When researchers need to prepare many reactions for polymerase chain reaction (PCR) amplification, they often create a "master mix" solution. A master mix contains the reagents common to all the planned PCR amplifications, regardless of the target DNA. Making a master mix is a way to minimize the number of pipetting steps. Suppose a researcher needs to PCR amplify seven different genes of interest from different organisms. The researcher prepares a master mix and dispenses it to seven different PCR tubes, one for each gene of interest. Select the PCR components the researcher must add to each of the seven tubes of master mix to selectively amplify each gene of interest. dNTPsMg2+-Mg2+-based bufferprimersDNA polymeraseDNA template

The master mix contains the following reagents: dNTPs, DNA Polymerase, PCR buffer and MgCl2.

Explanation:

The DNA templates are the gene fragments to amplify by PCR, thereby they have to be added separately in each tube. Moreover, the primer pairs are specific for each gene, thereby they have to be added separately in each tube.  

Deoxynucleotide triphosphates (dNTPs) are the building blocks of the DNA molecules: dGTP, dATP, TTP, and dCTP.

The PCR buffer provides a suitable medium for the activity of the DNA polymerase, often it contains Tris-Hcl and KCl.

MgCl2 is a cofactor for the activity of the DNA Polymerase.

The DNA Polymerase is an enzyme that amplifies DNA by adding nucleotides to the 3' end.

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