Nucleus and Nucleolus - MCAT Biological and Biochemical Foundations of Living Systems

The nucleolus is a structure within the nucleus, and is the site of rRNA synthesis. The individual subunits of the ribosome assemble in the nucleus, are exported through the nuclear pores, and are finally brought together into function units in the cytosol.

The nucleus contains DNA, the lysosome has enzymes that break down molecules, and the mitochondria are the site of ATP production. The rough endoplasmic reticulum has a surface that is studded with ribosomes, and works in coordination with these ribosomes to modify proteins after synthesis.

The nucleus contains the genetic code of each individual, and the mitochondria is a semiautonomous organelle that contains mitochondrial DNA (passed through the maternal line). mDNA codes specifically for the proteins involved in the electron transport chain, allowing for their implantation in the inner mitochondrial membrane without having to be synthesized elsewhere in the cell.

The nucleolus is located inside of the eukaryotic nucleus. It is responsible for the creation of the ribosomal subunits, as well as the rRNA that will be incorporated into the ribosomes. Ribosomal subunits are synthesized in the nucleolus and exported to the cytoplasm, where they can be assembled into functional translation complexes.

The rough endoplasmic reticulum contains ribosomes embedded in its membrane and is responsible for synthesizing certain proteins. The smooth endoplasmic reticulum is responsible for metabolizing toxins and synthesizing lipids. The Golgi apparatus plays a key role in protein sorting, packaging, and transport within vesicles.

The nucleolus is a special structure within the nucleus that is responsible for the transcription of rRNA genes and the formation of ribosomal subunits. Transcription of mRNA, tRNA, and miRNA occurs in other regions of the nucleus. Nuclear pores are highly regulated structures that allow for import and export of materials in the nucleus.

The only option that cannot pass the nuclear envelope from the nucleus is mitochondria. Mitochondria range from 0.5-1.0 micrometers, while nuclear pore complexes are about 120 nanometers in diameter. That being said, mitochondria are too large to fit through a nuclear pore complex.

Ribosomes, pre-mRNA, and proteins (such as transcription factors) are all perfectly capable of moving through the nuclear pore complexes, given the appropriate environment.

Ribosomal subunits are synthesized in the nucleolus, which is located inside the nucleus. The nucleus is surrounded by two membranes, known as the nuclear envelope, studded with large nuclear pore complexes. Import and export of substances through the pore complexes is highly regulated. Nuclei are supported by what is known as a nuclear lamina. This nuclear lamina is composed of intermediate filaments that provide support and structure for the nucleus.

It has recently been shown that chromosomes are not randomly distributed through the nucleus. Chromosomes occupy specific regions of the nucleus, which are currently referred to as "nuclear territories".

Lamins are intermediate filaments found inside the nucleus. Lamins attach to nuclear proteins and form a layer underneath the nuclear envelope (nuclear membrane) called the nuclear lamina. The nuclear lamina functions to position nuclear pores to let molecules pass between the cytoplasm and the nucleoplasm. It also functions to break and resynthesize the nuclear envelope during mitosis. Recall that the nuclear envelope is broken down during the prophase of mitosis; therefore, lack of lamins will directly affect this phase of mitosis.

In addition to the aforementioned functions, lamins are also involved in maintaining the integrity of chromosomes. Lack of lamins will decrease the stability of chromosomes. Cholesterol and most other lipids are synthesized in the smooth endoplasmic reticulum; therefore, lamins are irrelevant to cholesterol synthesis. Since they are involved in positioning nuclear pores, lamins are important for exchanging molecules between the nucleus and cytoplasm. Lack of lamins will decrease this trafficking.

The central dogma of molecular biology states that genetic information flows from DNA to RNA to protein. The first step is the replication of the genetic material, DNA, during the S phase of the cell cycle. The second step is the conversion of DNA to RNA. This process is called transcription and involves several enzymes that convert the DNA to mRNA. The final step is called translation, which involves the conversion of mRNA to protein.

DNA replication and transcription occur inside the nucleus because the enzymes required to carry out these processes are found in the nucleoplasm. Translation, on the other hand, occurs on ribosomes in the cytoplasm or on ribosomes on the rough endoplasmic reticulum.

Genetic information that codes for ribosomes is found on the nucleolus. Recall that the nucleolus is a specialized structure found inside the nucleus that functions to assemble ribosomes from proteins and ribosomal RNA (rRNA). The DNA that encodes for rRNA is on the nucleolus or in the vicinity of the nucleolus.

The periplasm is the space between the inner and outer cell membrane in gram-negative bacteria; it is irrelevant to this question. The nuclear envelope is the phospholipid bilayer that covers the nucleus. It does not contain any genetic information. Histones are proteins that organize and structure DNA strands; they don’t have any genetic information.

In the nucleolus, the transcripts synthesized are rRNA molecules. The unique aspect of rRNA molecules is that they are never converted to proteins; therefore, they never undergo translation. The rRNA molecules synthesized by the nucleolus are assembled with other proteins to create ribosomes; they themselves never undergo translation.

Transcription in the rest of the nucleus produces mRNA molecules that enter the cytoplasm and undergo translation to create proteins.

The nucleolus has genes for transcribing ribosomal RNA. RNA polymerase I in the nucleolus transcribes the gene for ribosomal RNA and causes formation of ribosomal complexes. mRNA and tRNA are transcribed elsewhere in the nucleus and use RNA polymerase II and III, respectively. Arachidonic acid is an omega-6 polyunsaturated fatty acid, which plays roles in cell-signaling, prostaglandin synthesis, and the immune response, and is synthesized in the cytosol.

The nucleolus, which is located in the nucleus, is directly involved in the production of rRNA. The rRNA composes the ribosome structure, and is directly involved in synthesis of protein.

The nucleolus is a structure within the nucleus, and is the site of rRNA synthesis. The individual subunits of the ribosome assemble in the nucleus, are exported through the nuclear pores, and are finally brought together into function units in the cytosol.

The nucleus contains DNA, the lysosome has enzymes that break down molecules, and the mitochondria are the site of ATP production. The rough endoplasmic reticulum has a surface that is studded with ribosomes, and works in coordination with these ribosomes to modify proteins after synthesis.

The nucleus contains the genetic code of each individual, and the mitochondria is a semiautonomous organelle that contains mitochondrial DNA (passed through the maternal line). mDNA codes specifically for the proteins involved in the electron transport chain, allowing for their implantation in the inner mitochondrial membrane without having to be synthesized elsewhere in the cell.

The nucleolus is located inside of the eukaryotic nucleus. It is responsible for the creation of the ribosomal subunits, as well as the rRNA that will be incorporated into the ribosomes. Ribosomal subunits are synthesized in the nucleolus and exported to the cytoplasm, where they can be assembled into functional translation complexes.

The rough endoplasmic reticulum contains ribosomes embedded in its membrane and is responsible for synthesizing certain proteins. The smooth endoplasmic reticulum is responsible for metabolizing toxins and synthesizing lipids. The Golgi apparatus plays a key role in protein sorting, packaging, and transport within vesicles.

The nucleolus is a special structure within the nucleus that is responsible for the transcription of rRNA genes and the formation of ribosomal subunits. Transcription of mRNA, tRNA, and miRNA occurs in other regions of the nucleus. Nuclear pores are highly regulated structures that allow for import and export of materials in the nucleus.

The only option that cannot pass the nuclear envelope from the nucleus is mitochondria. Mitochondria range from 0.5-1.0 micrometers, while nuclear pore complexes are about 120 nanometers in diameter. That being said, mitochondria are too large to fit through a nuclear pore complex.

Ribosomes, pre-mRNA, and proteins (such as transcription factors) are all perfectly capable of moving through the nuclear pore complexes, given the appropriate environment.

Ribosomal subunits are synthesized in the nucleolus, which is located inside the nucleus. The nucleus is surrounded by two membranes, known as the nuclear envelope, studded with large nuclear pore complexes. Import and export of substances through the pore complexes is highly regulated. Nuclei are supported by what is known as a nuclear lamina. This nuclear lamina is composed of intermediate filaments that provide support and structure for the nucleus.

It has recently been shown that chromosomes are not randomly distributed through the nucleus. Chromosomes occupy specific regions of the nucleus, which are currently referred to as "nuclear territories".

Lamins are intermediate filaments found inside the nucleus. Lamins attach to nuclear proteins and form a layer underneath the nuclear envelope (nuclear membrane) called the nuclear lamina. The nuclear lamina functions to position nuclear pores to let molecules pass between the cytoplasm and the nucleoplasm. It also functions to break and resynthesize the nuclear envelope during mitosis. Recall that the nuclear envelope is broken down during the prophase of mitosis; therefore, lack of lamins will directly affect this phase of mitosis.

In addition to the aforementioned functions, lamins are also involved in maintaining the integrity of chromosomes. Lack of lamins will decrease the stability of chromosomes. Cholesterol and most other lipids are synthesized in the smooth endoplasmic reticulum; therefore, lamins are irrelevant to cholesterol synthesis. Since they are involved in positioning nuclear pores, lamins are important for exchanging molecules between the nucleus and cytoplasm. Lack of lamins will decrease this trafficking.