changes/ new questions

- Pls note the 1 correction in the Summary ; contributed by YF
- the following are good questions posed by students over the hols:


Could I just find out, is the human genome considered a segmented genome? Segmented genome = genes arranged on more than 1 strand of nucleic acid?

I guess so if you used that definition alto i am not aware of the term being used for human genome. It is more likely to be a term used to differentiate the type of viral genome. Because virus, being so small and needed very few genes, it can also easily have all its genes on one strand of nucleic acid.
Diff fields in biology used terms differently.

What are the sources of the viral envelope? The notes mention nuclear, vacuolar and plasma membranes. Could it come from the other membraneous organelles like golgi or ER? Some websites mention that it could come from the golgi membrane too. Or are these three mentioned because there is greater tendency for viruses to find themselves surrounded by these membranes inside a cell but less likely (still possible?) to be surrounded by e.g. ER or a chloroplast?

Yes, in fact i am more towards having the viral envelope coming from nuclear membrane, ER or Golgi, plasma (and will ignore vacuolar).
And no chloroplast.
Where the envelope comes from deps on the types of virus but typically examples will refer to plasma membrane but let's not forget about the rest!!
I need to draw this out for you all to see how is this possible so we will leave it here first.

Does naked virus mean that it doesn't have an envelope or that it ONLY has a nucleocapsid and nothing else? So is a bacteriophage a naked virus? Since it doesn't have an envelope but it has other structures besides a nucleocapsid. Or is the tail fibre, base plate etc considered part of the capsid?

Naked virus is without envelope.
because you wouldnt have only naked DNA/RNA floating around so yes, the nucleic acid will be encloseed by capsid proteins.
tail fibres, base plates are not part of capsid but these are not impt in the classification. in other words, you can classify virus as naked or enveloped; with or without a membranous envelope.
So your phages are naked viruses despite having the tails and base plates.


For viruses, why do some bacteria undergo lytic cycle while some go lysogenic? wouldn't it make more sense to just lyse repeatedly and kill off as many cells as possible?

A temperature phage undergoes a lytic or lysogenic cycle depending on the environment its host is in. If the bacteria is in a environment with a lot of nutrients such that it is conducive for growth, the phage will see this as a great opportunity to undergo lytic cycle because it means that out there, there will be many bacteria to infect (since the bacteria are likely to be growing exponentially!) Joy to the phages which can infect many many more!. COnversely, if the bacteria is in a medium with little nutrients and are not actively dividing, the phage will do into the lysogenic cycle and wait for a better moment.
Another possible scenario is UV light. If there is irradiaton of bacteria by UV light, the phage will undergo lytic cycle. Reason being that to the phage, if the DNA of the bacteria is damaged, it may limit/impede cell division of the bacteria so it rather escape and find another host while it still can!!
PS: I don;t mean to make the virus sounds so lively cos we know that they are intermediate betw being a living and non-living things yah? They are pretty economical tho =)

The notes wrote that the SER catalyses the synthesis of carbohydrates, but er when i checked campbell reece it was stated that it metabolises carbohydrates and synthesises lipids): so which is it?

The notes also highlighted that the SER is also known to be involved in the synthesis of lipids. (impt!!)
As for carbohydrates, SER is known to metabolise it, rather than synthesise it.
Golgi Apparatus is where the bulk of carbo is synthesised tho.
But what other specific roles of SER do you know?

The classic example to study SER is liver cells where it is in abundance and have highly specific roles (which are what we study)

when they say that the RER is a membrane factory for the cell as it "adds membrane proteins and phospholipids to its own membrane".. what does it really mean? does it mean that the proteins produced by its ribosomes are directly incorporated into its membrane without going through modification by the golgi apparatus..? and i also read in campbell that the RER synthesises its own phospholipids [not in notes].. so are those the phospholipids that are incorporated into the membrane? >< (then what are the phospholipids produced by the SER for? D: ) does it matter anyway ><


It is inaccurate to talk about RER as membrane factory. we usually just refer to ER as the membrane factory. There is no need to make a distinction.
The basic idea is that phospholipids are synthesised at the ER where they are first incorporated. Through budding off and fusing, phospholipids may be added to any membrane-bound organelle involved in the endomembrane system (chloroplast and mitochondria are not included). For example if the plasma membrane needs to expand like after mitosis and the cell gets bigger in size to achieve optimal surface area: vol ratio, the additional PL are made at the ER and transported via Golgi A before fusing with exisiting plasma membrane.
Also, if the ER needs modified proteins, it can obtain them from vesicles budding off from the GA. the trafficking is two-way not just one-way. so that is not a problem.


in our notes, it was stated that a transport vesicle moves to the plasma membrane in the process of exocytosis. do we need to make the distinction between the type of vesicle it is here, and if so, shouldn't it be a secretory vesicle?

transport vesicle is very generic term. it can be a vesicle going to the plasma member or to other organelle like lysosomes or back to ER.
but as i mentioned in class, if you know the contents are going out of the cell via exocytosis, pls use secretory vesicles. that clearly defines its role and is awesome.