Cytology

• Prokaryotic Cells

  • Prokaryotic cells are cells without a nucleus.
  • Prokaryotic cells are found in single-celled organisms, such as bacteria
  • Organisms with prokaryotic cells are called prokaryotes.
• Eukaryotic Cells - are cells that contain a nucleus.
• • Eukaryotic cells are usually larger than prokaryotic
• Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to people.
• Organelles called mitochondria, for example, provide energy to the cell, and organelles called vacuoles store substances in the cell.
• Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can.
• Summary

  ● Prokaryotic cells are cells without a nucleus.

  ● Eukaryotic cells are cells that contain a nucleus.

  ● Eukaryotic cells have other organelles besides the nucleus. The only organelles in a prokaryotic cell are ribosomes.
• What are the two types of cell division ?

  1. Mitosis
  2. Meiosis
• Mitosis (Diploid) - Somatic

  • Mitosis refers to the process in which parent cells divide to produce two diploid daughter cells, which are exact replicas of themselves.
• Also known as vegetative cell division, this process is observed in all somatic or non-reproductive cells of the body (body cells) such as skin, hair, gut, organ, and muscle cells. Its primary role of mitosis is to repair and replace damaged, old, or dead cells.
• Meiosis (Haploid)/ Gametes

  • Meiosis is observed only in reproductive cells. In this cell division process, parent cells first divide to produce two diploid daughter cells.
  • The two daughter cells divide further into four haploid cells with half the number of chromosomes as the parent cell.
  • Each of the haploid cells contains a unique set of genetic information.
• Meiosis also allows genetic variation through a process of gene shuffling while the cells are dividing
• Stages of Mitosis

  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Interphase

  It undergoes the following phases when in interphase:

  ● G1 Phase: This is the period before the synthesis of DNA.

  ● S Phase: This is the phase during which DNA synthesis takes place.

  ● G2 Phase: This is the phase between the end of DNA synthesis and the beginning of the prophase.
• Prophase

  • Prophase immediately follows the S and G2 phases of the cycle and is marked by condensation of the genetic material to form compact mitotic chromosomes composed of two chromatids attached at the centromere.
• The completion of the prophase is characterised by the initiation of the assembly of the mitotic spindle, the microtubules and the proteinaceous components of the cytoplasm that help in the process.
• Metaphase

  • At this stage, the microtubules start pulling the chromosomes with equal force and the chromosome ends up in the middle of the cell. This region is known as the metaphase plate.
  • Thus, each cell gets an entire functioning genome.
• Anaphase

  • The splitting of the sister chromatids marks the onset of anaphase.
  • These sister chromatids become the chromosome of the daughter nuclei.
  • The chromosomes are then pulled towards the pole by the fibres attached to the kinetochores of each chromosome. The centromere of each chromosome leads at the edge while the arms trail behind it.
• Telophase

  • The chromosomes that cluster at the two poles start coalescing into an undifferentiated mass, as the nuclear envelope starts forming around it.
  • The nucleolus, Golgi bodies and ER complex, which had disappeared after prophase start to reappear.
• Telophase is followed by cytokinesis, which denotes the division of the cytoplasm to form two daughter cells. Thus, it marks the completion of cell division.
• Stages of Meiosis

  Meiosis I

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I
  • Cytokinesis

  Meiosis II

  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II
  • Cytokinesis II
• Meiosis I

  • Prophase I
  • The nuclear envelope disintegrates.
  • Chromosomes begin to condense.
  • Spindle fibres appear.
• Metaphase I

  ● The homologous chromosomes align at the equatorial plate ensuring genetic diversity among offspring.
• Anaphase I

  ● The homologous chromosomes are pulled towards the opposite poles.
• Telophase I

  ● Spindle fibres disappear.

  ● Nuclear envelope is reformed.
• Cytokinesis I

  ● The cytoplasm and the cell division result in 2 non-identical haploid daughter cells.
• Prophase II

  ● The chromatin condenses into chromosomes.

  ● Nuclear envelope disintegrates.

  ● Centrosomes migrate to either poles.

  ● Spindle fibres are reformed.
• Metaphase II

  ● The chromosomes align along the equatorial plate. On the contrary, the chromosomes in metaphase I were in homologous pairs.
• Anaphase II

  ● Sister chromatids are pulled to the opposite poles.
• Telophase II

  ● Nuclear envelope redevelops and the spindle fibres disappear.
• Cytokinesis II

  ● The cytoplasm and cell divide producing 4 non-identical haploid daughter cells.