By the end of this lesson
- Describe the outcome of mitosis.
- Sequence the key stages of cell division.
- Explain why chromosome replication is essential.
BIG QUESTION
How can one cell produce new cells with the same genetic information?
DNA is replicated before division so each daughter nucleus receives one copy of every chromosome.
DNA replicates before division
Before mitosis, each chromosome is replicated to form two sister chromatidsThe two identical copies of a chromosome, joined at the centromere, formed when DNA is copied before cell division.. Replication copies the DNA sequence so both chromatids carry the same genes.
The cell still has the same chromosome number at this point because joined sister chromatids count as one replicated chromosome.
Chromatids separate into two nuclei
Chromosomes condense, line up and sister chromatids are pulled to opposite poles. New nuclear membranes form around each complete set.
Cytokinesis then divides the cytoplasm, producing two daughter cells.
Mitosis conserves genetic information
Each daughter cell receives one chromatid from every replicated chromosome, so both are genetically identical to the original cell apart from mutation.
In a human somatic cell, mitosis produces two diploid cells with 46 chromosomes each.
Chromosome number through mitosis
Distinguish DNA amount from chromosome number.
| Stage | Chromosome/DNA state | Outcome |
|---|---|---|
| Before replication | One chromatid per chromosome. | Diploid parent cell. |
| After replication | Two joined sister chromatids. | DNA copied. |
| Chromatid separation | Sister chromatids move apart. | One set toward each pole. |
| After cytokinesis | One chromatid per chromosome in each cell. | Two identical diploid cells. |
Language that earns precision
- mitosis
- Nuclear division producing genetically identical daughter nuclei.
- cytokinesis
- Division of the cytoplasm into separate cells.
- sister chromatid
- One of two replicated copies of a chromosome joined before separation.
- somatic cell
- Body cell that is not a gamete.
Common misconceptions
Claim: Mitosis makes four cells.
Correction: One mitotic division normally produces two daughter cells.
Claim: Chromosome number doubles after replication.
Correction: DNA amount doubles, but joined sister chromatids are counted as one replicated chromosome.
Claim: Daughter cells receive random chromosome sets.
Correction: Each receives one copy of every chromosome.
Match the answer to the command word
Two genetically identical diploid cells.
Replication before separation ensures each cell receives a complete set.
Recall, apply, then justify
Describe the outcome of mitosis in a human body cell.
Hint
Number, identity and chromosome number.
Suggested marking points
- Two daughter cells are produced.
- They are genetically identical to each other and the parent cell apart from mutation.
- Each daughter cell has 46 chromosomes/is diploid.
Explain why DNA replication occurs before mitosis.
Hint
Two copies, separation, complete set.
Suggested marking points
- Each chromosome is copied to form sister chromatids.
- The chromatids can separate to opposite poles.
- Each daughter cell receives a complete set of genetic information.
Predict the consequences if spindle fibres failed to separate one chromosome’s sister chromatids.
Planning hint
Track chromosome distribution.
What a strong design includes
- The sister chromatids would not move to opposite poles normally.
- One daughter nucleus may receive an extra copy.
- The other may lack that chromosome copy.
- The cells would have abnormal chromosome numbers.
- Gene dosage could be altered.
- Cell function, survival or later division may be affected.