This unit covers how cells copy their DNA and divide through mitosis or meiosis. By the end, you should be able to track chromosomes through each phase, count chromosomes at each stage, and explain why meiosis usually produces more genetic variation than mitosis.
Read the big ideas first, then work through each topic note and answer its checkpoint question before you look at the answer.
Cell division is how one fertilized egg becomes a body with trillions of cells, and how that body makes sperm or eggs for the next generation. This unit compares two ways of dividing that start from the same copied DNA but end with very different cells.
Honors Biology Unit 6 covers cell division. You learn the cell cycle (G1, S, G2, and M phase), the four phases of mitosis (prophase, metaphase, anaphase, telophase), cytokinesis in plant and animal cells, and the checkpoints and proteins like cyclins and p53 that control when a cell divides. You then learn meiosis, which typically uses two divisions to turn one diploid cell into four haploid cells. Crossing over and independent assortment can produce different combinations of alleles in those cells, while random fertilization combines gametes from two parents. The unit ends by comparing mitosis and meiosis by number of divisions, number of cells produced, chromosome number, and genetic identity.
After S phase, each replicated chromosome has two sister chromatids joined at the centromere. The chromatids are usually identical copies before mitosis or meiosis begins. The divisions that follow only separate copies that already exist.
A human body cell with 46 chromosomes divides into two cells with 46 chromosomes each. Your body uses mitosis to grow, to heal a cut, and to replace skin and blood cells. When the checkpoints that control mitosis fail, cells can divide out of control and form a tumor.
A human diploid cell with 46 chromosomes typically produces four haploid cells with 23 chromosomes each through meiosis. When a typical sperm and egg fuse, the zygote has 46 chromosomes. Crossing over and independent assortment can produce different combinations of alleles, though two gametes can have the same combination.
Mitosis and meiosis II both separate sister chromatids, so the chromosome number does not change in that step. Meiosis I separates homologous chromosomes instead, which is the step that cuts the number in half. If you know which structures separate in anaphase and the starting chromosome number, you can predict the chromosome number in the resulting cells. The exact allele combinations also depend on events such as crossing over and independent assortment.
The cell cycle has two parts.
Meiosis typically takes one diploid cell (2n) and produces four haploid cells (n).
Both processes start after interphase, with chromosomes already copied into sister chromatids.
The cell cycle has two parts. Interphase is the long stretch where the cell grows (G1), copies its DNA (S), and gets ready to divide (G2). M phase is the short stretch where the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis).
Mitosis runs in four phases, often remembered as PMAT. In prophase, chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and spindle fibers form. Spindle fibers attach to structures called kinetochores at the centromeres. In metaphase, the chromosomes line up along the middle of the cell. In anaphase, sister chromatids separate and move to opposite poles. In telophase, nuclear envelopes reform and chromosomes loosen back into chromatin. In animal cells, a cleavage furrow pinches the cell in two, while plant cells build a cell plate because their rigid wall can't pinch inward.
Checkpoints at G1, G2, and M stop the cycle if something is wrong. Cyclins bind to and activate CDKs (cyclin-dependent kinases), which push the cell into the next phase. Tumor suppressors like p53 can pause the cycle to repair DNA. Mutations that break these controls can lead to cancer.
Worked example. A human cell in G1 has 46 chromosomes, each one chromatid, so 46 chromatids total. After S phase it still has 46 chromosomes, but each has two sister chromatids, so 92 chromatids. In anaphase the sisters split, giving 92 separate chromosomes in the cell. Each daughter cell ends with 46 chromosomes, matching the parent.
Check yourself: A cell in G2 has 12 chromosomes. How many chromatids does it have, and how many chromosomes will each daughter cell have after mitosis? (Answer: 24 chromatids, since each chromosome has two sisters after S phase; each daughter cell gets 12 chromosomes.)
Meiosis typically takes one diploid cell (2n) and produces four haploid cells (n). In animals, these cells contribute to gamete production, though human egg production normally yields one egg and polar bodies. In plants, meiosis produces spores. In humans, 2n = 46 and n = 23. When a 23-chromosome sperm fuses with a 23-chromosome egg, the zygote has 46. Without meiosis, the chromosome number would double every generation.
DNA is copied once, before meiosis I. Meiosis I is the reduction division. In prophase I, homologous chromosomes (one from each parent) pair up in synapsis, forming a tetrad of four chromatids. Non-sister chromatids swap segments at points called chiasmata. This is crossing over. In metaphase I, homologous pairs line up together, and each pair flips randomly, which is independent assortment. In anaphase I, homologs separate, so each cell gets one of each pair.
Meiosis II looks like mitosis. Sister chromatids separate, and the two haploid cells become four haploid cells. There is no DNA replication between meiosis I and II.
Worked example. A fruit fly has 2n = 8, so it has four homologous pairs and n = 4 chromosomes. Each pair can line up two ways in metaphase I, so independent assortment alone gives possible gametes. For humans, million. Random fertilization combines one parent's options with the other parent's options, giving trillion possible chromosome combinations in a zygote before accounting for crossing over.
Check yourself: A plant has 2n = 20. How many chromosomes are in each cell after meiosis I, and after meiosis II? (Answer: 10 after meiosis I, since homologs separate; still 10 after meiosis II, since only sister chromatids separate.)
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| What separates in anaphase | Homologous chromosomes | Sister chromatids |
| Synapsis and crossing over | Yes, in prophase I | No |
| Chromosome number (human) | 46 to 23 per cell | Stays 23 per cell |
| Cells at the end | 2 haploid cells | 4 haploid cells |
Both processes start after interphase, with chromosomes already copied into sister chromatids. The difference is what happens next.
Mitosis has one division and usually makes two cells with the same chromosome number and genetic information as the parent cell. Homologous chromosomes never pair up; each chromosome lines up on its own at the metaphase plate. Your body uses mitosis for growth, tissue repair, and replacing worn-out cells, and some organisms use it for asexual reproduction.
Meiosis has two divisions and typically makes four haploid cells. Crossing over and independent assortment can make these cells genetically different from one another. Homologs pair up and cross over in prophase I, then separate in anaphase I. Meiosis happens only in reproductive tissue, like the testes and ovaries in humans or anthers and ovules in flowering plants.
A fast way to tell them apart in a diagram is to look at metaphase. If chromosomes line up single file, it's mitosis or meiosis II. If they line up in pairs, it's meiosis I.
Check yourself: A diagram shows a cell with chromosomes lined up in pairs across the middle. Which process and phase is it? (Answer: Metaphase I of meiosis, because only meiosis I lines up homologous chromosomes as pairs.)
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Cells produced | 2 | 4 |
| Chromosome number (human) | 46 (diploid) | 23 (haploid) |
| Homologs pair up? | No | Yes, in prophase I |
| Crossing over? | No | Yes |
| Genetic identity | Usually genetically identical to parent | Can differ genetically from each other |
| Purpose | Growth, repair, asexual reproduction | Producing haploid cells for sexual life cycles |
| Term | Definition |
|---|---|
| Cell cycle | The full sequence of growth, DNA replication, and division that a cell goes through, made up of interphase and M phase. |
| Mitosis | Division of the nucleus that usually produces two genetically identical nuclei with the same chromosome number as the parent nucleus. |
| Meiosis | Two rounds of division that typically turn one diploid cell into four haploid cells, which can differ genetically. |
| Centromere | The chromosome region where sister chromatids are held together and where a structure called the kinetochore forms. Spindle fibers attach to the kinetochore. |
| Spindle fibers | Protein fibers made of microtubules that attach to chromosomes and pull them to opposite poles of the cell. |
| Tetrad | A pair of homologous chromosomes joined during prophase I, containing four chromatids in total. |
| Chiasma | The X-shaped point where non-sister chromatids have exchanged segments during crossing over. |
| Zygote | The diploid cell formed when a haploid sperm and a haploid egg fuse at fertilization. |
| p53 | A tumor suppressor protein that halts the cell cycle when DNA is damaged, and whose mutation is common in cancer. |
After S phase an X-shaped chromosome looks like two pieces, so it's easy to count it twice. Count centromeres instead. One centromere means one chromosome, whether it has one chromatid or two.
Because meiosis has two divisions, it seems like it needs two rounds of copying. It doesn't. DNA is copied once before meiosis I, with no replication before meiosis II. The cells become haploid when homologous chromosomes separate in meiosis I.
Sister chromatids are copies made in S phase. They are usually identical when first made, but crossing over or replication errors can make them different. Homologous chromosomes are a matched pair from two different parents and can carry different alleles. Crossing over happens between non-sister chromatids of homologs, which is why it creates new allele combinations.
The halving happens in meiosis I, when homologs separate. Meiosis II separates sister chromatids, just like mitosis, so the number of chromosomes per cell stays the same in that step.