36.4
Meiosis vs. Mitosis
Cell division is necessary for growth and reproduction in organisms. Mitosis aids cell growth and development by dividing somatic cells. In contrast,…
Mitosis and meiosis primarily differ from each other with respect to the number of steps involved, genetic recombination events, and final chromosome and daughter cell numbers.
Most eukaryotic cells divide by mitosis, an equational division, where each diploid parent cell produces two identical diploid daughter cells.
However, each diploid germ cell divides into four genetically distinct haploid daughter cells through meiosis, a reductional division.
Mitosis is a four-stage process – prophase, metaphase, anaphase, and telophase. Meiosis goes through these four stages twice without an intermediate DNA synthesis phase.
The prophase stage of mitosis is shorter and does not involve the pairing of homologous chromosomes or any recombination. Therefore, all the daughter cells are genetically identical.
In meiosis, prophase I is the longest phase consisting of five substages. In prophase I, two homologous chromosomes pair to form a synaptonemal complex and subsequently recombine to produce four genetically diverse chromatids.
During mitotic metaphase, the individual chromosomes assemble along the equator, whereas during metaphase I of meiosis, pairs of homologous chromosomes align on the equator.
The sister chromatids of each chromosome are held together by cohesin complexes.
The cohesin complexes are completely removed by the end of the mitotic metaphase allowing the sister chromatids to separate and move towards opposite poles.
However, in meiosis I, only the cohesin complexes present on the chromosome arms get detached from the chromatid at the start of the anaphase I, whereas those surrounding the kinetochore remain intact.
Thus, even as the homologous chromosomes are separated and pulled to opposite poles, the sister chromatids remain attached and migrate together.
The second cell division in meiosis II is similar to mitosis. Mitotic anaphase and anaphase II of meiosis involve the separation of two sister chromatids of each homologous chromosome, and subsequent division into two daughter cells - identical and diploid for mitosis but non-identical and haploid for meiosis II.
The daughter cells arising from meiosis are more likely to have chromosome segregation errors than the ones arising from mitosis. Such chromosome segregation errors produce aneuploid cells with an incorrect number of chromosomes which can lead to diseases like Down syndrome.
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Q1: How does meiosis differ from mitosis in terms of cell division outcomes?
Mitosis produces two identical diploid daughter cells for growth and repair, while meiosis produces four genetically unique haploid gametes for sexual reproduction. Meiosis involves two consecutive divisions and includes genetic recombination, whereas mitosis involves a single division with no genetic variation between daughter cells.
Q2: What is the primary purpose of meiosis in organisms?
Meiosis generates haploid gametes—sperm and egg cells—that carry half the genetic material of parent cells. When gametes fuse during fertilization, they restore the full diploid chromosome number in offspring while introducing genetic diversity essential for sexual reproduction and species survival.
Q3: How does crossing over increase genetic variation during meiosis?
Crossing over occurs during prophase I when homologous chromosomes exchange segments of DNA, shuffling alleles between maternal and paternal chromosomes. This recombination creates new genetic combinations in gametes, ensuring each offspring inherits a unique mix of traits from both parents.
Q4: Why does meiosis require two divisions instead of one?
Meiosis I separates homologous chromosome pairs, reducing the chromosome number from diploid to haploid. Meiosis II then separates sister chromatids, similar to mitosis. Two divisions are necessary to achieve both the reduction in chromosome number and the genetic recombination required for sexual reproduction.
Q5: What happens when nondisjunction occurs during meiosis?
Nondisjunction is the failure of chromosomes to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers. This produces gametes with extra or missing chromosomes, leading to aneuploid offspring with genetic disorders or developmental abnormalities that can affect survival and health.
Q6: How do somatic cells and gametes differ in chromosome composition?
Somatic cells are diploid, containing two complete sets of chromosomes from both parents. Gametes are haploid, containing only one set of chromosomes. Meiosis converts diploid somatic cells into haploid gametes, ensuring that when gametes combine during fertilization, the diploid state is restored in offspring.