Final answer:
The abbreviation 1n refers to haploid cells, which have one set of chromosomes and are crucial in sexual reproduction. Haploid cells, represented through meiosis, differ from diploid somatic cells which contain two sets of chromosomes.
Step-by-step explanation:
1n is an abbreviation for haploid, which is option a). Haploid refers to a cell with one set of chromosomes, such as a gamete. In the context of the cell cycle and mitosis, chromatids and chromosome duplication are important concepts. Haploid cells, designated as 1n, are significant in sexual reproduction. They are involved in meiosis, a process by which a diploid cell divides to form four unique haploid daughter cells.
In humans and many other organisms, most cells are diploid (2n), meaning they have two sets of chromosomes. These include somatic cells, which are all the cells that make up the body except for the reproductive cells. Haploid cells, however, contain only one set of chromosomes and are typically found in gametes—sex cells like eggs and sperm. When two haploid cells—such as an egg and a sperm—combine during fertilization, a diploid zygote is formed, which then undergoes mitosis to develop into a multicellular organism.
The process of forming haploid cells from diploid ones is called meiosis, which consists of two rounds of cell division—meiosis I and meiosis II. Meiosis I is commonly referred to as reduction division because it reduces the chromosome number by half, creating haploid cells. In contrast to meiosis, mitosis is a single round of cell division that results in two daughter cells that are genetically identical to the parent cell.