descride the functions of nucleus
Answers
Function of nucleus.
The nucleus is an organelle found in
eukaryotic cells. Inside its fully-enclosed nuclear membrane, it contains the majority of the cell's genetic material. This material is organized as DNA molecules, along with a variety of proteins, to form chromosomes.
The nucleus is made up of a double membrane nuclear envelope that keeps the entire organelle encased, isolating its contents from the rest of the cell, and the nucleoskeleton which supports the cell as a whole. The nucleus maintains the security of the genes and controls the functions of the entire cell by regulating gene expression. This is why the nucleus is sometimes referred to as the control center, or the "brain," of the cell.
Since large molecules cannot get inside the nucleus through the nuclear membrane, small holes called nuclear pores dot the surface area of the envelope. These pores regulate the transport of those molecules by carrier proteins embedded in the double layer of the membrane. Small molecules and ions are able to pass through the membrane freely, however.
The nucleus is the site for genetic transcription, while keeping it separated from the cytoplasm. This means gene regulation is taking place in eukaryotic cells that have a nucleus, but that this gene regulation isn't available to prokaryotes. That means the main function of the nucleus is to govern gene expression and facilitate DNA replication during the cell cycle.
Answer:
The nucleus is often called the control centre of the cell, as it is a double membrane organelle that contains DNA, which allows specific proteins to be synthesised (via protein synthesis) which determine the structure and function of the cell.
Specific epigenetic modifications (e.g. DNA methylation at a CpG motif or histone acetylation to form acetyl-lysine/ histone demethylation via PHF8 which removes a methyl group from lysine on the floppy tail of histone H3 at position 20) can cause certain genes to be switched on and expressed, and certain genes to be switched off (e.g. via a protein from the egg cytoplasm in embryogenesis that binds to the promoter region of a gene and attracts a complex of other proteins which may include epigenetic enzymes such as DNMT and MeCp2 which result in the DNA being methylated, and this methylation being interpreted by the cell due to the reader protein which also attracts a complex of proteins that help to switch the gene off, which could result in the permanent switching off of a gene and reduction in transcriptional noise. Alternatively, other developmental cues or environmental signals could result in histone methylation via epigenetic writers such as MLL2 attaching a methyl group to a lysine on the floppy tail of histone H3 at position 4, or via Trim28 which forms a complex with other proteins and methyates the histone. Epigenetic readers can then bind to the site and allow the cell to interpret the methylation marks, as well as building up a complex of other proteins on the nucleosome that help to switch the gene off. This complex may include DNMT, which could result in permanent methylation to adjacent DNA). Specific genes will be switched on (depending on the cues/signals the cell receives, e.g. the presence of steroid hormone-receptor complexes which can act as transcription factors and bind to the promoter region of a gene at a certain motif and attract a complex of other proteins/enzymes that includes RNA polymerase, to form the transcription initiation complex. Transcription factors can also be activated by intracellular second messenger molecules which are activated by peptide hormone-receptor complexes) therefore specific strands of mRNA will be produced in transcription (as the transcription initiation complex including RNA polymerase is able to bind to a certain motif at the promoter region upstream of a certain gene that isn't methylated). Therefore when the double stranded DNA unzips, the mRNA strand produced (the sense transcript) will be complementary to the original strand of DNA (thus will contain specific codons/triplets of bases) and attached to the DNA via hydrogen bonds between complementary bases. The mRNA produced will then detach from the DNA and move out of the nucleus (through a pore) and enter the cytoplasm. It will then bind to a ribosome, where translation occurs. tRNA anticodons that are complementary to the mRNA codons bring specific amino acids to the ribosome (as they have specific amino acid binding sites). Two tRNA molecules can be present at the ribosome at any one time and they hold the amino acids in place, where a peptide bonds forms between them in a condensation reaction to form a dipeptide. This process is repeated to form a polypeptide chain/protein via condensation polymerisation that has a specific primary structure as it has a specific sequence/order of amino acids which folds in a certain way due to specific interactions (e.g. ionic bonds, disulfide bridges, covalent bonds, and hydrogen bonds) between specific R groups, forming the secondary structure which can be an alpha helix or a beta pleated sheet, and finally the functional tertiary (3D) protein that has a particular structure and therefore a particular function. It therefore affects the structure and function of a cell, causing it to become specialised. The tertiary structure can undergo further modifications to form a quaternary structure. This occurs when the tertiary structure becomes associated with another polypeptide chain (e.g. collagen is a fibrous protein consisting of three polypeptide chains wound around each other and joined together by hydrogen bonds), or another non polypeptide group via covalent bonding or London forces/permanent dipole forces/ion dipole forces to form a conjugated protein (e.g. the conjugated globular protein haemoglobin contains the prosthetic group Fe2+). Thus, specific proteins are produced in translation that have a particular structure and therefore function and therefore affect the structure and function of the cell, causing it to become specialised.
Therefore, differentiated/ specialised cells are those that have been subject to specific epigenetic modifications to the cell
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