>>14529994
What about double meanings? Nuclear? Plasma? Mobile? Transcription? NPC?
Synthesis of Messenger RNA: Transcription
Transcription is the synthesis of an RNA molecule called messenger RNA (mRNA), from one of the complementary strands of a DNA molecule. There are other types of RNA as well, the most common being tRNA (transfer RNA) and ribosomal RNA (rRNA), both of which play critical roles in translation at the ribosome.
The purpose of mRNA is to create amobile,encoded set of directions for the synthesis of proteins. A length of DNA that includes the"blueprint"for a single protein product is called a gene. Each three-nucleotide sequence carries the instructions for making a particular amino acid, with amino acids being the building blocks of proteins in the same way nucleotides are the building blocks of nucleic acids.
There are 20 amino acids in all, allowing for an essentially limitless number of combinations and hence protein products.
Transcription occurs in the nucleus, along a single strand of DNA that has become uncoupled from its complementary strand for purposes of transcription. Enzymes become attached to the DNA molecule at the start of the gene, notably RNA polymerase. The mRNA that is synthesized is complementary to the DNA strand used as a template, and thus resembles the template strand's own complementary DNA strand except that U appears in mRNA wherever T would have appeared were the growing molecule DNA instead.
mRNA Transport Within the Nucleus
After mRNA molecules are synthesized at the transcription site, they must make their journey to the sites of translation, the ribosomes. Ribosomes appear both free in the cell cytoplasm and attached to a membranous organelle called the endoplasmic reticulum, both of which lie outside the nucleus.
Before the mRNA can pass through the doubleplasmamembrane that makes up thenuclear envelope(or nuclear membrane), it must reach the membrane somehow. This occurs by the binding of the new mRNA molecules to transport proteins.
Before the resulting mRNA-protein (mRNP) complexes can move to the edge, they become thoroughly mixed inside the substance of the nucleus, so that those mRNP complexes that happen to form near the edge of the nucleus have no better a chance at exiting the nucleus at a given time after formation than do mRNP processes close to the interior.
When mRNP complexes encounter regions of the nucleus heavy in DNA, which in this environment exists as chromatin (i.e., DNA bound to structural proteins), it can become stalled, just like a pickup truck being bogged down in heavy mud. This stalling can be overcome by the input of energy in the form of ATP, which prods the bogged-down mRNP in the direction of the edge of the nucleus.
Nuclear Pore Complexes
The nucleus needs to protect the all-important genetic material of the cell, yet it also must have a means of exchanging proteins and nucleic acids with the cell cytoplasm. This is accomplished via"gates"consisting of proteins and known as nuclear pore complexes (NPC). These complexes have a pore running through the double membrane of the nuclear envelope and a number of different structures on either side of this "gate."
The NPC is enormous by molecular standards. In human beings, it has a molecular mass of 125 million Daltons. In contrast, a molecule of glucose has a molecular mass of 180 Daltons, making it about 700,000 times smaller than the NPC complex. Both nucleic acid and protein transport into the nucleus and the movement of these molecules out of the nucleus occur via the NPC.
On the cytoplasmic side, the NPC has what is called a cytoplasmic ring as well as cytoplasmic filaments, both of which serve to help anchor the NPC in place in the nuclear membrane. On the nuclear side of the NPC is a nuclear ring, analogous to the cytoplasmic ring on the opposite side, as well as a nuclear basket.
A variety of individual proteins participate in the movement of mRNA and a diverse variety of other molecular cargoes out of the nucleus, with the same applying to movement of substances into the nucleus.
https://sciencing.com/mrna-leave-nucleus-10050146.html