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The path from genes to proteins involves two steps:
If DNA can't leave the nucleus, how is the information accessed by the ribosomes so that the information can be used to make the necessary cellular components?
If you want to take information from a reference from the library, you make a copy. Similarly, if your cell needs to make a protein, it makes a copy of that protein's "recipe" via RNA polymerase, which is a lot like DNA polymerase.
You can think of DNA and mRNA, also known as messenger RNA, as being in the same chemical "language". They are both nucleic acids, and the "copy" mRNA makes of a gene's nucleotide sequence is complementary to the DNA the way the DNA's other strand is complementary. So you make a copy (an RNA message) of a gene (a DNA "recipe") within the nucleus, then the mRNA moves from the nucleus to the endoplasmic reticulum.
Using mRNA as the go-between for DNA and ribosomes has a lot of advantages. For example, if you only need to make one "recipe" (one protein), it would be silly to lug the entire "recipe book" (your genetic code—all of your DNA's gene "recipes") out of the "library" (the nucleus). You don't need to make every single recipe in the recipe book all the time; mRNA allows you to copy only those proteins you need at any given moment. Because mRNA is single-stranded (only one strand of the DNA codes for a particular gene, so you only need to copy one strand) and mRNA made of ribose sugar instead of deoxyribose (as DNA is), mRNA is much more unstable. This means that it will degrade quickly, so once you have enough protein, the mRNA message won't linger and force you to keep making a protein you don't need anymore.
At this point, you may be wondering, "If there's DNA that contains my genetic code, what else does it contain?" Quite a lot, actually. Indeed, about 98% of your DNA doesn't code for protein directly; rather, it performs subtler functions. For example, within a gene, you will have stretches of sequence that get translated into protein, called exons. Between them, you will have introns; instead of getting translated into protein, introns are nucleotide sequences that recruit regulator proteins. Regulatory proteins modulate the timing and amount of a gene's expression into protein. They can even alter the protein into different versions that perform similar but subtly different functions.
Source: This work is adapted from Sophia Author Amanda Soderlind