Final answer:
To silence a gene an organism can de-acetylate histones and methylate cytosine bases in the DNA, both of which condense the chromatin and hinder gene expression.
Step-by-step explanation:
An organism that no longer needs to express a particular gene could use several strategies to silence that gene. Silencing the expression of a gene involves modifying the chromatin structure, leading to a condensed state that is inaccessible to the transcription machinery. Two common epigenetic modifications that lead to gene silencing are histone deacetylation and DNA methylation.
Option (a), de-acetylate histones, is a process that involves removing acetyl groups from histones, which causes the chromatin to condense and becomes less accessible for transcription, leading to gene silencing. Option (c), methylate C bases in the genome, refers to the addition of methyl groups to the cytosine bases within the DNA, particularly in the promoter regions of genes, which can inhibit gene expression. On the other hand, option (d), acetylate histones, would actually increase gene expression by making the DNA more accessible to transcriptional machinery, not silencing it.
Therefore, the correct answers that could lead to gene silencing are options (a) de-acetylate histones and (c) methylate C bases in the genome, as both will lead to a more condensed chromatin structure and a decreased likelihood of gene expression.