Science lesson plan · 12th grade (Year 13 in England) · 45 minutes
Calculate DNA base percentages (A = T, C = G)
Cells, genetics and evolution
CCSS NGSS.HS-LS3-1CCSS NGSS.SEP 5England: GCSE Biology: DNA structureEngland: A level Biology: Nucleic acids
Objective and success criteria
Students will: Calculate DNA base percentages (A = T, C = G).
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Write complementary DNA and mRNA base sequences
Leads to: the next skill in this area.
Materials: Microscope images or slides, Cell diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Write complementary DNA and mRNA base sequences). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- One strand of DNA reads CGC TCG. Write the complementary DNA strand (letters only). → GCGAGCPrompt: A always pairs with T in DNA.
- A DNA template strand reads CCG GCT. Write the mRNA sequence transcribed from it (letters only). → GGCCGAPrompt: RNA has uracil (U) instead of thymine (T).
- Which base is found in RNA but not in DNA? (adenine (A) / guanine (G) / thymine (T) / uracil (U)) → uracil (U)Prompt: It replaces one of the DNA bases.
Hook · 5 min
- Show a microscope image of cells and ask what students think they are looking at.
- Share the objective: Calculate DNA base percentages (A = T, C = G). Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: In a sample of double-stranded DNA, 30 % of the bases are adenine (A). What percentage are guanine (G)?
- Step 1: A = T = 30 %, so A + T = 60 %
- Step 2: 100 − 60 = 40
- Step 3: 40 ÷ 2 = 20
- Step 4: So the answer is: 20 %
- Answer: 20 %
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- In a sample of double-stranded DNA, 30 % of the bases are adenine (A). What percentage are guanine (G)? → 20 %Prompt: A pairs with T, so they are equal; C pairs with G.
- A double-stranded DNA fragment has 5000 bases in total. 1000 of them are cytosine. How many are adenine? → 1500Prompt: C = G, so double the cytosine count.
You do: independent practice · 10 min
- Students work on the student sheet. Circulate and check the first two answers of every student early.
- Students who finish: write a question of their own for a partner, with the answer.
The 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- A double-stranded DNA fragment has 5000 bases in total. 1500 of them are cytosine. How many are adenine? → 1000Prompt: C = G, so double the cytosine count.
- In a sample of double-stranded DNA, 24 % of the bases are thymine (T). What percentage are cytosine (C)? → 26 %Prompt: A pairs with T, so they are equal; C pairs with G.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Write complementary DNA and mRNA base sequences.
- Use concrete materials: microscope images or slides.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.
Free lesson plan from eurekami.com/lesson-plan-builder