This document discusses genomic and cDNA libraries. Genomic libraries are made from genomic DNA and represent all genes in an organism. They require a minimum number of clones to ensure all genes are captured. cDNA libraries are made from mRNA and represent expressed genes, avoiding introns. Key steps in making cDNA libraries include mRNA isolation, cDNA synthesis, addition of linkers, and ligation into a vector. Screening methods to identify clones of interest include hybridization, expression screening, and hybrid arrest/release.

2. ContentsContents
 Genomic librariesGenomic libraries
Representative gene libraries, Size of library,
Genomic DNA, Vectors
 cDNA librariescDNA libraries
mRNA isolation, purification and fractionation,
Synthesis of cDNA, Treatment of cDNA, Ligation
to vector
 Screening proceduresScreening procedures
Screening, Colony and plaque hybridization,
Expression screening, Hybrid arrest and release,
Chromosome walking

3.  Genomic libraries —Genomic libraries —
Representative gene librariesRepresentative gene libraries
Genomic libraryGenomic library:: A collection of different DNA
sequence from an organism each of which has been
cloned into a vector for ease of purification, storage
and analysis .
Genomic librariesGenomic libraries
cDNA librariescDNA libraries
Gene library
(made from genomic DNA)
(made from cDNA- copy of mRNA)

4. Important consideration:
Making a representative library---
Containing all the original sequences
(1) Lacking restriction sites
(2) Does not contain sufficient clones
(3) Enrich certain sequences, lack others
Missing original sequence:
Too long for the vector used

5.  Genomic libraries —Genomic libraries — Size of librarySize of library
The formula to calculate the number of recombinants:
P: desired probability
f : the fraction of the genome in one insert

6. For a probability of 0.99 with insert sizes of 20kb
these values for the E. coli (4.6×106 bp) and human
(3×109 bp) genomes are :
Easy to make good genomic libraries from
prokaryotes in plasmids where the insert size is 5-
10kb, as only a few thousand recombinants will be
needed.

7.  Genomic libraries —Genomic libraries — Genomic DNAGenomic DNA
Purify genomic DNA
Correct size for cloning into the chosen vector:
Physical shearing and restriction enzyme digestion
EukaryotesEukaryotes
ProkaryotesProkaryotes
Clone the fragments into vectors

8. 1. Purification of genomic DNA
Remove protein, lipids and other unwanted
macro-molecules by protease digestion and
phase extraction （ Phenol-chloroform ） .
EukaryotesEukaryotes:: Prepare cell nuclei (fractionation, reduce
contamination from organelle DNA)
Prokaryotes:Prokaryotes: Extracted DNA directly from cells

9. 2. Break DNA into fragments randomly
(1) Physical shearing
Pipeting, mixing or sonication. The
choice of method and time of exposure
depend on the size requirement of the
chosen vector.

10. (2)Restriction enzyme digestion
Partial digestion:
Get a greater lengths of DNA fragments. Time of
digestion and ration of restriction enzyme to DNA
are dependent on the desired insert size range, the
DNA is not digested at every recognition sequence
that is present.
Sau3A: 5’-/GATC-3’,
BamH1: 5’-G/GATCC-3’

11. Ends produced (sticky or blunt) &
the cleaved ends of the vector to be
cloned
DNA modifications
Whether the enzyme is inhibited by DNA
modifications (CpG methylation in
mammals).

12. I1 Genomic libraries —I1 Genomic libraries — VectorsVectors
According to genome’s size, select a proper
vector to construct a library .
Vectors Plasmid phageλ cosmid YAC
insert (kb) 10 23 45 1000

13. λ replacement vector
2.Ligation
3. Packing with a mixture of
the phage coat proteins and
phage DNA-processing
enzymes
4. Infection and
formation of plaques
1.Preparation of arms and
genomic inserts
Library constructed

14.  cDNA libraries —cDNA libraries — mRNA isolation,mRNA isolation,
purification and fractionationpurification and fractionation
The most commonly chosen genomic cloning vectors are λ
replacement vectors which must be digested with restriction
enzymes to produce the two λ end fragment or λ arms between
which the genomic DNA will be ligated.
1. Characteristics of cDNA libraries
2. Methods to isolate mRNA
3. Check the mRNA integrity
4. Cloning the particular mRNAs

15. 1. Characteristics of cDNA libraries
(a) No cDNA library was made from prokaryotic
mRNA.
• Prokaryotic mRNA is very unstable
• Genomic libraries of prokaryotes are easier to make and
contain all the genome sequences.
(b) cDNA libraries are very useful for eukaryotic
gene analysis
• cDNAs represent the transcribed parts of the genome
(i.e. the genes rather than the nontranscribed DNA).
cDNAs have no introns → genes can be expressed in E.
coli directly
• Tissue or cell type specific (differential expression of
genes

16. mRNA isolation, purification
Check the RNA integrity
Fractionate and enrich mRNA
Synthesis of cDNA
Treatment of cDNA ends
Ligation to vector
2. Methods to isolate mRNA

17. • Most eukaryotic mRNAs are polyadenylated
at their 3’ ends.
• oligo (dT) can be bound to the poly(A) tail and
used to recover the mRNA.
3’-AAAAAAAAAAn5’- cap

18. (1)Traditional method was done by pass a preparation
of total RNA down a column of oligo (dT)-cellulose.
(2)More rapid procedure is to add oligo(dT) linked to
magnetic beads directly to a cell lysate and ‘pulling
out’ the mRNA using a strong magnet.
(3)Lying cells and then preparing mRNA-ribosome
complexes on sucrose gradients.
Three methods:

19. 100mM NaCl
rRNA and tRNA
10mM Tris, 1mM
EDTA, poly(A) and
-oligo(dT)

20. Make sure that the mRNA is not degraded.
Methods:
(1)Translating the mRNA : use cell-free translation
system as wheat germ extract or rabbit reticulocyte
lysate to see if the mRNAs can be translated
(2)Analysis the mRNAs by gel electrophoresis: Use
agarose or polyacrylamide gels
3. Check the mRNA integrity

21. 4. Cloning the particular mRNAs
Is useful especially one is trying to clone a
particular gene rather to make a complete cDNA
library
• Fractionate on the gel: Performed on the basis of size, mRNAs
of the interested sizes are recovered from agarose gels
•Enrichment: Carried out by hybridization.
•Example: make a cDNA library of the mRNA sequences that
are induced with a hormone (hybridization ， substrated cDNA
library)

22. I2 cDNA libraries —I2 cDNA libraries — Synthesis of cDNASynthesis of cDNA
• The first strand and Second strand synthesis

23.  cDNA libraries —cDNA libraries — Treatment of cDNATreatment of cDNA
Blunt end ligation of large fragments is not efficient,
so we have to use special linkers to create sticky ends
for cloning.
DNA linker:
DNA adaptor:
HO-CCGAATTCGGGGGG
3’-GGCTTAAGCCCCCC
HO - CGGGGGG
3’-TTAAGCCCCCC

25. The process :
Move protruding 3’-ends( strand-special
nuclease)
Fill in missing 3’ nucleotide( Klenow fragment of
DNA polyI and 4 dNTPs)
Ligate the blunt-end and linkers( T4 DNA ligase)
Restriction enzyme digestion( EcoRI )

26.  cDNA libraries —cDNA libraries — Ligation to vectorLigation to vector
Any vectors with an EcoRI site would suitable
for cloning the cDNA.
The process :
Dephosphorylate the vector
Ligate vector and cDNA with T4 DNA ligase
Plasmid or λ phage vector, short, plasmid vector;
cDNA libraries, λ phage vector;
λgt11 has EcoRI site placed near the C terminus of its
lacZ gene, enabling expression of the cDNA as part of a
large β-galactosidase fusion protein.

27.  Screening procedures —Screening procedures — ScreeningScreening
Screening: The process of identifying one particular
clone containing the gene of interest from among
the
 very large number of others in the gene library .
(1) Using nucleic acid probe to screen the library
based on hybridization with nucleic acids.
(2) Analyze the protein product

28. Screening libraries
Hybridization to identify the interested DNA or its RNA product.
(1) DNA radiolabeled probes which is
complementary to a region of the interested
gene.

29. DNA sequence information:
An oligonucleotide derived from the sequence of a
protein product of the gene.
A DNA fragment/oligo from a related gene of another
species.
Preparation methods:
Automated chemical synthesis (short probes)
PCR

30.  Screening procedures —Screening procedures —
Colony and plaque hybridizationColony and plaque hybridization
Transfer the DNA in the plaque or colony to a Nylon or
nitrocellulose membrane
Phage DNA bind to
the membrane directly
Bacterial colonies must be lysed to
release DNA on the membrane surface.
(Alkali treatment)
Hybridization (in a solution
containing Nucleic acid probe)
Wash to remove unhybri-
dized probe and visualize
X-ray film (radio-
actively labeled )
Antibody or enzyme
(modified nucleotide
labeled)
Line up the hybridizated region or
repeated hybridization

31. Transfer to nitrocellulose
or nylon membrane
Denature DNA
(NaOH). Bake
onto membrane
Probe with 32
p-labled DNA
complementary to
gene of interest
Expose to film
Select positive
from master plate
Keep master
Plate
Screening by plaque hybridization

33.  Screening procedures —Screening procedures —
Expression screeningExpression screening
If the inserts are cloned into an expression
sites, it may be expressed. Therefore, we can
screen for the expressed proteins.
Example: the EcoRI site of lgt11 vector. The
inserted genes have one in six possibilities
(1/6) to be in both the correct orientation (two
possibilities;  ) and reading frame (three
possibilities).

34.  Screening procedures —Screening procedures —
Hybrid arrest and releaseHybrid arrest and release
(1) Hybrid arrested translation
Individual cDNA clones or pools of clones can be used to
hybridize to mRNA preparation.
Translate the mRNA population directly, and the
inhibition of translation of some products detected.
（ 2 ） Hybrid release translation
Purify the hybrids and the hybridized mRNAs
released from them and translated, it identifies the
protein encoded by the cDNA clone

35.  Screening procedures —Screening procedures —
Chromosome walkingChromosome walking
Definition: To clone the desired gene by
repeated isolating adjacent genomic clones
from the library.

37. Multiple choice questionsMultiple choice questions
1. Which two of the following statements about genomic libraries are false?
A genomic libraries are made from cDNA.
B genomic libraries must be representative if they are to contain all the genes in an
organism.
C genomic libraries must contain a minimum number of recombinants if they are to
contain all the genes In an orgamsm.
D the DNA must be fragmented to an appropriate size for the vector that is used.
E genomic libraries made from eukaryotic DNA usually use plasmid vectors.
2. Which statement correctly describes sequential steps in cDNA cloning?
A reverse transcription of Mrna second strand synthesis cDNA end modification ligation to
vector.
B mRNA preparation cDNA synthesis using reverse transcriptase second strand synthesis
using terminal transferase, ligation to vector.
C mRNA synthesis using RNA polymerase reverse transcription of mRNA, second strand
synthe­sis, ligation to vector.
D double stranded cDNA synthesis restriction enzyme digestion addition of linkers
ligation to vector.

38. 3. Which one of the following is not a valid
method of screening a library?
A hybridization of colony / plaque­lifted DNA
using a nucleic acid probe.
B using antibodies raised against the protein of
interest to screen an expression library.
C screening pools of clones from an expression
library for biological activity.
D hybridization of colony/plaque­lifted DNA
using an antibody probe.