Video: Cell nucleus

Every team needs a leader. A team without a leader is like a ship without a captain, lacking direction, even though they've got the potential for great things. In our cells, the nucleus takes on the responsibility of being captain with all the organelles working together as the crew on this microscopic ship. And in this tutorial, we're going to learn more about our cell's captain, the nucleus.

The nucleus is the command center, and appropriately so. It tends to be the most prominent organelle in the cell. It's usually large, round, or oval in shape, located towards the center of the cell. However, not all cells look the same. Some cells, like those of skeletal muscles, are multinucleated, meaning they have multiple nuclei, and these are more peripheral in location. On the other hand, mature red blood cells have no nucleus at all.

In typical cells, the nucleus is a membranous, also known as membrane-bound, organelle. It consists of three main parts: the nuclear envelope, chromatin, and nucleoli. The nuclear envelope surrounds the nucleoplasm, a gel-like substance that contains water and other components like enzymes and nucleotides. Along with nucleoplasm, the nucleus contains DNA and proteins in the form of thread-like chromatin, along with specialized regions known as nucleoli.

Let's learn more about its structure, one part at a time. We'll start with the nuclear envelope.

Just like how the cell itself is surrounded by a cell membrane, also known as the plasma membrane, that separates the contents of the cell from the extracellular environment, the nucleus has a nuclear envelope made of a phospholipid bilayer that separates its contents from the cytoplasm. This is a selectively permeable barrier and consists of two membranes -- an outer nuclear and inner nuclear membrane -- separated by a perinuclear space.

The outer membrane faces the cytoplasm as the surface is studded with ribosomes. It's continuous with the rough endoplasmic reticulum, which also has ribosomes. This makes the perinuclear space continuous with the lumen of the rough endoplasmic reticulum. The inner membrane faces the nucleoplasm and is associated with a mesh of proteins known as the nuclear lamina. These intermediate filaments not only provide support to the nuclear envelope but also help maintain the shape of the nucleus.

The nucleus and cytoplasm communicate through pores in the envelope known as nuclear pores. These protein complexes allow substances to enter and exit the nucleus. Ions and small solutes can freely pass through while the transport of larger macromolecules is regulated.

Why would substances need to enter or leave the nucleus?

The nucleus stores most of the cell's genetic material in the form of double-stranded deoxyribonucleic acid, DNA. DNA contains the blueprint for cellular functions and inheritance in the form of genes. Genes direct protein synthesis necessary for cell activities. However, DNA cannot leave the nucleus, so it synthesizes ribonucleic acid, RNA, by a process known as transcription. This RNA then leaves the nucleus and guides protein synthesis in ribosomes of the cytoplasm.

One type of RNA, known as ribosomal RNA, gets transcribed in specialized structures of the nucleus known as the nucleolus. These are the ribosome factories of the cell. Here, ribosomal RNA gets transcribed and associated with ribosomal proteins. The early stages of ribosomal subunit assembly take place within the nucleus. They leave the nucleus to get fully assembled into mature ribosomes, ready for protein synthesis.

Aside from nucleoli, the nucleus contains a very important structure known as chromatin.

Chromatin is our DNA. A single human nucleus can contain up to around 2 meters of DNA. That's a lot to fit into our microscopic nucleus. It gets stuffed into the nucleus in a very organized manner, neatly folded and packaged in the form of chromatin. Let's see how that happens.

Chromatin is a complex of DNA and other proteins. The ones that help package it are known as histone proteins. DNA winds around the histone proteins forming a nucleosome. These are the smallest units of the chromatin structure. Nucleosomes are connected to each other by short stretches of linker DNA. These repeating units resemble beads on a string. The string of nucleosomes coils further to form chromatin fibers, which form loops on the protein scaffold.

Two forms of chromatin can be found in the nucleus. One form is loosely dispersed, open, and transcriptionally active. This is known as euchromatin. The other form is heterochromatin which is densely packed and generally inactive. However, some types of heterochromatin can become active when modified. Nuclei contain both euchromatin and heterochromatin in different proportions depending on their activity.

Chromatin is the loosely dispersed form of genetic material in undivided cells. During cell division, they condense to form chromosomes.

Human cells typically have 46 chromosomes. An exception to this is sex cells, the egg and sperm cell. They have half the number of chromosomes, because during fertilization, they unite to restore the chromosome number of the offspring back to 46.

Somatic cells, which are body cells, are diploid because they have 46 chromosomes in 23 pairs. Each biological parent contributes one chromosome in each pair. On the other hand, sex cells, or gametes, which have half the number of chromosomes, are haploid cells with 23 chromosomes. Twenty-two of the twenty-three pairs of chromosomes are autosomes. Each pair has one maternal and one paternal chromosome forming homologous chromosomes.

The 23rd pair are allosomes, or sex chromosomes, X and Y. Biologically, females have two X chromosomes; one maternal X and one paternal X chromosome. Males have one maternal X chromosome and one paternal Y chromosome.

The laboratory technique used to visualize and analyze the number and morphology of chromosomes is known as karyotyping. This is usually done during the metaphase of cell division since that's when chromosomes are the most visible. This is when they look like an X. These are not X chromosomes but are shaped like an X because they duplicate before the cell divides so that each daughter cell can get the same amount of chromosomes.

In this stage, each chromosome has a pair of identical sister chromatids which represent the duplicated DNA, held together at a constriction known as the centromere. The centromere divides the chromosome into two arms -- a short P arm and a long Q arm. The ends or tips of the chromosome have protective regions known as telomeres.

Once cell division is complete, chromosomes decondense and return to being a ball of yarn. It's back to chromatin again. Thus, the nucleus stores genetic material in the form of DNA and chromatin, has all the machinery required for DNA replication prior to cell division, and synthesizes RNA and ribosomal subunits, which exit the nucleus to execute protein synthesis.

That concludes this tutorial on our hardworking ship captain, the nucleus.

Don't stop here. Learn all about the cell and its organelles with our study units and quizzes at Kenhub.