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LESSON 3 OF 3 · FREE · ABOUT 4 MINUTES

Separate biological counts, locations and roles

Use precise definitions and explicit assumptions to eliminate plausible distractors.

The idea

Biology questions often place related ideas beside one another: chromosomes and chromatids, a membrane and its enclosed space, a vessel's direction and its oxygen content, or an enzyme that cuts and one that joins. State what the question actually counts or asks before selecting an option. In a numerical model, keep the biological assumptions visible; independent assortment or a stated transfer efficiency is a condition for the calculation, not a universal rule.

A method to try

  1. Underline the object being counted, the location requested or the process being described.
  2. Write the relevant definition in a short phrase before reading the answer options again.
  3. For probability, list the possible gametes or separate independent events. For a sequence of transfers, count the links.
  4. Test each distractor against the definition. A true statement about another stage, structure or process can still be the wrong answer.
WORKED EXAMPLE

Follow the reasoning.

A cell with 8 chromosomes and 4 pg of nuclear DNA completes S phase. Before mitosis, how many chromosomes, chromatids and picograms of nuclear DNA does it contain? Count chromosomes by centromeres.

  1. Replication doubles the nuclear DNA from 4 pg to 8 pg. Each of the original chromosomes now has two sister chromatids.
  2. There are still 8 chromosomes because sister chromatids remain joined at their centromeres. There are 16 chromatids and 8 pg of nuclear DNA.
  3. Keep these three quantities separate in your notes. The two chromatids of one duplicated chromosome do not become separate chromosomes until their centromeres separate during the relevant division stage.
CHECK YOUR UNDERSTANDING

Think it through first.

In Aa × aa, with normal segregation and equal viability, what fraction of offspring is expected to be aa?

Show the explanation

One-half.

The first parent supplies A or a with equal probability, while the second supplies only a. The possible offspring genotypes are Aa and aa, each with probability one-half.

Make it your own

For every distractor you reject, name the different stage or role it describes. Explain the correct option aloud without repeating the option's wording.

This lesson is one part of a short starter pathway. It does not cover the full exam syllabus or establish a predicted score.

Official references

These provider and publisher links explain the exam or underlying concepts. Our lessons and questions are original, independent practice.

View 4 references and checked dates

Original lesson and examples · Checked 2026-09-29 · No provider endorsement

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