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The most important function of the nucleus is to store the cell's genetic information in the form of DNA. DNA holds the instructions for how the cell should work. DNA stands for deoxyribonucleic acid. The molecules of DNA are organized into special structures called chromosomes.
Feb 24, 2012 · The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression. Gene expression is the process by which the information in a gene is "decoded" by various cell molecules to produce a functional gene product, such as a protein molecule. DID YOU KNOW?
Cell Nucleus - Commanding the Cell. The cell nucleus acts like the brain of the cell. It helps control eating, movement, and reproduction. If it happens in a cell, chances are the nucleus knows about it. The nucleus is not always in the center of the cell.
- Overview
- Introduction
- The nucleus
- Chromosomes and DNA
- Ribosomes
Structure and function of the nucleus and ribosomes of a cell. How they work together in the production of proteins.
Suppose that you have a very precious piece of information. Let’s imagine that this piece of information is a blueprint. In fact, it’s not just a blueprint for a house, or a car, or even a top-secret fighter jet. It’s a blueprint for an entire organism – you – and it not only specifies how to put you together, but also provides the information that enables every cell in your body to keep functioning from moment to moment.
Sounds important, right? You’d probably want to keep information this valuable in a secure spot, perhaps in a protected vault where you can keep an eye on it. In fact, that’s exactly what eukaryotic cells do with their genetic material, placing it in a membrane-enclosed repository called the nucleus.
Eukaryotic DNA never leaves the nucleus; instead, it’s transcribed (copied) into RNA molecules, which may then travel out of the nucleus. In the cytosol, some RNAs associate with structures called ribosomes, where they direct synthesis of proteins. (Other RNAs play functional roles in the cell, serving as structural components of the ribosome or regulating activity of genes.) Here, we’ll look in a little more detail at the structure of the nucleus and ribosomes.
[Do all cells have just one nucleus?]
The nucleus (plural, nuclei) houses the cell’s genetic material, or DNA, and is also the site of synthesis for ribosomes, the cellular machines that assemble proteins. Inside the nucleus, chromatin (DNA wrapped around proteins, described further below) is stored in a gel-like substance called nucleoplasm.
Enclosing the nucleoplasm is the nuclear envelope, which is made up of two layers of membrane: an outer membrane and an inner membrane. Each of these membranes contains two layers of phospholipids, arranged with their tails pointing inward (forming a phospholipid bilayer). There’s a thin space between the two layers of the nuclear envelope, and this space is directly connected to the interior of another membranous organelle, the endoplasmic reticulum.
Nuclear pores, small channels that span the nuclear envelope, let substances enter and exit the nucleus. Each pore is lined by a set of proteins, called the nuclear pore complex, that control what molecules can go in or out.
If you look at a microscope image of the nucleus, you may notice – depending on the type of stain used to visualize the cell – that there’s a dark spot inside it. This darkly staining region is called the nucleolus, and it’s the site in which new ribosomes are assembled.
How do you make a ribosome? Some chromosomes have sections of DNA that encode ribosomal RNA, a type of structural RNA that combines with proteins to make the ribosome. In the nucleolus, new ribosomal RNA combines with proteins to form the subunits of the ribosome. The newly made subunits are transported out through the nuclear pores to the cytoplasm, where they can do their job.
Some cell types have more than one nucleolus inside the nucleus. For instance, some mouse cells have up to 6 nucleoli1 . Prokaryotes, which do not have a nucleus, don't have nucleoli and build their ribosomes in the cytosol.
Now that we have a sense of the structure of the nucleus, let’s have a closer look at the genetic information stored inside it: the DNA. Most of an organism’s DNA is organized into one or more chromosomes, each of which is a very long string or loop of DNA. A single chromosome can carry many different genes.
In prokaryotes, DNA is typically organized into a single circular chromosome (a loop). In eukaryotes, on the other hand, chromosomes are linear structures (strings). Every eukaryotic species has a specific number of chromosomes in the nuclei of its body’s cells. For example, a typical human body cell would have 46 chromosomes, while a comparable fruit fly cell would have 8 .
Chromosomes are only visible as distinct structures when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes instead resemble an unwound, jumbled bunch of threads. In this form, the DNA is accessible to the enzymes that transcribe it into RNA, allowing the genetic information to be put to use (expressed).
In both their loose and compact forms, the DNA strands of chromosomes are bound to structural proteins, including a family of proteins called histones (see picture below). These DNA-associated proteins organize the DNA and help it fit into the nucleus, and they also play a role in determining which genes are active or inactive. The complex formed by DNA and its supporting structural proteins is known as chromatin. You can learn more about DNA, chromatin, and chromosomes in the DNA and chromosomes article.
As mentioned above, ribosomes are the molecular machines responsible for protein synthesis. A ribosome is made out of RNA and proteins, and each ribosome consists of two separate RNA-protein complexes, known as the small and large subunits. The large subunit sits on top of the small subunit, with an RNA template sandwiched between the two. (A ribosome looks a little like a hamburger with a puffy bun on top, an RNA “patty” threading through it.)
In eukaryotes, ribosomes get their orders for protein synthesis from the nucleus, where portions of DNA (genes) are transcribed to make messenger RNAs (mRNAs). An mRNA travels to the ribosome, which uses the information it contains to build a protein with a specific amino acid sequence. This process is called translation. Prokaryotes lack a nucleus, so their mRNAs are transcribed in the cytoplasm and can be translated by ribosomes immediately.
Eukaryotic ribosomes may be either free, meaning that they are floating around in the cytoplasm, or bound, meaning that they are attached to the endoplasmic reticulum or the outside of the nuclear envelope. (In the first diagram in this article, the red dots represent bound ribosomes; endoplasmic reticulum with bound ribosomes is known as rough endoplasmic reticulum.)
Because protein synthesis is an essential function of all cells, ribosomes are found in practically every cell type of multicellular organisms, as well as in prokaryotes such as bacteria. However, eukaryotic cells that specialize in producing proteins have particularly large numbers of ribosomes. For example, the pancreas is responsible for producing and secreting large amounts of digestive enzymes, so the pancreatic cells that make these enzymes have an unusually high number of ribosomes.
Final fun fact: in a testament to the importance of the ribosome, the 2009 Nobel Prize in Chemistry was awarded to three researchers who mapped its structure and movements down to the level of individual atoms using a technique called X-ray crystallography5 .
[Attribution and references]
The nucleus is home to the genetic information of the cell and this is stored as DNA (deoxyribonucleic acid). The DNA contains all of the instructions for the functioning of the cell. The DNA molecules are well organized into chromosome structures. DNA sections are called “genes” and this is where all of the hereditary information is contained.
Nov 30, 2012 · Nucleolus. The nucleus of many cells also contains a central region called the nucleolus. The job of the nucleolus is to build ribosomes. These ribosomes flow out the nuclear pores into the cytoplasm. Ribosomes are organelles that make proteins in the cytoplasm. See the composition of the nucleus pictured below ( Figure below ).
Learn about the nucleus of a cell, including its parts and functions. Discover the nuclear membrane and its pores, the cytoplasm surrounding the nucleus, nucleoplasm, the nucleolus, and...
