286 out of 320 — your best paper on record, and twenty-six clear of the danger line. Seventy-three right, six wrong, one left blank. The headline is not the total, it is the shape of what is left: every single error family that has cost you marks over the last six papers is now clean. Match questions, figures, ordering, look-alike pairs, reference-shift, five-statement sets — all of them perfect on this paper. Three of your six errors are the one habit that has never yet been fixed.
Where the marks went
Difficulty
Right
Out of
Accuracy
Easy
18
20
90%
90%
Moderate
36
36
100%
100%
Hard
19
24
79%
79%
Thirty-six out of thirty-six on the moderate questions. That is the first perfect band in the record. The easy block at eighteen of twenty is now the weaker of the two, and the one question you left blank was an easy one.
Section by section
Section
Questions
Right
The search for the genetic material
Q1–9
9 / 9
What a genetic material must be able to do
Q10–16
7 / 7
The structure of the DNA molecule
Q17–26
9 / 10
Packaging the DNA inside the nucleus
Q27–35
7 / 9
The replication of DNA
Q36–47
11 / 12
The lac operon
Q48–57
9 / 10
The human genome project
Q58–68
10 / 11
DNA fingerprinting
Q69–77
8 / 9
The RNA world and the order of the discoveries
Q78–80
3 / 3
Everything that used to cost you marks
Question type
Right
Out of
Match the column
8
8
lost 3 of 3 on the full paper to a C/D swap
Two-clue figures
9
9
9 of 30 errors on the full paper
Ordering and arrangement
12
12
two order reversals on the full paper
Look-alike pairs
11
11
7 of 12 errors on Kinetic Theory
Five-statement sets
7
7
the dominant family on Anatomy
Multi-statement sets
8
8
Reference shift
5
5
4 of 8 errors on Morphology
Questions carrying a figure
11
11
Assertion–Reason
3
6
the one that has never been fixed
Read that table twice. Eight families that have cost you marks paper after paper are at a hundred per cent. One is at fifty.
The habit that is left
Three errors · 15 marks · all three the same move
You answered (1) on five of the six
There were six Assertion–Reason questions. Two of them had (1) as the answer; you chose (1) five times. Every one of your three errors is an option (1): both true, and R is the correct explanation of A. This is the same shape that cost you marks on Anatomy, on Kinetic Theory, on Thermodynamics and on the full paper. It is the last thing standing between you and the target.
What the question was
How many
You got
R explains A — answer (1)
2
2 / 2
R true but not the explanation — answer (2)
2
0 / 2
A true but R false — answer (3)
2
1 / 2
Three questions, in this order, every time. Is A true? Is R true? Does R answer the question why A? Only if all three are yes is the answer (1). The paper prints a box for exactly this and you have never used it.
Q45 — you chose (1), the answer is (2)The substrate of polymerisation · hard
Assertion (A): The substrates for DNA polymerase are deoxyribonucleoside triphosphates and not monophosphates. Reason (R): DNA polymerase catalyses polymerisation only in the 5′ to 3′ direction.
(1) Both A and R are true and R is the correct explanation of AYOUR ANSWER
(2) Both A and R are true but R is NOT the correct explanation of ACORRECT
(3)A is true but R is false
(4)A is false but R is true
Both statements are true. But R is about the direction of polymerisation — 5′ to 3′ — while A is about which substrate the enzyme uses. The real reason A is true is that the two extra phosphates carry the energy that drives the bond formation. R is a true sentence about a different property, so the answer is (2).
Ask what question R is answering. If it answers a different question from the one A raises, it cannot be the explanation.
Q33 — you chose (1), the answer is (2)Staining of chromatin · hard
Assertion (A): Heterochromatin takes up stain more deeply than euchromatin does. Reason (R): Heterochromatin is transcriptionally inactive.
(1) Both A and R are true and R is the correct explanation of AYOUR ANSWER
(2) Both A and R are true but R is NOT the correct explanation of ACORRECT
(3)A is true but R is false
(4)A is false but R is true
Both true again. R says heterochromatin is transcriptionally inactive; A says it stains more deeply. The staining follows from how tightly packed it is, and the tight packing is also what makes it inactive. So the two facts share a cause — one does not cause the other. (2).
Two facts with the same underlying cause look like cause and effect. They are not.
Q75 — you chose (1), the answer is (3)Repeats and identification · hard
Assertion (A): Variable number tandem repeats are of use in establishing the identity of an individual. Reason (R): The number of repeat units present at a given such locus is the same in every individual of the population.
(1) Both A and R are true and R is the correct explanation of AYOUR ANSWER
(2) Both A and R are true but R is NOT the correct explanation of A
(3)A is true but R is falseCORRECT
(4)A is false but R is true
Here the reason is simply false. The whole value of a variable number tandem repeat is that the number of repeats differs from person to person — that is what makes a fingerprint possible. R states the opposite. Once R is false the answer can only be (3), whatever you think of A.
Read R on its own, as a sentence, before you read it as an explanation. This one contradicts the assertion it is supposed to support.
The two numericals
Q19 — you chose (3), the answer is (4)Hydrogen bonding · hard
A double-stranded DNA molecule 150 base pairs long contains 40 adenine–thymine pairs, the rest being guanine–cytosine pairs. The total number of hydrogen bonds holding its two strands together is
(1) 450
(2) 300
(3) 340YOUR ANSWER
(4) 410CORRECT
A–T pairs are held by two hydrogen bonds and G–C pairs by three. With 40 A–T and 110 G–C: (40 × 2) + (110 × 3) = 80 + 330 = 410. Your 340 is (40 × 3) + (110 × 2) — the two bond counts swapped over.
Three bonds for the pair with three letters in common: G–C is the stronger pair, and that is why GC-rich DNA has the higher melting point.
Q61 — you chose (3), the answer is (2)How much of the genome the genes occupy · hard
Taking the number of human genes as 30 000 and the average gene as 3 000 bases long, the total length of DNA accounted for by genes of average size is closest to
(1) 9 000 Mb
(2) 90 MbCORRECT
(3) 900 MbYOUR ANSWER
(4) 9 Mb
30 000 genes × 3 000 bases = 90 000 000 bases = 90 Mb. Your 900 Mb is the same multiplication with one zero too many. A mega-base is 106 bases, so 9 × 107 is 90 Mb, not 900.
Write the power of ten out before converting: 3 × 104 × 3 × 103 = 9 × 107.
The negative stem, and the one you left blank
Six of eight negative stems right, which is in line with your recent form. But the two losses in this family are the same thing twice: one answered wrongly, one abandoned — and both of them were easy questions. They are also your only two easy-band losses on the whole paper.
Q31 — you chose (2), the answer is (3)Heterochromatin · easy
Each of the following is true of heterochromatin, EXCEPT one. Identify the exception.
(1) It stains darkly with the stains ordinarily used on chromosomes
(2) It is transcriptionally inactiveYOUR ANSWER
(3) It is the region from which most of a cell's messenger RNA is transcribedCORRECT
(4) It is the more densely packed of the two kinds of chromatin
On an EXCEPT question the three true statements are the distractors. Heterochromatin is transcriptionally inactive, so that option is true and cannot be the exception. The exception is the one that is false: most messenger RNA comes from euchromatin.
Mark each option T or F in the margin, then pick the F. The habit takes five seconds and it is the whole question.
Q54 — left blank, the answer is (3)The lac operon · easy
Each of the following is true of the lac operon of E. coli, EXCEPT one. Identify the exception.
(1) The repressor is the product of the i gene
(2) The structural genes are transcribed together on a single messenger RNA
(3) The repressor binds the promoter and so blocks the attachment of RNA polymeraseCORRECT
(4) Lactose acts as the inducer of the operon
Left blank. The repressor binds the operator, not the promoter — that is the exception. The other three statements are straight from the lac operon as you have it: the i gene makes the repressor, the structural genes share one messenger RNA, and lactose is the inducer.
You knew three of the four were true. That alone identifies the fourth — worth four marks at no risk.
What this paper says
Across six marked papers the error families have fallen away one at a time. Statement questions went first, then negative stems and reference-shift, then the figures and the match questions. On this paper eight families are at a hundred per cent and the moderate band is perfect. Assertion–Reason is the only one that has survived every paper, and it is worth putting plainly: on this paper it cost you fifteen marks, and 286 + 15 is 301 — past the target with eleven to spare.
What to do before the next paper
Use the box.
Every Assertion–Reason question on every paper I build prints a dashed box reading A true? R true? R explains A? You have never written in it, on any paper. Write one letter in each blank before you look at the four options.
The third question is the one that matters.
You are reliable on whether A is true and whether R is true. What you skip is does R answer the question why A. When R is true but talks about a different property, the answer is (2).
Read R as a standalone sentence first.
In Q75 the reason flatly contradicts the assertion it was supposed to support. Reading it on its own would have caught that in two seconds.
Mark T or F beside each option on an EXCEPT question.
Both of your negative-stem losses were easy questions.
Keep doing whatever you did here.
Eight error families at a hundred per cent, and thirty-six out of thirty-six on the moderate band. That is the best block of work in the record.
Marked from the red-pen script. Every tick was read by its vertex, and all 80 answers were transcribed twice — once by a geometric ink-and-vertex pipeline, once by independent visual reading. The two reads disagreed on one question only, Q58, which was then settled by eye from the script itself: the tick is there, on option (1), and it is correct.