DNA (Deoxyribonucleic acid)

Formation of DNA (during cell division) requires nucleotides, molecules that are the building blocks for DNA. 

The purine bases (adenine and guanine) and pyrimidine bases (thymine and cytosine) are bound to deoxyribose and phosphate and incorporated as necessary. 

Normally, the nucleotides are synthetized de novo from amino acids and other precursors.

A small part of nucleotide production,  is ‘recycled’ from degraded DNA of broken-down cells: salvage pathway”.

DNA is composed of two strands of nucleotides coiled around each other, linked together by hydrogen bonds and running in opposite directions.

Each strand is composed of four complementary nucleotides – adenine (A), cytosine (C), guanine (G) and thymine (T) – with an A on one strand always paired with T on the other, and C always paired with G.

Each strand is used to reconstruct the other, an idea central to the passing on of hereditary information between generations.

The nucleosome is the basic repeating unit in chromatin, made up of 146 base pairs of 2 superhelical turns of DNA wrapped around an 8 histone core, the octamer.

The human genome can be broadly separated into coding and non-coding sequences.

Organized into genes, which are long segments of DNA that include protein-coding regions (exons) and non-coding regions (introns).

Exome refers to the portion of the human genome that contains the DNA sequence that directs protein synthesis, the exons.

The coding genome makes up a small portion of all the bases and encodes instructions for genes, some of which code for proteins.

Genome-wide association studies enable mapping phenotypes to variation in nucleotide bases in human populations.

Exons of most known genes referred to as the exome.

The 22,000 known genes are comprised of approximately 180,000 exons and represent about 3% of the genome.

Most errors in DNA sequence that lead to altered protein function in tumors are located in the exons, therefore, exome sequencing is an efficient method for tumor DNA sequence analysis to uncover genetic causes for tumor behavior.

The specificity is determined by the base sequence which is stored in duplicate form as a double helical structure that allows correction of sequence errors and replication of information during cell division.

A double-stranded helical molecule, with each strand composed of nitrogenous bases (two purines: adenine and guanine, and two pyrimidines: thymine and cytosine) on a background of deoxyribose sugar and phosphoric acid.

The base sequence of a single strand of DNA is the template for replication by DNA-dependent DNA polymerases that unwind the double helix and copy each strand.