Mitosis and meiosis: flashcards, quiz and summary

Flashcards

1. 1Why does a cell need mitosis, and what does it produce?

   Mitosis supports growth, tissue repair, and asexual reproduction. It produces two diploid daughter cells that are genetically identical to the parent cell.

2. 2Why does meiosis produce gametes instead of identical body cells?

   Meiosis makes gametes for sexual reproduction. It produces four haploid cells that are genetically different, so that offspring have a mix of genetic material from both parents.

3. 3What happens to chromosome number in mitosis and in meiosis?

   In mitosis, chromosome number stays the same, going from 2n to 2n. In meiosis, chromosome number is halved, going from 2n to n.

4. 4What happens during S phase of interphase, and why does it matter for division?

   During S phase, DNA is replicated so each chromosome has two sister chromatids joined at the centromere. This must happen before either mitosis or meiosis can separate the chromosomes correctly.

5. 5What happens in prophase I of meiosis that does not happen in mitosis?

   Homologous chromosomes pair up in synapsis to form tetrads, and crossing over swaps pieces of DNA between them. This is a key source of genetic variation.

6. 6How does independent assortment create variation in meiosis?

   In metaphase I, each homologous pair lines up at the middle in a random orientation. The way each pair faces determines which chromosome from each pair goes into each gamete, so gametes get different combinations.

7. 7What is nondisjunction, and what is one example?

   Nondisjunction is when chromosomes fail to separate properly, so a gamete ends up with one too many or one too few chromosomes. Trisomy 21, which causes Down syndrome, means a person has three copies of chromosome 21.

8. 8How are cell plates and cleavage furrows involved in cytokinesis?

   In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms and becomes a new cell wall between the two daughter cells.

Practice quiz

Pick an answer to check it.

1. 1Which statement correctly describes the result of mitosis in a human body cell?

   1. Four diploid cells that are genetically identical to the parent
   2. Two haploid cells that are genetically different
   3. Four haploid cells that are genetically different
   4. Two diploid cells that are genetically identical to the parent
   Show the answer

   D. Two diploid cells that are genetically identical to the parent Mitosis has one division and produces two diploid daughter cells, which are genetically identical to the parent. Four cells with different genes is the result of meiosis, not mitosis.

2. 2During which phase of meiosis does crossing over occur?

   1. Metaphase I
   2. Prophase I
   3. Prophase II
   4. Anaphase I
   Show the answer

   B. Prophase I Crossing over happens in prophase I, when homologous chromosomes pair up to form tetrads and exchange DNA. Metaphase I is when independent assortment happens, so it is easy to confuse the two.

3. 3What is the chromosome number of a human gamete?

   1. 92
   2. 11.5
   3. 23
   4. 46
   Show the answer

   C. 23 Human body cells are diploid with 46 chromosomes, and gametes are haploid with 23. The value 46 is the body cell number, so it is a common wrong pick.

4. 4Which event separates sister chromatids in meiosis?

   1. Anaphase I
   2. Metaphase I
   3. Telophase I
   4. Anaphase II
   Show the answer

   D. Anaphase II Anaphase II separates sister chromatids, much like anaphase in mitosis. Anaphase I separates homologous chromosomes, while sister chromatids stay joined.

5. 5Which process adds genetic variation after meiosis has already produced the gametes?

   1. Random fertilization
   2. Cytokinesis
   3. Independent assortment
   4. Crossing over
   Show the answer

   A. Random fertilization Random fertilization happens after meiosis, when any sperm can join any egg. Crossing over and independent assortment both occur during meiosis itself, so they are not the answer.

6. 6Which structure forms when homologous chromosomes pair up during prophase I?

   1. A tetrad
   2. A cell plate
   3. A centromere
   4. A cleavage furrow
   Show the answer

   A. A tetrad Homologous chromosomes pair up during prophase I to form a tetrad. A centromere is the joining point of sister chromatids, so it is part of a chromosome rather than a pairing structure.

Test traps

What teachers like to test on this, and the mistakes students make.

• Confusing the number of divisions: mitosis has one division and meiosis has two, and students often lose points by swapping these in a comparison question.
• Saying meiosis produces two cells: meiosis produces four haploid cells, and mitosis produces two diploid cells.
• Mixing up which phase handles which event: crossing over belongs to prophase I, independent assortment to metaphase I, and sister chromatid separation to anaphase II.
• Thinking anaphase I separates sister chromatids: in meiosis I, homologous chromosomes are separated while sister chromatids stay together until anaphase II.
• Writing that crossing over happens in mitosis: it happens only in meiosis, and it is a common trick question on comparison tables.
• Describing nondisjunction as a random mutation: it is a failure of chromosomes to separate properly, which causes a gamete to have an extra or missing chromosome.

Summary

• Mitosis is used for growth, tissue repair, and asexual reproduction, and it produces two diploid cells that are genetically identical to the parent.
• Meiosis is used to make gametes for sexual reproduction, and it produces four haploid cells that are genetically different from each other.
• Interphase, which includes G1, S, and G2, comes before either division, and DNA is copied during S phase so each chromosome has two sister chromatids.
• Mitosis has four phases in order: prophase, metaphase, anaphase, and telophase, followed by cytokinesis that splits the cytoplasm.
• Meiosis has two divisions. Meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids.
• Crossing over happens in prophase I, independent assortment happens in metaphase I, and random fertilization adds more variation after meiosis.
• Nondisjunction means chromosomes fail to separate correctly, so a gamete ends up with an extra or missing chromosome, as in trisomy 21.
• Cancer is linked to cells that keep dividing by mitosis when the cell cycle checkpoints fail to stop them.

Key terms

Diploid (2n)

A cell with two sets of chromosomes, one from each parent. Human body cells are diploid with 46 chromosomes.

Haploid (n)

A cell with one set of chromosomes. Human gametes are haploid with 23 chromosomes.

Homologous chromosomes

A matching pair of chromosomes, one from each parent, with the same genes in the same places but possibly different alleles.

Sister chromatids

The two identical copies of one chromosome that form after DNA replication, joined at the centromere.

Tetrad

A structure formed in prophase I when homologous chromosomes pair up during synapsis.

Crossing over

The exchange of DNA pieces between homologous chromosomes during prophase I, which creates new combinations of alleles. It happens only in meiosis.

Independent assortment

The random way homologous pairs line up in metaphase I, which determines which chromosomes end up in each gamete.

Nondisjunction

A failure of chromosomes to separate properly, so a gamete has one too many or one too few chromosomes.

How to study it

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Questions