IEEE注册管理委员会为每一个网卡生产厂商分配Ethernet物理地址的前几个字节

A．2

B．3

C．4

D．5

Eukaryotic repressors抑制因子work invarious ways, just as they do in bacteria However, the simplest and most common mechamsm seen in bacteria is for the repressor to bind to a site overlapping the promoter, thus blocking binding RNA polymerase. That mechanism is not typically seen in eukaryotes. Most commonly, cukaryotic repressors work by recruiting(招募)histone modifiers that reduce transcription. For example, where as a histone acetylase(乙酰比酶)is typicallyassociated with activation, a histone deacetylase (去乙酰化梅)——that is，anenzyme that removes acetyl groups——acts to repress a gene. In some cases, long stretches of nucleosomal DNA can be kept in a relatively insert(无活性)state by appropriate nucleosome modification, most notably decetylation and methylation.