This cheat sheet compares mitosis and meiosis, the two major ways eukaryotic cells divide. Students need it because these processes explain growth, repair, reproduction, inheritance, and genetic variation. It helps organize the stages, chromosome changes, and final products in a clear side-by-side way.
Mitosis produces two genetically identical diploid body cells for growth and repair. Meiosis produces four genetically different haploid gametes for sexual reproduction. The most important ideas are chromosome number, DNA replication before division, homologous chromosome separation in meiosis I, and sister chromatid separation in mitosis and meiosis II.
Key Facts
- Mitosis has one cell division and produces 2 genetically identical diploid cells.
- Meiosis has two cell divisions and produces 4 genetically different haploid cells.
- Diploid cells have two sets of chromosomes, written as 2n, while haploid cells have one set, written as n.
- DNA replication happens once during interphase before both mitosis and meiosis.
- In mitosis, sister chromatids separate during anaphase, so chromosome number stays the same.
- In meiosis I, homologous chromosomes separate, reducing chromosome number from 2n to n.
- In meiosis II, sister chromatids separate, similar to mitosis, but the cells are already haploid.
- Crossing over in prophase I and independent assortment in metaphase I create genetic variation.
Vocabulary
- Mitosis
- Mitosis is nuclear division that produces two genetically identical body cells with the same chromosome number as the parent cell.
- Meiosis
- Meiosis is cell division that produces four genetically different gametes with half the chromosome number of the parent cell.
- Diploid
- A diploid cell has two sets of chromosomes, one set from each parent, and is written as 2n.
- Haploid
- A haploid cell has one set of chromosomes and is written as n.
- Homologous chromosomes
- Homologous chromosomes are matching chromosome pairs that carry genes for the same traits, with one chromosome inherited from each parent.
- Crossing over
- Crossing over is the exchange of DNA between homologous chromosomes during prophase I of meiosis.
Common Mistakes to Avoid
- Confusing sister chromatids with homologous chromosomes is wrong because sister chromatids are identical copies, while homologous chromosomes are a matched pair from two parents.
- Saying mitosis makes gametes is wrong because mitosis makes body cells for growth and repair, while meiosis makes sperm or egg cells.
- Forgetting that DNA replicates before division is wrong because chromosomes must copy during interphase before mitosis or meiosis begins.
- Thinking meiosis keeps chromosome number the same is wrong because meiosis reduces chromosome number from diploid 2n to haploid n.
- Placing crossing over in mitosis is wrong because crossing over normally occurs during prophase I of meiosis and increases genetic variation.
Practice Questions
- 1 A human body cell has 46 chromosomes. How many chromosomes are in each daughter cell after mitosis?
- 2 A diploid cell has 12 chromosomes. How many chromosomes will each gamete have after meiosis?
- 3 If one parent cell completes meiosis, how many daughter cells are produced, and are they genetically identical or different?
- 4 Why is meiosis important for sexual reproduction while mitosis is better suited for growth and tissue repair?
Understanding Mitosis vs Meiosis
A chromosome is not always shaped like the familiar X. Most of the time, it is a long, loose DNA molecule wrapped around proteins. After DNA is copied, each chromosome has two matching sister chromatids joined at a centromere.
The X shape is easiest to see when the cell has packed its DNA tightly for division. Homologous chromosomes are different from sister chromatids. A homologous pair contains one chromosome inherited from each parent.
They carry the same kinds of genes in the same places, but they can carry different versions of those genes. Keeping these terms separate makes the stages much easier to follow.
Cell division depends on a structure called the spindle. Spindle fibers attach near each chromosome's centromere and pull chromosomes toward opposite sides of the cell. The cell must attach the fibers correctly before it separates DNA.
Checkpoints slow or stop the process if chromosomes are not lined up or attached properly. This control matters because each new cell needs a complete set of genetic instructions. After the chromosomes move apart, the cell membrane pinches inward in animal cells.
Plant cells build a cell plate that becomes a new wall between the two cells. This splitting of the cell contents is called cytokinesis.
The special events of the first meiotic division explain why offspring from the same parents are not genetically identical. During crossing over, matching sections of DNA can be exchanged between homologous chromosomes. This creates chromosomes with new combinations of inherited gene versions.
Independent assortment adds another source of variety. Each homologous pair lines up independently of the other pairs, so maternal and paternal chromosomes are distributed in many possible combinations.
Fertilization then joins one gamete from each parent, increasing the possible combinations again. These processes help populations contain variation, which can affect traits and survival over many generations.
Mistakes in chromosome separation can have serious effects. If a pair fails to separate, one resulting cell may receive an extra chromosome while another receives too few. This error is called nondisjunction.
It can happen during either meiotic division and may lead to gametes with an unusual chromosome number. If such a gamete takes part in fertilization, the embryo may have a chromosomal condition, may not develop normally, or may not survive.
Scientists study these errors in genetics, fertility care, and prenatal testing. The risk of some nondisjunction events rises with parental age, especially the age of the egg-producing parent.
When studying diagrams, track one chromosome pair with two colors from start to finish. First identify whether each chromosome is copied. Then ask what separates at that stage, homologous chromosomes or sister chromatids.
Count chromosome sets, not just X shapes, because a copied chromosome still counts as one chromosome until its sister chromatids separate. Notice that DNA is copied only once before the divisions begin.
Drawing a simple sequence of cells often works better than memorizing stage names alone. The main goal is to connect chromosome movement with the genetic outcome of each cell.