Meiosis
Meiosis is a specialized form of eukaryotic cell division that halves chromosome number and creates genetically varied haploid cells used in sexual reproduction.
What meiosis is
Meiosis is a specialized type of cell division used in sexual reproduction. It starts with a diploid cell, meaning a cell with two sets of chromosomes, and produces haploid cells with one set. In animals, those haploid cells contribute to gametes such as sperm and eggs.
Why chromosome number is halved
Sexual reproduction combines genetic material from two parents. If gametes kept the full chromosome number, chromosome sets would double every generation. Meiosis prevents that by reducing chromosome number before fertilization, so the diploid number can be restored when gametes fuse.
Meiosis I
Meiosis I is the reduction division. Homologous chromosomes pair with each other, exchange segments in crossing over, line up as pairs, and then separate to opposite poles. Sister chromatids usually remain joined during this first division, so each daughter cell receives one chromosome from each homologous pair.
Meiosis II
Meiosis II resembles mitosis more closely. The sister chromatids of each chromosome separate, producing cells with one copy of each chromosome. Because the DNA was copied only once before the two divisions, the final products are haploid rather than diploid.
Crossing over
Crossing over occurs when homologous chromosomes exchange matching DNA segments during prophase I. This can create chromosomes with new combinations of alleles. The process helps explain why offspring are not simple copies of either parent, even before mutation or random fertilization are considered.
Independent assortment
During meiosis I, homologous chromosome pairs can orient in different ways before separation. The maternal and paternal versions of different chromosomes are assorted into gametes in many possible combinations. This independent assortment is another major source of genetic variation.
Meiosis and errors
Meiosis must pair, recombine, and separate chromosomes accurately. Errors in chromosome separation can produce cells with missing or extra chromosomes, a condition called aneuploidy. Some errors prevent development, while others are associated with infertility, miscarriage, or genetic conditions.
Why it matters
Meiosis matters because it links cell biology to heredity and evolution. It preserves chromosome number across generations while creating genetic diversity for natural selection to act on. Without meiosis, sexual reproduction, inheritance patterns, and much of eukaryotic diversity would look entirely different.