How would gene expression differ in a cell from an animal that is homozygous for a mutation in the argonaute gene, which encodes the main component of the miRISC complex? Assume there is only one copy of this gene in the genome.

Gene expression involves the process by which information from a gene is used to synthesize functional gene products such as proteins. One important regulator of gene expression is the RNA-induced silencing complex (RISC), which is guided by microRNAs (miRNAs) to bind target messenger RNAs (mRNAs) and regulate their stability and translation.

In this context, Argonaute is a key protein within the miRISC complex. It plays a critical role in guiding the complex to specific mRNA targets based on the sequence complementarity with the miRNA component. When Argonaute is mutated, especially when the organism is homozygous for such a mutation (meaning both copies of the gene are the mutant version), several downstream effects on gene expression can occur:

Loss of miRNA Functionality: The Argonaute protein is essential for loading miRNAs onto the RISC complex. A mutation in the Argonaute gene could prevent miRNAs from being correctly incorporated into the complex, impairing their function.
Failure to Degrade or Inhibit mRNAs: miRISC primarily functions by either degrading target mRNAs or by inhibiting their translation. If Argonaute is non-functional due to a mutation, these processes might be inefficient, leading to improperly regulated gene expression.
Dysregulation of Gene Expression: The inability to control target mRNA levels properly can lead to the overexpression or improper expression of various genes. This could affect cellular processes and potentially lead to developmental abnormalities or diseases, depending on what genes are misregulated.
Phenotypic Consequences: Since miRISC is involved in regulating thousands of genes, the mutation could result in wide-ranging phenotypic effects, depending on which mRNA are not properly regulated. These effects might include developmental defects or a predisposition to certain diseases if key regulatory pathways are affected.

In summary, a mutation in the Argonaute gene could lead to significant disruptions in gene expression by impairing the miRNA-mediated regulatory pathways. This disruption could result in widespread effects on cellular function and organismal development.