Nucleus and ribosomes (article) | Khan Academy
♢ Nucleolus is a suspended entity inside nucleoplasm of the nucleus. It is made up of RNA and protein. Its major known function is to synthesize. Ribosomes are cell organelles that consist of RNA and proteins. Ribosomal subunits are synthesized in the nucleolus and cross over the nuclear Ribosomes link amino acids together to form polypeptide chains, which are further Endoplasmic Reticulum - synthesizes carbohydrates and lipids. The nucleus contains the cell 's DNA and directs the synthesis of ribosomes and proteins. Ribosomes are large complexes of protein and ribonucleic acid (RNA ) which plants and other photoautotrophs generate carbohydrates and oxygen .
The inner membrane is is folded into cristae, and the walls of these cristae are studded with enzyme rich proteins which are involved in the production of ATP. Between the two membranes is the gel of the outer matrix - rich in enzymes for the processing of high energy hydrogen to ATP.
In the inner space is the inner matrix which contains numerous enzymes involved in the breakdown of Glucose, fats and proteins for energy. Mitochondria have several anomolies. They are self replicating and in fact have their own unique DNA.
When a mitochondria replicates, a new one buds off from an old one, similar in fashion to the way a bacteria replicates. This process is under the control of the mitochondria's own DNA.From DNA to protein - 3D
The mitochondrial DNA is not stored in a nucleus, but is in the form of a double circular molecule containing 16, base pairs compared with over a billion base pairs in the cells nucleus.
This is similar to the way DNA is stored in bacteria. There are other features similar to bacteria which has led to the postulation that mitochondria evolved from an aerobic bacterium that got ingested by a eukaryotic cell and then learned to live in symbiosis with it.
Mitochondrial DNA signature[ edit ] Sperm contain four mitochondria which are carried with the sperms tail. These are shed when the sperms head fuses with the ovum. This means that all mitochondrial DNA is descended from the female line.
Nucleus and ribosomes
Mitochondrial DNA also has few of the reparative mechanisms of eukaryotic DNA, which means that the mutation rate is much faster than inn eukaryotic cells. This has allowed paleo-geneticists to trace the ancestry of humans, and show that we had a common female ancestor about 60 million years ago.
In passing it should be noted that a similar analysis of the Y chromosome shows we have a common male ancestor about 60, years ago.
Centrioles[ edit ] Transmission electron microscope image of a thin section cut through the developing brain tissue telencephalic hemisphere of an This high magnification image of "Embryonic brain " show a spindle centriole and some spindle microtubules visible in the cytoplasm of a mitotic cell at the luminal surface of the telencephalon. Near the nucleus are found the centrioles, two structures usually at right angles to each other made up of microtubules. These are important in cell-division.
Let's take a closer look at the different parts of the endomembrane system and how they function in the shipping of proteins and lipids.
The endoplasmic reticulum The endoplasmic reticulum ER plays a key role in the modification of proteins and the synthesis of lipids. It consists of a network of membranous tubules and flattened sacs. The discs and tubules of the ER are hollow, and the space inside is called the lumen.
Rough ER The rough endoplasmic reticulum rough ER gets its name from the bumpy ribosomes attached to its cytoplasmic surface. As these ribosomes make proteins, they feed the newly forming protein chains into the lumen. Some are transferred fully into the ER and float inside, while others are anchored in the membrane.
Inside the ER, the proteins fold and undergo modifications, such as the addition of carbohydrate side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell. If the modified proteins are not destined to stay in the ER, they will be packaged into vesicles, or small spheres of membrane that are used for transport, and shipped to the Golgi apparatus.
The rough ER also makes phospholipids for other cellular membranes, which are transported when the vesicle forms. Since the rough ER helps modify proteins that will be secreted from the cell, cells whose job is to secrete large amounts of enzymes or other proteins, such as liver cells, have lots of rough ER.
Smooth ER The smooth endoplasmic reticulum smooth ER is continuous with the rough ER but has few or no ribosomes on its cytoplasmic surface. Functions of the smooth ER include: Synthesis of carbohydrates, lipids, and steroid hormones Detoxification of medications and poisons Storage of calcium ions In muscle cellsa special type of smooth ER called the sarcoplasmic reticulum is responsible for storage of calcium ions that are needed to trigger the coordinated contractions of the muscle cells.
There are also tiny "smooth" patches of ER found within the rough ER.
These patches serve as exit sites for vesicles budding off from the rough ER and are called transitional ER 1 1. The Golgi apparatus When vesicles bud off from the ER, where do they go?
Shortly after the synthesis of this sequence is completed, the synthesis stops until the ribosome docks at the rough endoplasmic reticulum. Once docked, the synthesis continues, with the new protein threading into the rough endoplasmic reticulum. The signal sequence is then cleaved from the polypeptide chain. Often, too, further enzymes cut the protein in other places. Most secreted proteins are modified before secretion.
Next, vesicles containing the protein bud from the rough endoplasmic reticulum and move too nearby the nearby Golgi apparatus. This is comprised of a stack of large, flattened vesicles. It is often likened to a stack of pita breads. The vesicles from the rough endoplasmic reticulum fuse with one end, adding their proteins to the first flattened vesicle. In turn, small vesicles bud from this structure and transfer the protein to the next layer of the stack.
This continues until the protein winds up at the opposite end of the Golgi apparatus. Once the protein has moved through the entire Golgi apparatus, secretion vesicles containing the protein bud off.
These vesicles move to the plasma membrane, attach to the membrane, and then release their contents into the extracellular fluid through the process of exocytosis.
Often a signal is required to initiate the release. Collagen Collagen is a secreted protein we will encounter frequently, especially in lab. It is the most abundant protein in the body. A tendon, for example, is almost entirely collagen. And it is abundant in skin, bone and indeed in extracellular spaces throughout the body, where it holds the cells of the body together.
A fibroblast is often the cell making the collagen. The figure to the right is from an earlier webpage and shows a small segment of the molecule. The center portion of the molecule is a repetition of a dozens of segments exactly like this. As you can see, the molecule consists of three polypeptide chains. The tightly packed structure makes the center of the molecule stiff and linear. During the synthesis, the three chains are synthesized separately, and then, of course, the signal sequences are removed.
Next, while still in the endoplasmic reticulum, the three polypeptide chains wind around each other into the quaternary structure shown.