4.2 Storing and Transmitting Information
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Recognize the structure and describe the function of the nucleus, ribosomes and centrosomes
- Outline how and where the cell’s genetic information is stored
- Understand the functional relationship between the nucleus and the ribosome
- Evaluate the role of centrosomes in transmitting genetic information during cell division
- Give examples of how the abundance or structure of nuclei, ribosomes, and centrosomes varies among different cell types
The storage and transmission of information is necessary for the growth and development of the cell. Genetic information is stored in DNA, but this information is only useful if it can be used to make functional macromolecules (like proteins) within a cell. This information must also be copied and transmitted from one cell to another, e.g., during cell division. Multiple organelles play a key role in the transmission and storage of cell information. In this section, the structure and function of the nucleus, the ribosome and the centrosomes will be examined to uncover how cells store and transmit information.
The Nucleus
The nucleus (plural = nuclei) is a relatively large organelle in the cell, with a diameter of around 6 – 10 mm (Figure 4.5). This important organelle plays a critical role in multiple aspects of storing and transmitting information. In most cells, the nucleus is the most prominent organelle, and some cells have more than one nucleus. The nucleus is surrounded by a nuclear envelope and stores the cell’s chromatin (DNA and associated proteins) in its nucleoplasm (semi-solid fluid located inside the nucleus). The nucleus is the site of DNA replication. Prior to cell division, DNA is replicated within the nucleus, which allows genetic information to be copied and transferred to new cells. The nucleus is also the site of RNA synthesis (transcription), which is an important step in transmitting the information from DNA into a form that can travel out into the cytosol. Once in the cytoplasm (outside of the nucleus), this RNA provides the instructions for the synthesis (translation) of proteins by ribosomes. Proteins then carry out most cellular functions. Although ribosomes function in the cytoplasm, they are actually partially assembled in a region of the nucleus called the nucleolus.
The Nuclear Envelope
The nuclear envelope is a double-membrane structure that constitutes the nucleus’ outermost portion (Figure 4.5). Both the nuclear envelope’s inner and outer membranes are phospholipid bilayers. The nuclear envelope is structurally-supported by the nuclear lamina, a fibrous network of proteins under the inner membrane. The nuclear envelope contains many nuclear pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm.
Chromatin and Chromosomes
Chromatin consists of the DNA stored in the nucleus, along with the proteins (histones) that associate with that DNA (Figure 4.6). Chromatin is organized into discrete structures called chromosomes. In eukaryotes, chromosomes are linear structures (Figure 4.6), each of which contains many genes. Each gene codes for a different RNA or protein, which each have distinct functions in the cell. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide because the chromatin condenses. During the rest of the cell cycle, chromatin is relatively uncondensed, and resembles an unwound, jumbled bunch of threads that are usually too thin to see. Actively transcribed, loosely packed chromatin is referred to as euchromatin while the more densely packed, non-transcribing regions are called heterochromatin.
The Nucleolus
We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have genes that encode ribosomal RNAs (rRNA). The nucleus contains an area called the nucleolus (plural = nucleoli) that aggregates the rRNA with associated proteins to assemble the ribosomal subunits in eukaryotic cells. There are two distinct ribosomal subunits in eukaryotic cells: the large (60S) and the small (40S). These subunits are then transported out of the nucleolus and nucleoplasm to the cytoplasm through the nuclear pores located in the nuclear membrane.
Ribosomes
Ribosomes are relatively small organelles, having a diameter of approximately 20-30 nm, and are responsible for protein synthesis. That is, ribosomes convert the information stored in DNA into a functional form. Protein synthesis is an essential function of all cells. Some types of proteins include enzymes, hormones, antibodies, pigments, structural proteins, and surface receptors. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the human pancreas is responsible for producing several digestive enzymes and the cells that produce these enzymes contain many ribosomes.
Ribosomes are one of the only eukaryotic organelles that are not membrane-bound. Under an electron microscope, ribosomes appear as either clusters (polyribosomes) or as single, tiny dots that float freely in the cytoplasm. They may also be attached to the cytoplasmic side of the endoplasmic reticulum and the nuclear outer membrane (Figure 4.5). Electron microscopy shows us that ribosomes consist of a large and a small ribosomal subunit (Figure 4.7). These subunits are large complexes of proteins and ribosomal RNA (rRNA). Ribosomes receive their “orders” for protein synthesis from the nucleus where DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, exiting the nucleus through the nuclear pores. The ribosomes then translate the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein (Figure 4.7). A third type of RNA – transfer RNA (tRNA) – is important for bringing these amino acids to the ribosome.
Link to Learning
You can watch an animation explaining how proteins are created from DNA here from “yourgenome”
The Centrosome
The centrosome is a microtubule-organizing center found near the nucleus in animal cells. While not an organelle, it is important for transmission of information during cell division and so we discuss it here. The centrosome contains a pair of centrioles, two structures that lie perpendicular to each other (Figure 4.8). Each centriole is a cylinder of nine triplets of microtubules. The centrosome is replicated before a cell divides, and the centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell. Therefore, the centrosome is important for ensuring that genetic information (chromosomes) is properly transmitted into new cells. However, the centrosome’s exact function in cell division isn’t clear. This is because animal cells that have had their centrosome removed can still divide. Plant cells also completely lack centrosomes and are still capable of cell division.
Connecting the Dots
How do these three organelles (nucleus, ribosome, centriole) cooperate to regulate the storage and transmission of genetic information? Let’s review. The nucleus is the storage site of DNA and houses the nucleolus. The nucleolus (in the nucleus) is the location where ribosomal subunits are formed, followed by release into the cytoplasm through the nuclear pores. Within the nucleus, DNA is used to synthesize RNA (via transcription), which then travels into the cytoplasm in the form of mRNA. In the cytoplasm, ribosomes then synthesize proteins (translation) using the mRNA as a template. The nucleus is also the site of DNA replication for cell division. During cell division the chromosomes are moved to opposite end of the cells with the help of centrosomes (at least, in animal cells). Thus, the nucleus, the ribosomes and the centrosomes are all connected through DNA.