rmmcdonald's blog

The structure and formation of the ER outlines how the ER assists in protein and lipid synthesis among other biological processes. There are two types of ER, as seen in Figure 2, the rough ER and the smooth ER. The name of the rough ER originates from the fact that ribosomes spot the outer membrane. In contrast, the outer membrane of the smooth ER contains no ribosomes. Depending on the type of cell, the ratio of smooth to rough ER will vary drastically (“The Endoplasmic Reticulum”). Both types of ER, however, have a similar structure of a large, continuous membrane that folds to create intramembrane spaces of different sizes. This intramembrane space is known as the cisternal space and connections exist between the cisternal spaces themselves in addition to the nucleus. The whole organelle is supported by the cytoskeleton of the cell which allows the ER to maintain its structure (English).

    In eukaryotic cells, the smooth ER is a major site where membrane lipids are synthesized (Cooper). Membrane lipids must be synthesized on the cytosolic side of the ER membrane, or outside of the ER membrane (Cooper). After the lipids are synthesized they follow a similar pathway to proteins where vesicles transport the lipids to their final destinations. The smooth ER more specifically synthesizes three important membrane lipids: ceramide, cholesterol, and glycerol phospholipids (Cooper). Ceramide undergoes further processing in the Golgi to become sphingomyelin. Therefore the smooth ER produces a variety of lipids that either act as precursors that require more processing or are final products (Cooper).

The main function of the rough ER is to synthesize, process, and transport proteins. This biological process begins in the cytosol where a ribosome initiates the translation of an mRNA strand. The ribosome may translate a short signal sequence on N-terminus that marks the polypeptide as destined for the ER. A signal recognition particle, or SRP, will recognize this sequence and bind to the polypeptide (“Molecular Biology”). Figure 3 reflects the process of translation in two different scenarios: when a polypeptide expresses an ER signal sequence or no signal sequence. With the SRP guiding, the complex is then localized to a translocon on the rough ER membrane. As the ribosome creates a protein strand based off of the mRNA transcript, the channel protein allows the polypeptide chain to enter the ER lumen where protein folding commences. This process where the still synthesizing polypeptide enters the ER is known as co-translation (Cooper). Within the ER lumen are a variety of chaperone proteins that help fold polypeptides. These chaperone proteins contain an ER retention sequence so that they remain within the ER (Cooper). Other proteins that have entered the ER to be processed contain only a temporary ER signal sequence and exhibit another signal sequences for a different cellular location. In that case, proteins will be packaged into a vesicle, a membrane bound sack, that buds off of the ER. This vesicle travels along microtubules, the “backbones” of the cell, until it fuses with the membrane of the Golgi (“Transport from the ER”). The Golgi acts as the final step to process and transport the proteins to their ultimate location in the cell. Figure 4 reflects the general pathway that proteins follow depending on the individual protein’s signal sequence. 

The structure and formation of the ER outlines how the ER assists in protein and lipid synthesis among other biological processes. There are two types of ER, as seen in Figure 2, the rough ER and the smooth ER. The name of the rough ER originates from the fact that ribosomes spot the outer membrane. In contrast, the outer membrane of the smooth ER contains no ribosomes. Depending on the type of cell, the ratio of smooth to rough ER will vary drastically (“The Endoplasmic Reticulum”). Both types of ER, however, have a similar structure of a large, continuous membrane that folds to create intramembrane spaces of different sizes. This intramembrane space is known as the cisternal space and connections exist between the cisternal spaces themselves in addition to the nucleus. The whole organelle is supported by the cytoskeleton of the cell which allows the ER to maintain its structure (English).

The endoplasmic reticulum is an important organelle that makes up 10% of the cell's volume. The endoplasmic reticulum, or ER, surounds the nucleus and spreads through out the cell. There are two types of ER: smooth and rough. The smooth ER contains no ribosomes on its membrane, hence the name "smooth". This smooth structure is because the main purpose of the smooth ER is to synthesize lipids and decrease toxicity of the cell. In contrast the rough ER has ribosomes scattered along the membrane because the main purpose of the rough ER is to synthesize, process, and export protiens. In terms of the relationship of the ER to ribosomes, a ribosome will recgonize a translation initation sequence of an mRNA and begin synthesizing the protein. This complex will then bind a translocon located on the membrane of the ER. This channel protein will direct the newly synthesized protein into the lumen of the ER. The protein is then processed and can possibly be retained in the lumen, secreted into the cytoplasm, or released in a vesicle. 

The endoplasmic reticulum makes up 10% of the cell's volume and surounds the nucleus. There are two types of ER: smooth and rough. The smooth ER contains no ribosomes on its membrane, hence the name smooth. This is because the main purpose of the smooth ER is to synthesize lipids and decrease toxicity of the cell. In contrast the rough ER has ribosomes scattered along the membrane because the main purpose of the rough ER is to synthesize, process, and export protiens. In terms of the relationship of the ER to ribosomes, a ribosome will recgonize a translation initation sequence of an mRNA and begin synthesizing the protein. This complex will then bind a tranlocon located on the membrane of the ER. This channel protein will direct the newly synthesized protein into the lumen of the ER. The protein is then processed and can possibly be retained in the lumen, secreted into the cytoplasm, or released in a vesicle. 

The fitness of a trait expressed an individual in a population is determined by a few factors. In broad sense heritablilty, the proportion is measured by genetic variation to genetic variation and environmental variation. The greater the environmental factors, the less likely that the trait is influenced by genetics and less likely that the trait can be passed down. In terms of narrow sense heritability, other genetic factors are measured to determine if internal/external environmental conditions contribute. If a trait has high fitness, relatively high survivial rate and produce a large number of offspring, then traits with high heritability will likely be passed to offspring. This will result in a selection differential between the mean of the general population and the population of breeders, creating a directional slection pattern if the traits are graphed. The reaction to selection may also be quantified by multiply the heritability of a trait with the selection differential. Fitness does not necessesarily correlate with high frequency. Sometimes traits with low frequency express a higher fitness which is known as negative frequency dependent selection. 

I find it interesting how Roman rulers follow a similar, cyclical pattern over time. A strong, diplomatic ruler will be in command and have complete trust of the Empire, like Augustus or Trajan. These rulers create buildings and events that benefit the public in addition to having a healthy relationship with the senate. In the case of Trajan and many other Roman rulers, when it came to pick an heir he never quite fully endorsed an individual. Hadrian rose to power nonetheless and fell into the downfalls of princeps that previously followed great rulers. Like Tiberius, Hadrian attacked the senate and any insubordinate military officers. He also separated himself from Rome, residing in a  "massive villa he built for himself at Tivoli". Again, Hadrian follows all the steps of a reckless and apathetic princep that passed poor rulers took. By ignoring the senate and the people of Rome Hadrian intensified his bad reputation. 

In the early, mythic years of Rome, the military seemed intrinsically linked to Romulus and as the mythological leader, he set the tone for future evolution. Romulus supposedly tied the defense of the city to the population that established it. Romulus recruited the population of Rome by offering asylum and “a promiscuous young crowd of freemen and slaves eager for change, fled thither from the neighboring states” to join Rome (Livy 125). The influx of young, motivated men helped not only build the city, but supplied the basis of an informal military. Even though these individuals came from all different states and backgrounds, these men made up a Roman State that was “so strong that was a match for any of its neighbors in war” (Livy 125). The mythical establishment of a militia style military seemed to reflect the characteristics that Romans of the Republic desired their military to embody. This includes the characteristic of the initial Roman military being accepting of those who were willing to fight for the city. Romans of the Republic looked back and established this fable in order to reflect the ideals they hope to aspire in the contemporary military institution.

In the early, mythic years of Rome, the military seemed intrinsically linked to Romulus and as the mythological leader, he set the tone for future evolution. Romulus supposedly tied the defense of the city to the population that established it. Romulus recruited the population of Rome by offering asylum and “a promiscuous young crowd of freemen and slaves eager for change, fled thither from the neighboring states” to join Rome (Livy 125). The influx of young, motivated men helped not only build the city, but supplied the basis of an informal military. Even though these individuals came from all different states and backgrounds, these men made up a Roman State that was “so strong that was a match for any of its neighbors in war” (Livy 125). The mythical establishment of a militia style military seemed to reflect the characteristics that Romans of the Republic desired their military to embody. This includes the characteristic of the initial Roman military being accepting of those who were willing to fight for the city. Romans of the Republic looked back and established this fable in order to reflect the ideals they hope to aspire in the contemporary military institution.