2013 Group 3 Project
- 1 Golgi Apparatus
- 2 Introduction
- 3 Behavior of the Golgi during Mitosis
- 4 How does the Golgi differ from other organelles?
- 5 Where GA is located in cells
- 6 Structure
- 7 function
- 8 history
- 9 Morphology of GA prior to cell division
- 10 morphology during each part of cell divisions
- 11 how the above process differs in other organisms
- 12 limitations of current models
- 13 Why the GA participates in cell division differently compared to other membrane bound organelles
- 14 References
- 15 Links
The Golgi Apparatus discovered in 1897 by physician Camillo Golgi is a vital cellular organelle which is found in almost all eukaryotic cells. It is found in the cytoplasm and facilitates the formation and direction of membrane bound vesicles, mainly formed from proteins directed from the rough endoplasmic reticulum before they reach the plasma membrane. The Golgi is capable of regulating cellular transport and secretion depending on the volume and density of the vesicles and their contents. Structurally, it consists of a series of stacked components of cisternae and has two identified faces- a cis face and a trans face. 
Behavior of the Golgi during Mitosis
Despite being one of the earliest organelles to be identified and studied, there are still grey areas concerning the division of the Golgi Apparatus. Some existing models of division for the Golgi suggest that it interacts with the endoplasmic reticulum during several points throughout cell division. This is due to their structural similarities during the interphase stage of mitosis in mammalian cells. Both consist of interconnect membrane networks and studies have found that both Golgi and ER proteins can be found together in bound vesicles. An experiment conducted by Jesch SA et. al. compares the distribution of the two organelles during interphase using innumnoflourecence microscopy, velocity gradient fractionation and density gradient fractionation. They concluded that the Golgi and the ER do not combine and furthermore that mitotic cells are unable to facilitate the fusion of the two. Instead the Golgi dismantles directly into vesicles and disperses equally throughout the dividing cell so that the resultant daughter cells will contain the necessary constituents.
How does the Golgi differ from other organelles?
Where GA is located in cells
The Golgi apparatus is a relatively large organelle and thus one of the easiest cell structures to study in detail . The organelle is located nearby the cell nucleus and is closely associated with the endoplasmic reticulum. By observing via metallic impregnation, it can be seen through phase contrast microscopy that the Golgi has a convoluted, dense and “ill-formed” morphology . Initial studies have shown that the organelle has great variance in its form dependent on the type of cell it is in as well as the state of activity that the cell is in. There are roughly around 40-100 Golgi apparatus ‘stacks’ within a mammalian cell .
Overall, the Golgi apparatus is made of 4-8 flattened, membrane-bound sacs that are stacked upon one another . These are known as cisternae. The Golgi also includes associated nearby vesicles. Each cisterna primarily contains products from the endoplasmic reticulum, which enter the Golgi at the cis face – the end that is closest to the ER and accepts incoming vesicles (5). The cis face is where new cisternae are formed. The products eventually pass through two more functional regions (medial Golgi and endo Golgi networks) and then are exported via outgoing vesicles at the trans face of the organelle. The trans face is where formed proteins are sent off and is the face furthest away from the ER. There is a constant and relatively consistent distance kept between cisternae of the Golgi apparatus .
Morphology of GA prior to cell division
morphology during each part of cell divisions
how the above process differs in other organisms
limitations of current models
A key problem with this line of reasoning is that micro- injection of the identical Sar1p mutant protein (GDP- or GTP- bound form) in interphase cells leads to the fragmentation of the Golgi apparatus into vesicles and tubules very similar to those comprising the MGCs.
This suggests that in itself cessation of transport from the ER is a detrimental factor for the maintenance of the Golgi apparatus and is independent of the localization of Golgi enzymes. One possibility is the requirement for a factor that needs to be transported to a downstream location to regulate maintenance/reassembly of the Golgi apparatus is accumulated in the ER. It is perhaps therefore not possible for normal Golgi reassembly to occur at the end of the mitosis in cells micro-injected with Sar1p DN .
The recycling of Golgi enzymes to the ER observed during interphase could take place to a certain extent during mitosis. More sensitive electron microscopy methods need to be developed to detect endogenous and transfected and epitope-tagged proteins that are expressed at low levels. These enzymes, when present in the ER, could indeed be partitioned in an ER-dependent manner, while others Golgi components, the structural Golgi proteins (see the Introduction), could be partitioned with the MGCs.
Why the GA participates in cell division differently compared to other membrane bound organelles
It was once theorised that the Golgi apparatus could be formed de novo within daughter and mitotic cells. It has since been discovered that in animal cells, the organelle cannot be synthesised de novo and thus, must divide when a cell divides (1). Through cytochemical (2) and immunofluorescence (3) light microscopy, it has been shown that the Golgi would undergo breakdown early on in mitosis, giving rise to many Golgi fragments (4). These fragments (in the form of small tubules and vesicles) would be dispersed throughout the cytoplasm at random. The partitioned Golgi would segregate into even groups and end up in the daughter cells to serve as a template to new Golgi apparatuses(5).
- HW BEAMS, RG KESSEL - The Golgi Apparatus: Structure And Function. International Review of Cytology Vol. 23 1968
- Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G