Only the cells of advanced organisms, known as eukaryotes , have a nucleus. Generally there is only one nucleus per cell, but there are exceptions, such as the cells of slime molds and the Siphonales group of algae. Simpler one-celled organisms prokaryotes , like the bacteria and cyanobacteria, don't have a nucleus.
In these organisms, all of the cell's information and administrative functions are dispersed throughout the cytoplasm. The spherical nucleus typically occupies about 10 percent of a eukaryotic cell's volume, making it one of the cell's most prominent features. A double-layered membrane, the nuclear envelope, separates the contents of the nucleus from the cellular cytoplasm. The envelope is riddled with holes called nuclear pores that allow specific types and sizes of molecules to pass back and forth between the nucleus and the cytoplasm.
It is also attached to a network of tubules and sacs, called the endoplasmic reticulum, where protein synthesis occurs, and is usually studded with ribosomes see Figure 1. The semifluid matrix found inside the nucleus is called nucleoplasm. Within the nucleoplasm, most of the nuclear material consists of chromatin, the less condensed form of the cell's DNA that organizes to form chromosomes during mitosis or cell division. The nucleus also contains one or more nucleoli, organelles that synthesize protein-producing macromolecular assemblies called ribosomes, and a variety of other smaller components, such as Cajal bodies, GEMS Gemini of coiled bodies , and interchromatin granule clusters.
Chromatin and Chromosomes - Packed inside the nucleus of every human cell is nearly 6 feet of DNA, which is divided into 46 individual molecules, one for each chromosome and each about 1. Packing all this material into a microscopic cell nucleus is an extraordinary feat of packaging. It's in the middle of the cell, and the nucleus contains all of the cell's chromosomes, which encode the genetic material. So this is really an important part of the cell to protect. The nucleus has a membrane around it that keeps all the chromosomes inside and makes the distinction between the chromosomes being inside the nucleus and the other organelles and components of the cell staying outside.
The ribosomes assemble, and translation of RNA and protein synthesis occurs in the cytoplasm. Protein synthesis occurs on free ribosome or on ribosomes attached to the endoplasmic reticulum rough endoplasmic reticulum , in which case a pore is formed so that newly synthesized proteins move into the cisterna of the rough endoplasmic reticulum.
The proteins synthesised on ribosomes attached to the ER, are then transported to the Golgi, and packaged for secretion. Cells are normally diploid - this means that they have a pair - two sets of homologous chromosomes, and hence two copies of each gene or genetic locus. However, cells can be haploid, polyploid or aneuploid.
Haploid : only has one set of chromosomes - i. Polyploid : Contains more than two sets of homologous chromosomes. Aneuploid : Have atypical chromosome numbers - can either have one or two extra chromosomes, or can lose chromosomes. This is abnormal, and can be diagnostic of cancer. The karyotype - is a count of how many pairs of chromosomes there are. This picture shows a 'G-banded' karyotype from a diploid mouse cell in metaphase. Each cell in your body with the exception of germ cells contains the complete set of your DNA.
The following section will explore the structure of the nucleus and its contents, as well as the process of DNA replication. Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope.
This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores.
A nuclear pore is a tiny passageway for the passage of proteins, RNA, and solutes between the nucleus and the cytoplasm. Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. The nucleolus is a region of the nucleus that is responsible for manufacturing the RNA necessary for construction of ribosomes.
The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins Figure 3. Chromatin is a stringy, fibrous form of DNA that allows efficient packaging of DNA within the nucleus while maintaining a structure that allows early stages of proteins synthesis. Along the chromatin threads, the DNA is wrapped around a set of histone proteins.
It is estimated that humans have almost 22, genes distributed on 46 chromosomes. A set of three major organelles together form a system within the cell called the endomembrane system. These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice.
The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions Figure 3.
Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in very different amounts depending on the type of cell. A ribosome is an organelle that serves as the site of protein synthesis. It can be found free floating in the cytoplasm or attached to the ER.
It is composed of two ribosomal RNA subunits that wrap around mRNA to start the process of translation, a stage of protein synthesis. Protein synthesis consists of two stages: transcription and translation. The mRNA leaves the nucleus via nuclear pores and goes to the ribosome. Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it by a process called glycosylation before it is transported within a vesicle to the next stage in the packaging and shipping process: the Golgi apparatus.
One of the main functions of the smooth ER is in the synthesis of lipids. The smooth ER synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones. For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition to lipid synthesis, the smooth ER also sequesters i.
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