins. Each
of the estimated 30,000 genes in the human genome produces an average of three
proteins.
What is Human Genome Sequencing and how to sequence the
genome?
Human Genome Sequencing, which means determining the exact sequence of base
pairs in a DNA fragment. Human chromosomes range in size from about 50,000,000
to 300,000,000 base pairs. Because bases exist in pairs, and the identity of one base in
the pair determines another member of the pair, scientists don’t have to report the two
bases of the pair.
The primary method by which HGP is used to generate the finished version of the
human genetic code is map-based or BAC-based sequencing. BAC is an acronym for
"bacterial artificial chromosomes." Human DNA is fragmented into relatively large
but still controllable fragments (between 150,000 and 200,000 base pairs). These
fragments are cloned into bacteria that can store and replicate human DNA, so that a
sufficient amount of DNA can be prepared for sequencing. If carefully chosen to
minimize overlap, approximately 20,000 different BAC clones are required to contain
3 billion pairs of bases of the human genome. A collection of BAC clones containing
the entire human genome is referred to as a "BAC library."
In the BAC-based approach, each BAC clone is "mapped" to determine the location
of the DNA in the BAC clone from in the human genome. Using this approach
ensures that scientists know the precise location of the DNA letters sequenced from
each clone and their spatial relationship to human DNA sequenced in other BAC
clones.
For sequencing, each BAC clone is cut into smaller fragments that are approximately
2,000 bases in length. These pieces are called "subclones." These subclones are
subjected to a "sequencing reaction". The product of the sequencing reaction is then
loaded into a sequencer (sequencer). The sequencer produces about 500 to 800 base
pairs of A, T, C and G from each sequencing reaction, so that each base is sequenced
about 10 times. The computer then assembles these short sequences into a contiguous
sequence of sequences representing human DNA in the BAC clone
Whose DNA is sequenced for Human Genome Sequencing?
Information is kept confidential for volunteers who provide DNA samples for
sequencing. This sequence is derived from the DNA of several volunteers. To ensure
