And a reminder: homework is exercise 3, about the dihybrid cross and linkage. And don’t forget to get the grid filled in and signed! It’s due on Monday.

That’s the first Tuesday back after the holidays – we’ll spend the Monday’s lesson on revision, but then it’s straight into the NAB. Remember to revise over the holidays. And, of course, if you have any questions at all about Unit 1 or the NAB, post them here and I’ll reply as quickly as I can.

]]> http://drgorman.wordpress.com/2009/09/28/photosynthetic-pigments/feed/ 2 Helen Pigments mind map http://drgorman.wordpress.com/2009/09/27/virus-essay-mark-scheme/ http://drgorman.wordpress.com/2009/09/27/virus-essay-mark-scheme/#comments Sun, 27 Sep 2009 17:42:31 +0000 Helen http://drgorman.wordpress.com/?p=26 ]]> Last week’s essay was the following question: “Give an account of the events that occur during the invasion of a host cell by a DNA virus.”

Here’s the mark scheme that we went over in class. As you can see, there are 11 potential marks, of which you could get a maximum of 10.

General background information (must be relevant to viral invasion, not just general information about viruses).

• Viral structure – DNA surrounded by protein capsid or a diagram showing this
• Lacks machinery to replicate – must use host cell
• Once inside, depends on host for ATP, nucleotides and amino acids

Steps of viral invasion:

1. Virus contacts specific host cells and binds

2. Thrusts tail through cell wall

3. Injects DNA into cell

4. Viral DNA replicates using host nucleotides

5. Viral DNA transcribed into viral mRNA

6. mRNA directs protein synthesis (translation) using host amino acids – protein coats

7. Viral DNA is enclosed in protein coat

8. Lysis of host cell

On the whole I was really pleased with the standard of the essays. Most people got at least 6 out of 10, and those scores can be improved by the inclusion of more biological detail. You’ve got to be really specific about including key points and using the correct vocabulary. You will also need to be concise – you won’t have long in the exam to churn out two essays. So don’t be tempted to waffle!

As for this week’s homework, remember that it is to revise for our end-of-topic quiz on Wednesday. This will include viruses, the immune system, and cellular defence in plants. I’m looking for much better scores than last time!

Give an account of the events that occur during the invasion of a host cell by a DNA virus.

As usual, it’s worth 10 marks, so I’m looking for at least 10 good key points – preferably more, just to be on the safe side. I’m looking for Greens on this one – in other words, 7 out of 10 or more.

Remember, I’ll be at the Scottish Learning Festival on Wednesday, so you have two extra periods to catch up on any missing notes (and, if you’re very organised, do your homework!).

As most of the class was at the Rothes Halls event this afternoon, we didn’t do the quiz on DNA and protein synthesis – we will do it first thing during Friday’s lesson, before the presentations.

Next week we start learning about viruses and the immune system.

Have a good weekend!

“Describe the main processes that occur during the replication of a molecule of DNA.”

Today we went over the mark scheme in class – and here it is for future reference.

Any of the following background information gets a mark:

• Requires: DNA template, 4 types of free nucleotides, DNA polymerase and ATP
• Semi-conservative – each new molecule is made of half the parent molecule, half new DNA
• Occurs during interphase (the phase of mitosis before cell division starts) – each new molecule forms a sister chromatid (accounting for the typical X-shape we associate with chromosomes)
• Quantity of genetic material doubles but chromosome number stays the same

Plus there are marks for a description of the stages of DNA replication:

• DNA unwinds
• Weak H-bonds between the bases break
• Free nucleotides complementary pair with the newly exposed bases
• Weak H-bonds form between the free nucleotides and the bases
• Strong chemical bonds form between sugar and phosphate
• Formation of these strong bonds is catalysed by DNA polymerase
• Daughter molecules wind up into double helix

Remember, when writing a Higher essay, you must make at least 10 good key points (unless there are marks for coherence and relevance – more on that at a later date). It’s safer to make a couple of extra points just in case some of the ones you have made aren’t on the examiners’ mark scheme. Here I have given marks for relevant background information (ie, information about replication, not just about DNA in general) because there isn’t really enough you can say about the actual process to make 10 good points.