Mitosis: Cell Division Stages

• Cell division involves the multiplication of cells for growth, reproduction, and replacement of old cells
• There are two types of cell division.

Mitotic Cell Division

• Leads to the formation of two daughter cells
• Daughter cells contain the exact same number of chromosomes as the parent cells.
• Mitosis occurs in five stages

Stages of Mitosis

• Interphase
• Prophase
• Metaphase
• Anaphase
• Telophase
• Crucial events take place at each stage
• The process takes about an hour

Interphase

• Described as a resting stage
• Involves protein synthesis, formation of organelles, and replication of DNA
• Cell builds up a sufficient store of energy

Chromosomes During Interphase

• Chromosomes are not visible under a light microscope or electron microscope
• Centrioles are among the most prominent organelles just before mitosis

Prophase - Early

• Chromosomes become visible as they contract
• The nucleus shrinks
• Centrioles move to opposite sides of the nucleus
• Spindle fibers start to form

Prophase - Late

• Chromosomes become shorter and fatter
• Each chromosome consists of a pair of chromatids joined at the centromere
• The nucleolus disappears
• Prophase ends with the breakdown of the nuclear envelope

Metaphase

• Chromosomes arrange themselves on the equator of the spindle

Anaphase

• Chromatids separate and migrate to opposite poles of the cell
• The centromere is leading

Telophase

• Chromosomes unwind
• Nuclear envelope and nucleoli form

Silent Features of Mitosis

• DNA replicates before mitosis
• Chromosomes arrange themselves at the equator before migration to opposite sides

Importance of Mitosis

• Genetic stability is ensured, daughter cells carry the exact genetic material as the parent cells
• Growth results in an increase in the number of cells
• Body repair occurs as worn-out cells are replaced
• Asexual reproduction occurs

Meiosis

• Parent cell divides into four haploid cells
• Leads to the formation of gametes
• Ensures that the zygote has a normal diploid condition after fertilization
• Meiosis occurs in two successive divisions, resulting in a total of four daughter cells

Stages in Meiosis - Meiotic Division 1

• Interphase: Cells are in normal non-dividing conditions with chromosomes long and threadlike

Prophase 1

• Longest phase
• Subdivided into 5 stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis

Leptotene

• Chromosomes are single, long, and scattered

Zygotene

• Single chromosomes begin to pair up with homologous chromosomes
• Process is called synapsis, and each pair is called a bivalent

Pachytene

• Chromosomes become intimately coiled around each other
• Chromosomes replicate into chromatids that move slightly apart
• Chromatids of homologous chromosomes remain in contact at certain points called chiasmata
• Chiasmata is the site of genetic exchange called crossing over

Diplotene

• Chromosomes in the bivalents begin to separate
• Remain joined at the chiasmata in each bivalent
• The number of chiasmata varies but tends to be more on long chromatids

Diakinesis

• Internal coiling continues
• Bivalents are at their most contractive stage

Metaphase 1

• Homologous chromosomes, bivalents arrange themselves opposite each other on the equator of the spindle

Anaphase 1

• Homologous chromosomes (each made up of a pair of chromatids joined at the centromere) move towards opposite poles
• Sister chromatids also separate along their length except at the centromere

Telophase 1

• Chromosomes reach their respective poles
• The cell starts to divide across the middle
• Nuclear envelope forms around the two new nuclei

Meiotic Division II

• Separation of homologous chromosomes that make up a bivalent is achieved by the first meiotic division
• The purpose of the second division is to separate the chromatids from one another

Prophase II

• Two daughter cells prepare for the second meiotic division
• Centrioles have replicated
• A new spindle is formed

Metaphase II

• Chromosomes move to the equator of the spindle
• Chromatids are oriented towards opposite poles

Anaphase II

• Chromatids separate and move apart from each other

Telophase II

• Chromosomes reach the end of the spindle
• The cell divides
• The spindle apparatus disappears
• Chromosomes begin to regain their threadlike form
• New nuclear envelopes and nucleoli form

Significance of Meiosis

• Sexual reproduction: Meiosis results in half the number of chromosomes
• During fertilization, the nuclei of the two gametes fuse to form a zygote
• Restores the diploid number of chromosomes for its species
• Genetic variation occurs due to new recombination of genes
• Crossing over and independent assortment of chromosomes into the gametes at metaphase 1 occur

Differences between Mitosis and Meiosis:

• Homologous chromosomes:
  • Mitosis: Homologous chromosomes remain separate
  • Meiosis: Homologous chromosomes pair up
• Crossing over:
  • Mitosis: Does not occur
  • Meiosis: Occurs
• Chiasmata formation:
  • Mitosis: No formation
  • Meiosis: Formation of chiasmata
• Division:
  • Mitosis: Single division
  • Meiosis: Two series of division
• Daughter cells:
  • Mitosis: Results in two diploid daughter cells
  • Meiosis: Results in four haploid daughter cells
• Location:
  • Mitosis: Occurs in somatic cells
  • Meiosis: Occurs in gonads