are two epigenetic mechanisms that can prevent DNA from being accessible to transcription molecules. When the DNA is unwound, molecules that bind to determine when and how to transcribe genes. When gene transcription begins, a special type of protein known as binds to a promoter and assembles RNA. An activator is a transcription factor that binds to and promotes transcription. A repressor is a transcription factor that binds to and prevents transcription. Even if a gene is transcribed, its ultimate translation and expression can be silenced by

microRNAs

Explanation:

Within the cell, there are different types of regulatory non-coding RNAs (ncRNAs) such as microRNAs (miRNAs), small-interfering RNAs (siRNAs), piwi-interacting RNAs (piRNAs), long ncRNAs, etc., which are able to regulate gene expression. miRNAs are evolutionary conserved small ncRNAs (typically 18-23 nucleotides in length) known to inhibit gene expression by base-pairing with complementary target mRNAs, and thus trigger RNA interference (RNAi) pathways (e.g., mRNA degradation, inhibition of translation, etc). These miRNAs form part of a ribonucleoprotein complex named RNA-induced silencing complex (RISC) that acts to trigger RNAi mechanisms.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.