Question

1.List the basic sequence of cellular events resulting in protein production and export from the cell, beginning at DNA in the nucleus and ending with vesicular exocytosis. Include the following terms and describe what happens at each organelle or cellular structure: ribosomes, mRNA, DNA, rough the endoplasmic reticulum, exocytosis, nuclear pore complex, Golgi complex (cis and trans sides), nucleus, microtubules, vesicle.

2. Be able to label and describe the function of the cellular features of the eukaryotic mitochondria.

3. Be able to label and describe the function of the cellular features of the chloroplast.

4. Describe the endosymbiont theory and its importance to the evolution of multicellular organisms.

5. Be able to list and briefly describe the major roles of membrane proteins.

Answer 1

1. In eukaryotes the protein synthesis occurs via process called translation following transcription (production of mRNA).

Transcription is the process of synthesis of mRNA from DNA, that occurs in following steps.

a)Binding of RNA polymerase along with transcription factors (regulation of gene expression), thus creating a transcription bubble for breaking the hydrogen bonds in between the two strands of DNA helix.

b)Addition of nucleotide bases complementary to one strand of DNA by RNA polymerase thus creating a DNA-RNA hybrid.

c)After the formation of RNA strand, the hydrogen bonds in between the DNA-RNA hybrid breaks and RNA obtained is now free, which consists of exons (coding sequences) and introns (non-coding sequences). The introns are removed by restriction enzymes and the exons join to form single sequence called as the mRNA, the process so called as splicing.

d)The mRNA is capped with poly adenylate residues on 3' end and a guanosine cap is attached at 5' end thus forming the hnRNA. All the above mentioned processes occur inside nucleus.

This mRNA is exported to cytosol via the Nuclear Pore Complex (NPC) which requires additional conserved mRNA export factors including mex67/Tap and Mtr2/p15, and the mRNA proceeds for translation.

Translation

Translation is process of protein synthesis from mRNA and occurs in cytoplasm. The synthesis takes place inside ribosomes present on Rough Endoplasmic Reticulum. Ribosomes consist of two subunits one of which reads mRNA (smaller subunit), and the other joins amino acids, thus forming polypeptide chains). tRNA that is present in cytoplasm acts as an adapter molecule which reads the codons of mRNA at one end and consists of two sites A and P sites. Initiator RNA occupies P site and A site ready to receive aminoacyl tRNA. The elongation of peptide takes place by positioning of correct aminoacyl-tRNA in A site of ribosome, forming peptide bond and shiftiing mRNA by one codon relative to ribosome. Thus protein is synthesized and released into cytosol.

Microtubules are the tube like structures in cytoplasm that guide the movement of certain molecules inside the cell. These microtubules help the proteins from the endoplasmic reticulum enter the cis face of golgi apparatus where processing of the proteins occur, which leave via the trans face and exit in the form vesicles, and are released into the extracellular matrix by the process of exocytosis.

2. Eukaryotic mitochondria is the cell organelle that produces energy in the form of Adenosine Triphosphate (ATP) molecules which is utilized in the metabolic processes. It is also referred as the 'power-house' of the cell. The mitochodrion consists of double membrane structure inner and the outer membrane. The mitochondrial matrix fills in the inside of mitochondria where the citric acid cycle takes place and the inner membrane consist of folds known as cristae that increase the surface area, resulting in faster production of ATP resulting from the oxidative phosphorylation.. It is the inner membrane which intakes the electrons via chemiosmosis. Mitochondria consist of their own DNA (mtDNA),which is different from the chromosomal DNA and their replication process is still unknown. Mitochondria of offspring is often inherited from the maternal side.

  3. Chloroplast is a cell organelle present in eukaryotic green plants and some of the unicellular microbes including cyanobacteria and in some algae such as spirulina, blue-green algae etc. It is the organelle which produces green pigmentation due to the presence of chlorophyll. It belongs to the class of plastids and there are two other plastids found in plants viz. leucoplast (colourless) and chromoplast (colours other than green). The chloroplast consists of network of stack structures called the granum and each stack consists of cylindrical structure called thylakoids, which consist of chlorophyll. The chlorophyll absorbs protein from the sunlight and stores it in energy storage molecules ATP and NADPH, where oxygen is released free from water in plants. Like mitochondria the chloroplasts also contain their own DNA which is thought to have inherited from ancestor.

4. Endosymbiont theory describes about the organisms that live within the cells of other organisms (host) and show mutualism. The theory further explains the horizontal and vertical transmissions, where horizontal transmission refers to acquiring of free living symbionts from outside environment, which probably leads to more genetic variations between the species, rate of evolution is higher.

Vertical transmission is the transfer of symbiont from parent to offspring where chances of evolution is less, Since the genotype of offspring would be similar. Example of endosymbionts include rhizobia living in root nodules of leguminous plants.

5. Membrane proteins are of great significance and about one third of proteins in the human body are the membrane proteins. They are involved in cell signalling pathways, facilitate the movement of ions and molecules across the cell membrane and provide mechanical support along with cell-cell adhesion.

Integral membrane proteins are permanently attached to membrane which include the receptor molecules, transmembrane proteins.

Peripheral membrane proteins attach to lipid bilayer and dissociate when treated with polar reagent.