The shape of the lesson
By the end of the lesson, learners can
- Phosphate, sugar, base: Click through the four chips in the simulator: DNA nucleotide, RNA nucleotide, purines and pyrimidines, and ATP.
- Antiparallel strands, paired bases: Click any base pair in the 20-pair helix and read off the bases and the number of hydrogen bonds.
- Messenger RNA, a working copy: RNA uses the same backbone chemistry as DNA, but its molecules are built for a different job: a short-lived copy of one gene that can leave the nucleus.
- Purines (A, G) double ring; pyrimidines (C, T, U) single ring.
- Phosphodiester bonds within a strand; hydrogen bonds between strands.
- A-T: 2 hydrogen bonds; C-G: 3.
- Antiparallel: 5′ to 3′ alongside 3′ to 5′.
One picture every idea returns to
A nucleotide is a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA) and a nitrogen-containing base. Adenine and guanine are purines with a double ring; cytosine, thymine and uracil are pyrimidines with a single ring. ATP is a phosphorylated nucleotide: adenine, ribose and three phosphates. Nucleotides are joined into a polynucleotide by phosphodiester bonds (condensation reactions) between the phosphate of one and the sugar of the next. DNA is a double helix of two antiparallel strands, one running 5′ to 3′ and the other 3′ to 5′, held together by hydrogen bonds between complementary bases: A with T (two hydrogen bonds) and C with G (three). mRNA is a single, short, unpaired polynucleotide with ribose and uracil in place of deoxyribose and thymine.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Four letters, two strands.. DNA and RNA are polymers of nucleotides. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Phosphate, sugar, base | Click through the four chips in the simulator: DNA nucleotide, RNA nucleotide, purines and pyrimidines, and ATP. Project tab 1 of the lab (Nucleic acids lab, nucleotides and ATP) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Antiparallel strands, paired bases | Click any base pair in the 20-pair helix and read off the bases and the number of hydrogen bonds. Project tab 2 of the lab (Nucleic acids lab, the DNA double helix) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Messenger RNA, a working copy | RNA uses the same backbone chemistry as DNA, but its molecules are built for a different job: a short-lived copy of one gene that can leave the nucleus. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 17 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Messenger RNA, a working copy, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Do not say thymine is a purine because it pairs with adenine. | Pairing tells you nothing about the class: every pair is one of each. Learn the ring count instead (memory aid: pure As Gold): adenine and guanine are the double-ringed purines; C, T and U are single-ringed pyrimidines. |
| Antiparallel does not just mean the strands are side by side. | It means they run in opposite directions: where one strand has its 5′ end, the other has its 3′ end. |
The structure of nucleic acids: worksheet
6 questions, 17 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the structure of an ATP molecule and state how it differs from a DNA nucleotide.
Explain why the two strands of a DNA double helix are described as complementary and antiparallel.
Analysis of a double-stranded DNA sample shows that 29% of the bases are thymine. Calculate the percentage of cytosine.
A purine always pairs with a pyrimidine in DNA. Explain why this is important for the structure of the double helix.
A length of DNA contains 200 base pairs. 35% of the bases are guanine. Calculate the total number of hydrogen bonds between the two strands.
Compare the structure of a DNA molecule with the structure of an mRNA molecule.
Answer key and marking notes
- ATP is a nucleotide made of the base adenine, the pentose sugar ribose and three phosphate groups.
- A DNA nucleotide has deoxyribose instead of ribose.
- A DNA nucleotide has only one phosphate group (and could have any of four bases).
- Complementary: each base pairs only with one other (A with T, C with G), so the sequence of one strand determines the sequence of the other.
- Antiparallel: the strands run in opposite directions.
- One strand runs 5′ to 3′ and the other 3′ to 5′, so the 5′ end of one lies opposite the 3′ end of the other.
- T = A = 29%, so A + T = 58% and C + G = 42%.
- C = G, so cytosine = 21%.
- A purine has a double ring and a pyrimidine a single ring, so every base pair is the same total width (three rings).
- The two sugar-phosphate backbones therefore stay a constant distance apart along the whole molecule, giving a regular helix.
- G = C = 35%, so C-G pairs = 70% of 200 = 140 pairs; A-T pairs = 60.
- Hydrogen bonds = (140 × 3) + (60 × 2) = 420 + 120.
- Total = 540 hydrogen bonds.
- Both are polynucleotides with a sugar-phosphate backbone joined by phosphodiester bonds; both contain A, C and G.
- DNA is double-stranded (a double helix with hydrogen-bonded base pairs); mRNA is single-stranded.
- DNA contains deoxyribose and thymine; mRNA contains ribose and uracil.
- DNA is very long (many genes); mRNA is short, a copy of one gene.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
The diagram shows one nucleotide. Component R has a double ring structure. Which row identifies P, Q and R correctly if the nucleotide came from RNA?
- AP phosphate; Q ribose; R adenine or guanine
- BP phosphate; Q deoxyribose; R adenine or guanine
- CP phosphate; Q ribose; R cytosine or uracil
- DP base; Q ribose; R phosphate
Which feature is shared by ATP and an RNA nucleotide containing adenine?
- Aboth contain the sugar ribose
- Bboth contain three phosphate groups
- Cboth contain the sugar deoxyribose
- Dboth are joined by phosphodiester bonds into polynucleotides
One strand of a DNA molecule has the sequence 5′ GATTCA 3′. What is the sequence of the complementary strand, written 5′ to 3′?
- A5′ TGAATC 3′
- B5′ CTAAGT 3′
- C5′ TGAAUC 3′
- D5′ ACTTAG 3′
Which type of bond holds the two polynucleotide strands of a DNA double helix together?
- Ahydrogen bonds between complementary bases
- Bphosphodiester bonds between the two backbones
- Cpeptide bonds between the bases
- Dionic bonds between phosphate groups
A double-stranded DNA molecule contains 4000 nucleotides. 1200 of them contain cytosine. How many contain adenine?
- A800
- B1200
- C1600
- D2800
DNA is heated until its two strands separate (melting). DNA from species X separates at a higher temperature than DNA from species Y of the same length. Which conclusion is best supported?
- AX has a higher proportion of C-G base pairs than Y
- BX has a higher proportion of A-T base pairs than Y
- CX has more phosphodiester bonds per strand than Y
- DX contains uracil while Y contains thymine
A sample of nucleic acid contains A 24%, G 31%, C 19% and U 26%. Which is the best description of the molecule?
- Asingle-stranded RNA, because it contains U and A does not equal U
- Bdouble-stranded DNA, because the four percentages add to 100
- Cdouble-stranded RNA, because it contains uracil
- Dsingle-stranded DNA, because G does not equal C
A single polynucleotide strand of 120 nucleotides is built from free nucleotides. How many water molecules are released as the strand forms?
- A119
- B120
- C240
- D60
Quiz key
Every nucleotide is a phosphate (P), a pentose sugar (Q) and a base (R). RNA contains ribose, so deoxyribose is wrong. A double ring means a purine: adenine or guanine. Cytosine and uracil are single-ringed pyrimidines. The phosphate is the small group at the end, not the base.
ATP is adenine + ribose + three phosphates; the RNA nucleotide (AMP) is adenine + ribose + one phosphate, so ribose is shared. Only ATP has three phosphates. Deoxyribose is found in DNA nucleotides, not in either. ATP is a free molecule and is not built into a polynucleotide chain.
Pairing gives 3′ CTAAGT 5′; reading this 5′ to 3′ gives TGAATC. CTAAGT is the right bases written in the wrong direction (it ignores antiparallel strands). TGAAUC contains U, which is not found in DNA. ACTTAG is just the original strand reversed, with no pairing.
The strands are held by hydrogen bonds between base pairs (2 for A-T, 3 for C-G). Phosphodiester bonds link nucleotides within one strand, not across. Peptide bonds join amino acids in proteins. The phosphate groups are on the outside of the helix and repel, so they do not bond the strands.
C = G = 1200, so C + G = 2400 and A + T = 1600. A = T, so A = 800. 1200 assumes all bases are equal to cytosine. 1600 is A + T together. 2800 is 4000 − 1200, which forgets guanine.
C-G pairs are held by three hydrogen bonds and A-T pairs by two, so DNA richer in C-G needs more energy to separate. More A-T pairs would lower the melting temperature. Strands of the same length have the same number of phosphodiester bonds, and these are not broken when strands separate. DNA does not contain uracil.
Uracil means RNA, and because A is not equal to U (and G is not equal to C) the bases are not paired, so it is single-stranded, like mRNA. All base percentages add to 100 in any sample, so that proves nothing. Double-stranded RNA would show A = U and G = C. DNA contains thymine, not uracil.
Each phosphodiester bond releases one water molecule, and a chain of 120 nucleotides has 119 links between neighbours, so 119. 120 counts one bond per nucleotide, but the last nucleotide has nothing further to join to. 240 counts two per nucleotide. 60 treats the strand as pairs of nucleotides, which applies to base pairs between strands, not to the backbone.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.