Biology · Introductory biology · Concept
Meiosis: from diploid cells to gametes
Meiosis makes gametes, sperm and eggs, with half the chromosome number of the parent cell. A diploid cell, 2n, with pairs of homologous chromosomes copies its DNA and then divides twice. Meiosis I separates the homologous pairs, which halves the chromosome number; meiosis II separates sister chromatids, as mitosis does. The four haploid cells, n, differ from each other and from the parent cell, because homologs exchange segments by crossing over and each pair lines up independently of the others.
Diploid and haploid
Most body cells are diploid: they carry two sets of chromosomes, one from each parent, so each chromosome has a homologous partner with the same genes, though possibly different alleles. Human body cells have 46 chromosomes, 23 pairs. Gametes are haploid, with one of each pair, so fertilization restores 46.
Meiosis I: separating homologs
Before meiosis the DNA replicates, so each chromosome has two identical sister chromatids. In prophase I the homologs pair up and exchange segments by crossing over. In metaphase I the pairs line up at the middle of the cell, and in anaphase I the homologs of each pair go to opposite poles. Each of the two new cells has n chromosomes, each still of two chromatids.
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | One | Two |
| Cells made | Two | Four |
| Chromosome number | The same as the parent (2n) | Half the parent’s (n) |
| Daughter cells identical? | Yes | No: crossing over and independent assortment |
| Homologs pair up? | No | Yes, in prophase I |
| Role | Growth and repair | Gametes for sexual reproduction |
Meiosis II: separating sister chromatids
Meiosis II works like mitosis, with no new round of DNA replication: the chromosomes line up, the sister chromatids separate, and each cell divides again. One diploid cell gives four haploid cells.
Where variation comes from
Independent assortment: each homologous pair lines up in metaphase I independently of the others, so a cell with n pairs can make 2ⁿ combinations of chromosomes. For humans that is 2²³, more than 8 million, before crossing over adds new combinations of alleles along each chromosome. Random fertilization then joins one of a mother’s possibilities with one of a father’s.
When separation fails
Nondisjunction is a failure of homologs to separate in meiosis I or of sister chromatids to separate in meiosis II. Some gametes then carry an extra chromosome and others miss one, and fertilization gives a trisomy or a monosomy, such as trisomy 21, which causes Down syndrome.
Common mistakes
- Saying meiosis II halves the chromosome number: meiosis I does, by separating homologs; meiosis II separates sister chromatids.
- Counting chromatids as chromosomes: after replication a chromosome still counts as one until its chromatids separate.
- Saying the four cells from meiosis are identical: crossing over and independent assortment make them differ.
- Confusing homologous chromosomes with sister chromatids: homologs come from different parents and can carry different alleles; sister chromatids are copies made by replication.
Key terms
- Meiosis
- Two divisions that turn one diploid cell into four haploid cells, the gametes in animals. Homologs separate in meiosis I; sister chromatids separate in meiosis II.
- Ploidy
- The number of chromosome sets in a cell. Haploid means one set; diploid means two. Chromosome replication alone does not double ploidy.
- Homologous chromosomes
- A matched pair of chromosomes, one from each parent, with the same genes at the same places. Their alleles can differ.
- Sister chromatids
- The two identical copies of a chromosome made when its DNA is replicated. They stay joined at the centromere until they separate in mitosis or meiosis II.
- Gamete
- A haploid sex cell, an egg or a sperm, with one chromosome from each pair. Two gametes fuse at fertilization to make a diploid zygote.
- Crossing over
- Exchange of corresponding chromosome segments between nonsister chromatids of homologs during meiosis. Exchanges can generate new allele combinations without creating new alleles.
- Independent assortment
- Alleles of different genes go into gametes independently of each other, because each chromosome pair lines up on its own in meiosis. Genes close together on one chromosome usually don’t assort independently.
- Allele segregation
- The separation of a gene’s two alleles into different gametes during meiosis. A heterozygote (Aa) passes A to half its gametes and a to the other half.
- Recombinant
- A chromosome, gamete or offspring with a combination of alleles that neither parental chromosome carried, made by crossing-over. The recombinant fraction shows how far apart two linked genes are.
- Nondisjunction
- A failure of homologous chromosomes or sister chromatids to separate during cell division. Gametes then carry an extra or a missing chromosome, and fertilization gives a trisomy or a monosomy.
- Allele
- One version of a gene. A diploid organism usually carries two copies of each gene, one from each parent, and the two can be different alleles without looking different.
Work through an example
A cell from an organism with 2n = 8 enters meiosis. How many chromosomes and chromatids does it have after DNA replication, after meiosis I and after meiosis II? Ignoring crossing over, how many combinations of chromosomes can its gametes carry?
Count chromosomes through meiosis →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Study it, then try it
Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.
Study the 11 key terms Open the cell in Chromosomes & meiosis Open worked example on a board Genetics in Biology ReferenceYour existing work stays on this device. Examples open as editable copies.