Identify the vector suitable for cloning long DNA fragments:
a. Phage vector
b. Bacterial plasmid
c. Yeast plasmids
d. Cosmids

Hint: Vectors are a transmission agent, which is a self replicating DNA molecule and is used for indirect gene/piece of DNA transfer from one host to another. Cosmid is a type of plasmid vector constructed by the insertion of cos sequences, yeast plasmids are more efficient for eukaryotic gene transfer.

Complete answer:
A vector is a transmission agent, which is a self replicating DNA molecule and is used for indirect gene/piece of DNA transfer from one host to another.
There are different types of vectors which are:
> Phage vector- these are viruses that attack the bacteria and are used to clone a moderate DNA segment e.g. lambda phage and M13. These clone the DNA segments up to 10 kB.

> Bacterial plasmid- these generally occur in bacteria. These were the first genetically modified vectors or carriers for cloning a small piece of DNA in rDNA experiments. They clone the DNA segments up to 20 kB

> Yeast plasmid- these are not used as a cloning vector, instead, these are used as expression vectors. These can express DNA segments up to 500 kB

> Cosmids- these are hybrid vectors or recombinant plasmid having a DNA sequence of lambda phage and plasmid DNA. It allows the cloning of large DNA segments up to 50 kB.

Hence, the correct answer is option (D).

Note: Some of the important features of cloning vectors are-
- Vectors have a DNA sequence called the origin of replication from where replication starts.
- A good vector has a selectable marker that differentiates coding and non-coding parts of DNA segments.
- A good vector should contain several different cloning sites.