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In molecular biology
Molecular biology

Molecular biology is the study of biology at a molecule level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry....
, a library is a collection of molecules in a stable form that represents some aspect of an organism. Two common types of libraries are cDNA libraries
CDNA library

A cDNA library is a collection of cloned cDNA fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of the organism....
 (formed from Complementary DNA
Complementary DNA

In genetics, complementary DNA is DNA synthesized from a mature mRNA template in a reaction catalyzed by the enzyme reverse transcriptase. cDNA is often used to clone eukaryote genes in prokaryotes....
) and genomic libraries
Gene library

A gene Library is a population of organisms, each of which carries a DNA molecule that was inserted into a cloning vector. Ideally, all of the cloned DNA molecules represent the entire genome of the organism....
. The nucleotide sequences of interest are preserved as inserts to a plasmid
Plasmid

File:plasmid .svgA plasmid is an extra-chromosomal DNA molecule separate from the chromosome which is capable of replicating independently of the chromosomal DNA....
 or the genome of a bacteriophage
Bacteriophage

A bacteriophage is any one of a number of viruses that infection bacteria. The term is commonly used in its shortened form, phage.Typically, bacteriophages consist of an outer protein hull enclosing genetic material....
 that has been used to infect bacterial cells.

A cDNA library
CDNA library

A cDNA library is a collection of cloned cDNA fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of the organism....
 represents all of the mRNA present in a particular tissue, which has been converted back to a DNA template by the use of the enzyme reverse transcriptase
Reverse transcriptase

In biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcription single-stranded RNA into double-stranded DNA....
.






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In molecular biology
Molecular biology

Molecular biology is the study of biology at a molecule level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry....
, a library is a collection of molecules in a stable form that represents some aspect of an organism. Two common types of libraries are cDNA libraries
CDNA library

A cDNA library is a collection of cloned cDNA fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of the organism....
 (formed from Complementary DNA
Complementary DNA

In genetics, complementary DNA is DNA synthesized from a mature mRNA template in a reaction catalyzed by the enzyme reverse transcriptase. cDNA is often used to clone eukaryote genes in prokaryotes....
) and genomic libraries
Gene library

A gene Library is a population of organisms, each of which carries a DNA molecule that was inserted into a cloning vector. Ideally, all of the cloned DNA molecules represent the entire genome of the organism....
. The nucleotide sequences of interest are preserved as inserts to a plasmid
Plasmid

File:plasmid .svgA plasmid is an extra-chromosomal DNA molecule separate from the chromosome which is capable of replicating independently of the chromosomal DNA....
 or the genome of a bacteriophage
Bacteriophage

A bacteriophage is any one of a number of viruses that infection bacteria. The term is commonly used in its shortened form, phage.Typically, bacteriophages consist of an outer protein hull enclosing genetic material....
 that has been used to infect bacterial cells.

A cDNA library
CDNA library

A cDNA library is a collection of cloned cDNA fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of the organism....
 represents all of the mRNA present in a particular tissue, which has been converted back to a DNA template by the use of the enzyme reverse transcriptase
Reverse transcriptase

In biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcription single-stranded RNA into double-stranded DNA....
. It represents the genes that are transcribed in particular tissues under particular physiological, developmental, or environmental conditions. cDNA libraries are useful in reverse genetics, but should not be confused with a genomic library, as it does not represent the entire genome, only a very small (less than 1%) portion which is being transcribed.

A genomic library is a set of clones, packaged in the same vector, that together represents all regions of the genome of the organism in question. The number of clones that constitute a genomic library depends on (1) the size of the genome in question and (2) the insert size tolerated by the particular cloning vector system.

Usually a cDNA library is created when reproducing eukaryotic genomic material, whereas genomic libraries are often created when working with genomic target material from bacteria and viruses.