[求助]请教

The substances that Gross studied are maternal

(40) messenger RNA’s --products of certain of the maternal

genes. He and other biologists studying a wide variety

of organisms have found that these particular RNA’s

direct, in large part, the synthesis of histones, a class

of proteins that bind to DNA.

很困惑,想请问一下,为什么要用RNA'S而不直接RNA呢? 所有格在这里表明什么意思啊?

Passage 3也有, MESBIC's are the result of the belief of ...

真的不明白为什么要用所有格在这些地方...

难不成这里RNA's是指RNA的复数???