SBI4U · Unit 3 test review

Molecular Genetics: Test Review

Everything for the Unit 3 test on one page: the key facts from all 5 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

DNA: Discovery and Structure

Griffith (1928): transformation. Avery–MacLeod–McCarty (1944): the transforming principle is DNA. Hershey–Chase (1952): phage DNA ( $^{32} P$ ), not protein ( $^{35} S$ ), enters the cell.
Chargaff's rules for double-stranded DNA: $% A = % T$ and $% G = % C$ , so purines = pyrimidines = 50%.
A–T pairs have 2 hydrogen bonds and G–C pairs have 3. A purine (A, G) always pairs with a pyrimidine (C, T).
The strands are antiparallel: 5'→3' pairs with 3'→5'. The 5' end has a free phosphate; the 3' end has a free –OH.
Double helix: about 2 nm wide, 0.34 nm per base pair, about 10 base pairs (3.4 nm) per turn.
Eukaryotic DNA wraps around histone octamers to form nucleosomes, then chromatin; prokaryotes usually have one circular chromosome without nucleosomes.

DNA Replication and Repair

Replication is semiconservative: each daughter helix has one parental strand and one new strand (Meselson and Stahl, 1958, using $^{15} N$ and $^{14} N$ ).
DNA polymerase adds nucleotides only to a free 3'-OH, so new strands grow 5'→3', the template is read 3'→5', and a primer is required.
At the fork: helicase unwinds, SSBs stabilize, topoisomerase relieves twisting, primase primes, pol III extends, pol I replaces primers, ligase seals.
Leading strand: continuous, toward the fork. Lagging strand: discontinuous Okazaki fragments, built away from the fork.
Error control: proofreading (during), mismatch repair (just after), excision repair (damage at any time); final error rate about 1 in $1 0^{9}$ .
Telomerase extends telomere repeats (TTAGGG in humans) at the ends of linear chromosomes.

Protein Synthesis: Transcription and Translation

Central dogma: DNA $\to$ RNA $\to$ protein (transcription, then translation).
RNA polymerase reads the template strand 3'→5' and builds mRNA 5'→3'; the coding strand matches the mRNA with T in place of U.
Eukaryotic pre-mRNA processing: 5' cap, 3' poly-A tail, and splicing (introns removed, exons joined).
Genetic code: triplet, $4^{3} = 64$ codons (61 for amino acids, 3 stops), redundant, unambiguous, nearly universal. Start AUG (Met); stop UAA, UAG, UGA.
Anticodons pair antiparallel with codons: codon 5'-AUG-3' pairs with anticodon 3'-UAC-5'.
Ribosome sites: A (new aminoacyl-tRNA enters), P (holds the growing chain), E (empty tRNA exits); the ribosome moves 5'→3' along the mRNA.

Gene Regulation and Mutations

Operon = promoter + operator + structural genes transcribed as one mRNA; a repressor bound to the operator blocks RNA polymerase.
lac operon (inducible): allolactose inactivates the repressor; CAP–cAMP (glucose low) boosts transcription. Fully on only when lactose is present and glucose is low.
trp operon (repressible): tryptophan is a corepressor that activates the repressor and switches the operon off.
Eukaryotic control points: chromatin (acetylation opens, methylation closes), transcription (transcription factors, enhancers), RNA processing, translation, and after translation.
Substitutions: silent (same amino acid), missense (different amino acid), nonsense (stop codon). Insertions or deletions that are not a multiple of 3 cause frameshifts.
Mutagens: UV (thymine dimers), ionizing radiation (strand breaks), chemicals (tobacco smoke, base analogues, intercalating agents).

Biotechnology and Its Issues

EcoRI cuts 5'-GAATTC-3' between G and A on each strand, leaving complementary AATT sticky ends; DNA ligase seals the sugar-phosphate backbone.
Recombinant DNA: cut gene and plasmid with the same enzyme, ligate, transform bacteria, and select on antibiotic agar.
Gel electrophoresis: DNA (negative) moves toward the positive electrode; smaller fragments travel farther.
PCR: denature about 95 °C, anneal about 55–65 °C, extend about 72 °C with Taq polymerase; copies $N = N_{0} \times 2^{n}$ .
Cutting a linear molecule at $k$ sites gives $k + 1$ fragments; cutting a circular plasmid at $k$ sites gives $k$ fragments.
Canada: higher life forms not patentable (Harvard mouse, 2002) but patented genes in plants are enforceable (Schmeiser, 2004); human cloning banned (AHRA, 2004); genetic discrimination banned (GNDA, 2017); GM labelling voluntary (EU: mandatory above 0.9%).

Where marks get lost

Applying Chargaff's rules to a single strand. %A = %T is true only for double-stranded DNA, because each A on one strand pairs with a T on the other; one strand alone can have any composition. (DNA: Discovery and Structure)
Writing the complementary strand without flipping its ends. The partner of 5'-AGGT-3' is 3'-TCCA-5', which in standard form is 5'-ACCT-3', not 5'-TCCA-3'. (DNA: Discovery and Structure)
Swapping purines and pyrimidines. Purines (A, G) are the larger double-ring bases; pyrimidines (C, T, U) are single rings. (DNA: Discovery and Structure)
Saying Hershey and Chase labelled DNA with sulfur. Sulfur is in protein and phosphorus is in DNA; the experiment works only because each element tags one kind of molecule. (DNA: Discovery and Structure)
Thinking the white threads in a DNA extraction are one DNA molecule, or that alcohol creates the DNA. Ethanol only makes DNA insoluble, so millions of molecules clump together where you can see them. (DNA: Discovery and Structure)
Saying the lagging strand is built 3'→5'. Both new strands are built 5'→3'; the lagging strand is discontinuous precisely because the polymerase cannot build in the other direction. (DNA Replication and Repair)
Mixing up polymerases I and III. Pol III does most of the synthesis; pol I removes RNA primers and fills those gaps. Ligase, not a polymerase, seals the final nick. (DNA Replication and Repair)
Thinking helicase breaks covalent bonds, or that topoisomerase unzips DNA. Helicase breaks the hydrogen bonds between bases; topoisomerase relieves the twisting strain ahead of the fork by cutting and rejoining the backbone. (DNA Replication and Repair)
Forgetting that primers are RNA. Primase is an RNA polymerase, which is exactly why the primers must later be removed and replaced with DNA. (DNA Replication and Repair)
Treating the Meselson–Stahl generation-1 result as full proof. One intermediate band rules out only conservative replication; the generation-2 bands were needed to rule out dispersive replication. (DNA Replication and Repair)
Writing the mRNA straight off a template strand given 5'→3', without reversing it. The mRNA is antiparallel to the template, so rewrite the template 3'→5' first, then pair. (Protein Synthesis: Transcription and Translation)
Putting T in mRNA or pairing A with T during transcription. RNA has no thymine; adenine in the template is matched by uracil. (Protein Synthesis: Transcription and Translation)
Looking up anticodons in the codon table. The table lists mRNA codons, so convert the anticodon to its codon first (anticodon 3'-UAC-5' pairs with codon 5'-AUG-3', Met). (Protein Synthesis: Transcription and Translation)
Counting the stop codon as an amino acid. Stop codons are read by a release factor, not a tRNA, so n codons from start to stop give n − 1 amino acids. (Protein Synthesis: Transcription and Translation)
Thinking the coding strand is the one transcribed. RNA polymerase reads the template strand; the coding strand is simply the one whose sequence matches the mRNA. (Protein Synthesis: Transcription and Translation)
Saying the inducer binds the operator. Allolactose binds the repressor protein and changes its shape, so the repressor lets go of the operator. (Gene Regulation and Mutations)
Thinking the lac operon is fully on whenever lactose is present. If glucose is also present, cAMP is low, CAP is not bound, and transcription stays low. (Gene Regulation and Mutations)
Getting the trp logic backwards. Tryptophan does not switch its own operon on; it is a corepressor that activates the repressor, turning the operon off when tryptophan is plentiful. (Gene Regulation and Mutations)
Calling every insertion or deletion a frameshift. Adding or removing a multiple of three bases keeps the reading frame and only adds or removes whole amino acids. (Gene Regulation and Mutations)
Assuming every mutation is harmful or inherited. Silent mutations change nothing, some mutations are beneficial, and somatic mutations are not passed to offspring. (Gene Regulation and Mutations)
Saying DNA moves toward the negative electrode, or that larger fragments travel farther. DNA is negative (phosphate groups) and runs toward the positive electrode, and the smallest fragments move farthest. (Biotechnology and Its Issues)
Counting fragments for a plasmid as if it were linear. Cutting a circle at k sites gives k fragments, not k + 1. (Biotechnology and Its Issues)
Using 2 × n instead of $2^{n}$ for PCR. The copies double every cycle, so 10 cycles multiply the DNA by $2^{10} = 1024$ , not by 20. (Biotechnology and Its Issues)
Cutting the gene and the plasmid with different enzymes whose sticky ends do not match. The ends must be complementary for the pieces to pair before ligase seals them. (Biotechnology and Its Issues)
Stating that Canada bans all patents on GM organisms. The Harvard mouse ruling stopped patents on the animal itself, but the Schmeiser ruling confirmed that patents on modified genes and cells are valid, even inside a plant. (Biotechnology and Its Issues)

Practice test

25 questions from across the unit. Each one shows the lesson it comes from.

DNA: Discovery and Structure
1
Avery, MacLeod and McCarty treated extracts of heat-killed S cells with different enzymes before mixing them with live R cells. Which result identified DNA as the transforming principle?
DNA: Discovery and Structure
2
A double-stranded DNA segment has 12 A–T pairs and 18 G–C pairs. How many hydrogen bonds hold the two strands together?
hydrogen bonds
DNA: Discovery and Structure
3
A double-stranded DNA molecule contains 2000 nucleotides in total, and 560 of them are thymine. How many are cytosine?
nucleotides
DNA: Discovery and Structure
4
Put these discoveries in chronological order, earliest first.
1. 1Hershey and Chase use labelled phages
2. 2Chargaff publishes his base-ratio rules
3. 3Watson and Crick publish the double helix
4. 4Miescher isolates nuclein
5. 5Griffith discovers transformation
6. 6Avery, MacLeod and McCarty identify DNA as the transforming principle
DNA: Discovery and Structure
5
In a DNA extraction lab, why is ice-cold ethanol layered on top of the filtered extract?
DNA Replication and Repair
6
In the Meselson–Stahl experiment, what did the DNA look like after two generations in $^{14} N$ medium?
DNA Replication and Repair
7
Match each protein to its job in replication.
Helicase
Topoisomerase
Primase
DNA polymerase III
DNA polymerase I
DNA ligase
DNA Replication and Repair
8
What are the short DNA segments made on the lagging strand called?
DNA Replication and Repair
9
Skin cells exposed to UV light develop thymine dimers. Which system removes this damage?
DNA Replication and Repair
10
Why do the chromosomes of most human body cells get slightly shorter each time they are replicated?
Protein Synthesis: Transcription and Translation
11
A template strand reads 5'-CTAGTGTTCCAT-3'. What polypeptide does it code for?
Protein Synthesis: Transcription and Translation
12
Of the 64 codons in the genetic code, how many specify an amino acid?
codons
Protein Synthesis: Transcription and Translation
13
Match each structure or molecule to its role.
A site
P site
E site
Release factor
Spliceosome
Aminoacyl-tRNA synthetase
Protein Synthesis: Transcription and Translation
14
Which property of the genetic code explains why many substitutions in the third base of a codon have no effect on the protein?
Protein Synthesis: Transcription and Translation
15
Why can bacteria begin translating an mRNA before its transcription is finished?
Gene Regulation and Mutations
16
An E. coli cell has low glucose and no lactose. What is the state of its lac operon?
Gene Regulation and Mutations
17
What term describes an operon that is normally on and is switched off when its end product builds up?
Gene Regulation and Mutations
18
A normal protein has 250 amino acids. A mutation deletes 6 consecutive nucleotides that make up exactly two complete codons. How many amino acids does the mutant protein have?
amino acids
Gene Regulation and Mutations
19
What is the usual effect of histone acetylation on a eukaryotic gene?
Gene Regulation and Mutations
20
In mRNA, the codon CAG (glutamine) changes at its first base and becomes a stop codon. Which base replaced the C?
Biotechnology and Its Issues
21
A linear DNA molecule has 3 recognition sites for a restriction enzyme. How many fragments form when it is cut completely?
fragments
Biotechnology and Its Issues
22
The restriction enzyme HaeIII recognizes the 4-base site GGCC. In a random DNA sequence, on average how many base pairs apart are its sites?
bp
Biotechnology and Its Issues
23
Match each tool or technique to its purpose.
PCR
Gel electrophoresis
Restriction enzyme
DNA ligase
Sanger sequencing
CRISPR-Cas9
Biotechnology and Its Issues
24
Which 2017 Canadian law makes it illegal to require someone to take a genetic test or disclose the results in order to get insurance or other services?
Biotechnology and Its Issues
25
At three STR loci, crime-scene DNA has repeat numbers (8, 11), (14, 14) and (6, 9). Suspect A has (8, 11), (14, 15), (6, 9). Suspect B has (8, 11), (14, 14), (6, 9). Suspect C has (7, 11), (14, 14), (6, 9). Which suspect's profile matches?

Endless drill

Fresh numbers every time, for the calculation questions in this unit.

Endless drillnew numbers every time

Generating a problem…