2. The group of enzymes involving the extending the telomere and prevention of chromosome shorting during DNA replication is called (A) primases； (B) helicases； (C) topoisomerases； (D) telomerases.

1. Skin cells and neuron cells in an individual show very distinct morphology and function, but they carry identical genome. How can this occurs? (A) Different mutations occur in different cell types； (B) Different parts of genome are lost in different cell types； (C) Different sets of genes are expressed in different cell types； (D) Different chromosomes are inactivated in different cell types.

2. The group of enzymes involving the extending the telomere and prevention of chromosome shorting during DNA replication is called
(A) primases；
(B) helicases；
(C) topoisomerases；
(D) telomerases.

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