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By the end of this section, you will be able to:
  • Describe three types of sequencing
  • Define whole-genome sequencing

Although there have been significant advances in the medical sciences in recent years, doctors are still confounded by some diseases, and they are using whole-genome sequencing to get to the bottom of the problem. Whole-genome sequencing is a process that determines the DNA sequence of an entire genome. Whole-genome sequencing is a brute-force approach to problem solving when there is a genetic basis at the core of a disease. Several laboratories now provide services to sequence, analyze, and interpret entire genomes.

For example, whole-exome sequencing is a lower-cost alternative to whole genome sequencing. In exome sequencing, only the coding, exon-producing regions of the DNA are sequenced. In 2010, whole-exome sequencing was used to save a young boy whose intestines had multiple mysterious abscesses. The child had several colon operations with no relief. Finally, whole-exome sequencing was performed, which revealed a defect in a pathway that controls apoptosis (programmed cell death). A bone-marrow transplant was used to overcome this genetic disorder, leading to a cure for the boy. He was the first person to be successfully treated based on a diagnosis made by whole-exome sequencing. Today, human genome sequencing is more readily available and can be completed in a day or two for about $1000.

Strategies used in sequencing projects

The basic sequencing technique used in all modern day sequencing projects is the chain termination method (also known as the dideoxy method), which was developed by Fred Sanger in the 1970s. The chain termination method involves DNA replication of a single-stranded template with the use of a primer and a regular deoxynucleotide    (dNTP), which is a monomer, or a single unit, of DNA. The primer and dNTP are mixed with a small proportion of fluorescently labeled dideoxynucleotides (ddNTPs). The ddNTPs are monomers that are missing a hydroxyl group (–OH) at the site at which another nucleotide usually attaches to form a chain ( [link] ). Each ddNTP is labeled with a different color of fluorophore. Every time a ddNTP is incorporated in the growing complementary strand, it terminates the process of DNA replication, which results in multiple short strands of replicated DNA that are each terminated at a different point during replication. When the reaction mixture is processed by gel electrophoresis after being separated into single strands, the multiple newly replicated DNA strands form a ladder because of the differing sizes. Because the ddNTPs are fluorescently labeled, each band on the gel reflects the size of the DNA strand and the ddNTP that terminated the reaction. The different colors of the fluorophore-labeled ddNTPs help identify the ddNTP incorporated at that position. Reading the gel on the basis of the color of each band on the ladder produces the sequence of the template strand ( [link] ).

A deoxynucleotide consists of a deoxyribose sugar, a base, and three phosphate groups. Dideoxyribose is identical to deoxyribose except that the hydroxyl (–OH) group at the 3' position is replaced by H. A 3' hydroxyl is necessary for elongation of the DNA chain, and the chain therefore stops growing if a dideoxyribose instead of deoxyribose is incorporated into the growing chain.
A dideoxynucleotide is similar in structure to a deoxynucleotide, but is missing the 3' hydroxyl group (indicated by the box). When a dideoxynucleotide is incorporated into a DNA strand, DNA synthesis stops.

Questions & Answers

What contribute to evolution of eukaryotes
Chiquita Reply
how transpiration occur in aquatic plants
Sajid Reply
what is the study of allelemorph
Faith Reply
what is protein
Majid Reply
any of a class of nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms, especially as structural components of body tissues such as muscle, hair, etc., and as enzymes and antibodies.
Anirban
what is DNA replication?
Anirban Reply
separation of the DNA to produce new daughter cell. mostly in the form of meiosis
Faith
what is xenia
Mani Reply
can i get a broader difference between inductive reasoning and deductive reasoning
Daniel Reply
what are the types of tissues and there functions
Daniel
what is signal cascade?
nur Reply
it is the process by which plants produce their fo
Getabalew Reply
what are the substrates of this process
Fiko
definition of photosynthesis
Fiko Reply
it is simply the process by which plants get there food from the sun through the use of chlorophyll
Daniel
what are the advantages and disadvantages of external and internal fertilization
Mohd Reply
which type of blood group can be tranfer easily after Accidents
Durgesh Reply
O positive cause it is a general donor
Daniel
It can give to other blood group except O negative that can only get from O negative
Daniel
which is the polygonum type of embryo sac in angiosperms
Madhura Reply
Describe how hormones regulate blood pressure, blood volume, and kidney function
junius Reply
2 Positive water potential is placed on the left side of the tube by increasing Ψp such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?
Sarah Reply
where are the enzymes required for electron transport system located
Madhura
how transpiration occur in aquatic plant
Sajid

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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