Question

[NE2000]

Hey I've got a question, are the X and Y chromosomes counted as homologous?

[Stephanie]

XX is classed as a homoglous pair, XY are non-homologous - they're hemizygous

[bucket]

When explaining why the fossil record is incomplete, do you only need to state that not all organism that ever lived has been fossilised? Some exams go into detail as to WHY, but they take different routes ie. fossils can be formed easier in aquatic environments, fossils normally only preserve hard parts of an organism etc. and it confuses me, and on top of that it'll be annoying and very time consuming to explain every possibility :S

[JL_91]

I think two points is enough to explain why the fossil record is incomplete, for exam purposes.

1) The point you made about the fact that not all organisms that ever lived were fossilised. Aquatic, rather than terrestrial environments, provide more ideal conditions for fossilisation. In addition, as you said, the likelihood of the hard parts of an organism being preserved is far more likely than the soft parts, which decay.

2) Some fossils that exist have not been found yet, and most will probably never be found.

[bturville]

...plus some get destroyed, and what about things like slugs and other invertebrates. Theres a whole range of reasons why, just state the fundamental/basic reasons.

[JL_91]

^ I forgot that one - scavenging by other animals.

Basically what I would do is, look at how many marks the question is worth, and only put enough to gain all the marks. If the question is worth 3 marks, then I'd make 3 points about why the fossil record is incomplete, etc.

[bucket]

thanks

[Mikey123]

An autosomal recessive inherited disorder runs in the Jones family. An allele specifying the production of a
normal protein (R) is 160 kbp in length and has a recognition sequence for Hind III halfway along its length.
The mutant allele (r) has a second restriction site, 60 kbp beyond the midway point of the gene.
b. Describe the DNA fragments resulting from
i. the restriction of the normal allele by Hind III.
____________________________________________________________________________
ii. the restriction of the mutant allele by Hind III.




Question B ii)

[bturville]

Drawing a quick line diagram helps, do that. If its a plasmid, make sure to draw a circle rather than a line, as it affects the number of pieces left after cutting

The mutant allele will be chopped into 3 pieces, one 80kb long, one 20 long, and one 60 long.

[Mikey123]

Ho do you get 20 and 60 sorry to be a pain lol

[JL_91]

ii) The mutant allele is also 160kbp long, and has the following two recognition sites: the same one as the normal allele, at 80kbp (the midway point of the 160kbp long protein). The second site is 60kbp beyond this midway point - at 140kbp.

--------I------I--
       site 1     site 2

Therefore the DNA fragments produced when the mutant allele is digested by Hind III are:
one 80kbp length piece
one 60kbp piece
one 20 kbp piece.

*Note: that diagram I typed out is to scale - 16 dashes. So to help you understand, just count the dashes

[Mikey123]

Thanks  You are a legend.