Why is it said that Meiosis I is a reduction division?
Meiosis is the cellular division that creates sex cells (sperm and egg) in organisms. This is in contrast to mitosis, which creates somatic (body) cells during growth and repair in multicellular organisms.
DNA is only replicated during the interphase. DNA does not undergo another replication during Meiosis II. Therefore, Meiosis I is called a reduction division because is results in the reduction of the diploid number of chromosomes being split into the haploid number.
The diploid number of chromosomes refers to the normal number of chromosomes that is characteristic to all somatic cells of a species (di = 2). The diploid number is often represented by a (2n). The haploid number refers to half of the diploid number. Sex cells contain the haploid number (represented by just an "n") of chromosomes.
For example, somatic cells of a normal human cell contain 46 chromosomes whereas a sex cell contains 23 chromosomes.
This reduction division is needed so that when the egg and sperm unite during fertilization, the diploid number of the species is restored.