Question pertains to comparison of level of mRNA expression from single gene copy in the genome to single-copy-single-plasmid system.
Kindly also explain the reasons for the same.
The short answer is "generally, yes".
The longer answer is to point out that copy number is only a single variable in a process controlled by many, many factors.
A single copy genomic gene with a strong promoter will give you far, far more expression than a plasmid gene with a weak promoter, even if you have multiple copies of the plasmid. A plasmid gene with a strong promoter that requires factors not present in the host cell will give almost no expression, even if you have multiple copies.
A single copy genomic gene might give you more or less expression than an ostensibly identical plasmid gene depending on whether the genomic copy lies in a more or less active region of the genome.
A single copy of a plasmid might express wildly differently between individual cells because of stochastic effects.
Biology is messy.
What is the specific question you are hoping to answer, here?
Sandeep K. Panda
Yes. Two things you have to keep in mind: (1) a plasmid is usually designed by researchers for special expression of a Gene-of-Interest (GOI). Promoter is changed and genetic elements are added for the purpose of GOI expression. (2) The environment is different when a specific gene is cut out from its native genome and placed in a plasmid, which can affect its expression level. Here, I listed 3 examples for that.
I. Promoter effect.
You have to think what promoter do you use in the plasmid. Promoter does all the trick. Some plasmids use very strong constitutive promoter (ex. double 35S promoter (for plant)), then the expression level will be very high, compared to use a *native* promoter for expression. Some researchers also modified the plasmid by adding some genetic elements which can also pump up a specific gene expression.
II. 'Modifier gene' effect.
There are 'Modifiers' (genes) in the genome, which can affect gene expression (see below). For instance, enhanced Gene X expression. If you only take Gene X out and clone into a plasmid (with a *native* promoter), w/o the existing of the Modifier. The expression level of Gene X can be different.
" Modifier genes are defined as genes that affect the phenotypic and/or molecular expression of other genes. Genetic modifiers can affect penetrance, dominance, expressivity, and pleiotropy (Nadeau, 2001)." [1]
[1] https://www.sciencedirect.com/topics/neuroscience/modifier-gene
III. Enhancer/Repressor effect.
"Enhancers can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or thousands of nucleotides away. When a DNA -bending protein binds to the enhancer, the shape of the DNA changes, which allows interactions between the activators and transcription factors to occur. [1]" So, if you only cut this gene out and stick into a plasmid, the *enhancer effect* would disappear, leading to low expression in the plasmid.
[1] https://courses.lumenlearning.com/boundless-biology/chapter/eukaryotic-gene-regulation/#:~:text=Enhancers%20can%20be%20located%20upstream,and%20transcription%20factors%20to%20occur.
I think the other respondents have covered the subject pretty well.
One last detail.
Some single-copy plasmids, like F, replicate shortlybefore the cell divides. That means that for much of the cell cycle, the copy number of chromosomal genes is twice that of genes on the F plasmid. In addition, in fast-growing cells, the time taken to replicate the chromosome is longer than the cell cycle, which the cell solves by starting replication in a previous cycle. This leads to nested replication forks; genes near the origin of replication can have four-times the copy number of those near the terminus.
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