Cell biology 1

 The nucleus is the brain of eukaryotic cells. It is only present in eukaryotic cells (which are eukaryotic because they have a nucleus) and there is only one of these organelles in each cell. 2

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 the nucleus is round and is the largest organelle in the cell. It is surrounded by a membrane, called the nuclear envelope, which is similar to the cell membrane.  The envelope is riddled with holes, called nuclear pores, that allow specific materials to pass in and out of the nucleus.  Attached to the nuclear envelope is the endoplasmic reticulum.  The nucleus is surrounded by the cytoplasm inside a cell. 4

 The nucleus houses the DNA (deoxyribonucleic acid) which stores genetic information for a cell.  The DNA contains instructions for the production of the cell's proteins and for reproduction 5

 To construct proteins, the DNA is copied to messenger RNA (ribonucleic acid) in the process called transcription.  The mRNA goes to the ribosomes, either in the nucleus or in the endoplasmic reticulum, where the actual construction of the proteins takes place. 6

 Structurally, the nucleus is composed of three main parts,  the nucleolus,  the nuclear envelope,  the chromatin. 7

 The nucleolus contains ribosomes, RNA, DNA, and proteins.  The nucleolus has some of the ribosomes that synthesize proteins (others are in the endoplasmic reticulum). 8

 The chromatin (meaning "colored substance") contains DNA and proteins formed into packets of code called chromosomes. 9

 This is the condensed form of chromatin organization. It is seen as dense patches of chromatin.  Heterochromatin is considered transcriptionally inactive 10

 Euchromatin is threadlike, delicate.  It is most abundant in active, transcribing cells. 11

 The nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus (DNA in particular) from the cytosol (cytoplasm). Many nuclear pores are inserted in the nuclear envelope, which facilitate and regulate the exchange of materials (proteins such as transcription factors, and RNA) between the nucleus and the cytoplasm.lipid bilayereukaryoticcellsDNAcytosolnuclear poresproteinsRNA 12

 Nucleus' functions:  Contolling center  Hereditary characteristics  Controls cell division 13