Question

8. RNA polymerases lack proofreading ability. Define proofreading ability and describe its importance in replication of DNA genomes.   
9. a. What advantage might its absence give RNA viruses like SARS-CoV-2?

10. Following protein synthesis, RNA genome and proteins assemble to form virus particles that then bud into the endoplasmic reticulum (ER) lumen and then on to the Golgi apparatus. Next, they are transported via Golgi vesicles to the cell membrane, where they are released via exocytosis. Define ER, Golgi apparatus, and exocytosis, and explain how this process undertaken by these viruses is comparable to processes regularly used by the cell.

Answer 1

The term proofreading is used in genetics as well as in molecular biology to refer to the error correcting processes.most of the mistakes during DNA replication are promptly corrected by DNA polymerase ( DNA pol ) by proofreading the base that has been just added

In proofreading, the DNA pol reads the newly added base before adding the next one, so the correction can be made

the polymerase checks whether the newly added base has paired correctly with the base in the template strand. if it is the right base the next nucleotide is added , if an incorrect base has been added the enzyme makes a cut at the phosphodiester bond and releases the wrong nucleotide

This is performed by the exonuclease action of DNA pol 3, once the incorrect nucleotide has been removed ,a new one will be added again using its polymerase activity

Some errors are not corrected during replication but are instead corrected after replication is completed, this type of repair is known as mismatch repair

CCCXTCACCAAAAA AAAAGACCA CTTAG GETTIGACE CCACCIA ACILICATICAATC AAAGACCACTA GTAGACE Daughter strands CCACCIA ACIIICATICAATC GEETA TAGASI TI GESTATALITI Figure 2. In mismatch repair, the incorrectly added base is detected after replication. The mismatch repair proteins detect this base and remove it from the newly synthesized strand by nuclease action. The gap is now filled with the correctly paired base.

In another type of repair mechanism ,nucleotide excision repair,enzyme replace incorrect bases by making a cut on both 3' and 5' ends of the incorrect base.this repair mechanism is often employed when UV exposure causes the formation of pyrimidine dimer

с А Т -Т с т G TAFF III iT À À À GTA A GAC Figure 3. Nucleotide excision repairs thymine dimers. When exposed to UV, thymines lying adjacent to each other can form thymine dimers. In normal cells, they are excised and replaced.

ENDOPLASMIC RETICULUM (ER)

ER is a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cell and serve multiple functions, being important particularly in the synthesis, folding, modification and transport of proteins. in animal cell the ER usually constitutes more than half of the membranous content of the cell. functionally there are two kind of ER ,namely rough ER (ribosomes are attached) and smooth ER (ribosomes are not attached)

GOLGI APPARATUS

It is also termed as Golgi complex or Golgi body, it is a membrane bound organelle of eukaryotic cell that is made up of a series of flattened, stacked pouches called cisternae

The Golgi apparatus is responsible for transporting, modifying and packaging proteins and lipids into vesicles for delivery to targeted destination. It is located in the cytoplasm next to the ER and near the cell nucleus

Many type of cells contain only one or several Golgi apparatus, plant cells can contain hundreds

EXOCYTOSIS

Exocytosis is the process of moving materials from within a cell to the exterior of the cell. this process requires energy and is therefore a type of active transport

In exocytosis, membrane bound vesicles containing cellular molecules are transported to the cell membrane. the vesicles fuse with the cell membrane and expel their contents to the exterior of the cell

Exocytotic vesicles are formed by he Golgi apparatus, endosomes,and pre synaptic neurons. this process is important for the removel of waste, for chemical messaging between cells, and for rebuilding the cell membrane

     Sever Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is the name given to the 2019 novel coronavirus. It have larger positive strand RNA genome that are 5' capped and 3' polyadenylated. during the course of infection the viral genome is incorporated with human genome and is replicated, replication followed by transcription as well as translation process, as a result viral protein is released .

The viral protein synthesis is more faster than that of the normal cells protein synthesis , this is because of the viral mRNA do not undergo post transcriptional modifications it directly undergo translation process while the human mRNA undergo post transcriptional modifications.