Mitosis and Meiosis: Cell Division Processes

Posted on May 20, 2024 in Biology

Mitosis

Mitosis is a process of nucleus division, through which two daughter cells with the same number of chromosomes as the parent cell are formed.

Phases:

  • Prophase: Chromosomes, replicated during the interphase, condense. The nuclear membrane and the nucleolus disappear. Centrioles separate and move to opposite poles of the cell. Filaments grow between them, forming the spindle apparatus, which allows chromosomes to move.
  • Metaphase: Chromosomes reach maximum condensation. They align perpendicular to the spindle apparatus and join to it by the centromeres.
  • Anaphase: Sister chromatids of each chromosome separate. Each chromatid travels to one end of the cell, forming two daughter chromosomes, one for each end.
  • Telophase: Daughter chromosomes migrate to the end of their corresponding cells. The spindle apparatus disappears, and the nucleolus and nuclear membrane are reconstituted. At the end, two nuclei are formed.

Meiosis

Sexual Reproduction

Sexual reproduction takes place when two sex cells from two parents of different sexes join together. There are three phases: formation of gametes or sex cells; the fusion of the gametes through fertilization, creating a new cell called a zygote; and the development of the zygote, which divides successively and creates a complete individual.

How are gametes formed?

There are two main types of cells:

  • Somatic cells: Cells that form the body of the organisms. They are diploid (2n) cells, meaning each pair of homologous chromosomes has one chromosome from the father and one from the mother. Cell division through mitosis causes the organism to grow and replaces dead cells.
  • Germ cells: Diploid cells that produce haploid (n) sex cells through meiosis.

Meiosis

Meiosis is a cell division process through which haploid cells are formed.

After meiosis, daughter cells have half the number of chromosomes (n) of the original cell. We call meiosis a reductional division.

Meiosis involves two consecutive cell divisions: the first meiotic division and the second meiotic division.

First Meiotic Division Phases:

  • Prophase I: The longest phase. Chromatin fibers replicated during the interphase condense, and chromosomes become visible. Each chromosome is made up of two chromatids. Chromosomes pair up (synapsis) and exchange fragments (crossover), which will make the daughter cells different from the parent cell. The nuclear membrane disappears.
  • Metaphase I: Pairs of homologous chromosomes, which remain joined at the crossover points, place themselves midway down the cell, attached to the spindle filaments.
  • Anaphase I: The fibers of the spindle apparatus become shorter, and the homologous chromosomes separate. Homologous chromosomes migrate to different ends of the cell.
  • Telophase I: Homologous chromosomes decondense, the nuclear membrane reforms, and the spindle apparatus disappears.

Second Meiotic Division Phases:

  • Prophase II: Division takes place in the two cells that are the result of the first division. The spindle apparatus reforms, the nuclear membrane disappears, and chromosomes recondense.
  • Metaphase II: Chromosomes gather midway down the cell and form a metaphase plate.
  • Anaphase II: The two chromatids in each chromosome separate, with one chromatid moving to each end of the cell.
  • Telophase II: Chromosomes decondense, and chromatin is formed. The nuclear membrane and the spindle apparatus are formed.
  • Cytokinesis: The cytoplasm divides, and the organelles are distributed between the daughter cells. The final result is cells that have half the chromosomes of the 2n parent cell.

Biological Significance of Mitosis

  • In multicellular organisms, the purpose of mitosis is to enable the individual to grow through successive divisions and to renew deteriorated cells.
  • In eukaryotic unicellular organisms, mitosis is a reproduction mechanism that enables individuals to increase in number.

Biological Significance of Meiosis

+Essencial process that halves the number of chromosomes during the formation of sex cells. Diploid (2n) cells with 2 complete sets of chromosomes produce haploid cells with one set of chromosomes.
+During meiosis, there is a exchange of sister chromatid fragments, between homologous chromosomes. this creates cells that are genetically different from the parent cell, which results in genetic variability
MITOSIS+  MEIOSIS*
+only takes places in somatic cells. *takes places in germ cell.
+ happens in both haploid and diploid cells. *happens only in cells with a diplod number of chromosomes.
+ short process. * long process.
+nucleus divides ones. *nucleus divides twice.
+no crossover. *crossover between homologous chromosomes.
+sister chromosomes separate during the anaphase. *in anaphase I homologous chromosomes separates .sister chromosomes separates during anaphase II.
+identical daughter cells are formed with the same chromosomes as the mother. *4 genetically different daughter cells are formed with half the chromosome of the parent.