Question

Problem 4

The following are questions regarding cDNA and genomic libraries

e. What sequence(s) is lacking in a genomic library that is present in a cDNA library?

f.Would you be likely to find an average longer ORF in cloned sequences from a genomic

library or from a cDNA library? Explain

g. If you sequenced a new organism how would estimate how many genes are in the

genome? List at least two ways you could do this.

Answer 1

Answers:

e) Considering this question, we can say that the genomic library contains everything present in genome (sequences of both coding and non coding regions). Whilst the cDNA (complementary DNA) library contains information about the expressed mRNA sequence only. Therefore we can say that cDNA library lacks information about introns, regulatory sequences, all other RNAs except mRNA, while genomic library has all the information.

f) The ORF will be longer in case of genomic library than if obtained from cDNA library. Because cDNA library lacks the intron sequences while genomic library will have information about introns, hence the length of ORF obtained from genomic library will be longer.

g) If you sequence a new organism, to know the number of genes expressed in this organism, you should prepare a cDNA library.

So to prepare a cDNA library follow the given steps:

1. Isolate total RNA from the organism.

2. Prepare cDNA to obtain only mRNA sequence information.

3. clone the cDNA in some cloning vector like lambdaEMBL etc.

4. Then transfer this cloned vector into some organism (e.g. E. coli) and store this for future use.