Restriction enzymes, ligase and vectors কী?

A type II restriction enzyme is a bacterial endonuclease that cuts double-stranded DNA at a specific sequence of $4\text{ to }8$ base pairs, usually a palindrome: EcoRI cuts G$^{\downarrow}$AATTC, leaving four-base single-stranded overhangs (sticky ends) complementary to any other EcoRI end; other enzymes leave blunt ends. DNA ligase seals two fragments whose ends fit. A vector is a DNA molecule that replicates in a host and can carry an inserted fragment: a plasmid of a few kilobases with an origin of replication, a selectable marker (an antibiotic resistance gene, so that only cells that took up the plasmid grow on the antibiotic) and a cluster of unique restriction sites for the insert; bacteriophage $\lambda$, cosmids, and bacterial or yeast artificial chromosomes for inserts of $20\text{ to }1000\,\mathrm{kb}$. DNA enters a bacterium by transformation, after a chemical or electrical shock that makes the membrane transiently permeable, with an efficiency of $10^{6}$ to $10^{9}$ transformants per microgram of plasmid. A recombinant molecule is one that joins DNA from two sources.