Cell Division and Cycle Quiz

Cell Division

• Cell division is the process of forming new daughter cells from a pre-existing mother cell.
• There are three main types: Amitosis, Mitosis, and Meiosis.

Cell Cycle

• The cell cycle encompasses the changes between successive mitotic cell divisions.
• The cell cycle involves cell growth and division.
• Examples include bacterial cells (20 minutes) and epithelial cells (8-10 minutes).
• The cycle is divided into Interphase, Karyokinesis, and Cytokinesis.

Interphase

• This is the longest phase, also known as the preparatory phase.
• It is divided into three sub-phases:
  • G1 phase (Gap 1): Cell size increases; carbohydrate, lipids, and functional proteins are made; RNA is formed.
  • S phase (Synthetic): DNA replicates; histone proteins are formed; each chromosome has two chromatids with a centromere.
  • G2 phase (Gap 2): Duplication of cell organelles; cell stores energy; proteins for spindle fibers are made.

Karyokinesis

• This is the division of the nucleus.
• It occurs in four phases: Prophase, Metaphase, Anaphase, and Telophase

Prophase

• Chromosomes shorten and thicken.
• Each chromosome has two chromatids joined by a centromere.
• Chromosomes are arranged irregularly in the nucleoplasm.
• The nuclear membrane and nucleolus disappear and spindle fibers start appearing.

Metaphase

• All chromosomes are arranged at the equatorial plane.
• Spindle fibers are formed and attached to centromeres from opposite poles.
• Chromosomes appear as a plate called metaphasic plate.

Anaphase

• The centromere of each chromosome divides, allowing chromatids to become individual chromosomes.
• Spindle fibers contract, pulling chromatids (now chromosomes) to opposite poles.
• The movement follows a V, U, L, J, or I shape.

Telophase

• Two groups of chromosomes become organized at opposite poles into two nuclei.
• Chromosomes elongate and overlap to form chromatin.
• The nuclear membrane and nucleolus reappear.

Cytokinesis

• The cytoplasm divides to form two daughter cells.
• Cell organelles are distributed equally.
• Methods:
  • Cell plate method: In plant cells.
  • Cleavage or constriction method: In animal cells.

Significance of Mitosis

• Enables growth and development of multicellular organisms.
• Creates genetically identical cells.
• Maintains chromosome number in cells.
• Repairs and regenerates cells (including lost body parts).
• Replaces old/worn-out cells.
• Promotes wound healing.

Meiosis

• This type of cell division creates four daughter cells, each with half the number of chromosomes of the mother cell.
• Also known as reductional cell division.
• Occurs only in reproductive cells for gamete formation.
• Meiosis involves two sets of divisions (Meiosis I and Meiosis II).
• Cells involved are called meiocytes.

Interkinesis

• A phase similar to interphase, occurring between Meiosis I and Meiosis II.
• DNA synthesis does not occur during interkinesis.

Meiosis I

• Divided into four phases: Prophase I, Metaphase I, Anaphase I, and Telophase I.

Prophase I

• This is a complex and lengthy phase, with five subphases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
  • Leptotene: Nucleus size increases; chromosomes shorten and thicken; swollen areas (chromomeres) present.
  • Zygotene: Homologous chromosomes pair (synapsis); pairs are called bivalents; held together by synaptonemal complex.
  • Pachytene: Nucleoprotein dissolves, forming four chromatids (tetrad); exchange of genetic materials (crossing over) between non-sister chromatids.
  • Diplotene: Synaptonemal complex dissolves; non-sister chromatids begin separating but remain connected at chiasmata; nuclear membrane and nucleolus begin disappearing.
  • Diakinesis: Chiasmata move towards the ends of the chromosomes (terminalisation); nuclear membrane and nucleolus completely disappear; spindle fibers start appearing.

Metaphase I

• Spindle fibers are complete and oriented toward opposite poles.
• Bivalents align along the equatorial plane in two planes.
• Metaphasic plates are formed.

Anaphase I

• Homologous chromosomes separate and move toward opposite poles (disjunction).
• Each chromosome contains two chromatids (dyads).
• Chromosomes take characteristic V, U, L, J, or I shape.

Telophase I

• Nuclear membranes and nucleoli reappear around the two groups of chromosomes at each pole.
• Chromosomes elongate.
• Two haploid nuclei are formed from a single diploid nucleus.

Meiosis II

• Similar to mitosis; chromosomes remain the same number.
• Consists of four phases: Prophase II, Metaphase II, Anaphase II, and Telophase II.

Prophase II

• Chromosomes shorten and thicken.
• Nuclear membrane and nucleolus disappear.

Metaphase II

• Dyad chromosomes line up along the equatorial plane in one plane.
• Centromeres attach to spindle fibers.

Anaphase II

• The centromeres split, and sister chromatids separate and move to opposite poles.

Telophase II

• Four haploid nuclei are formed.
• Nuclear membranes and nucleoli reappear.
• Chromosomes elongate.

Cytokinesis (Meiosis)

• Two distinct methods:
  • Successive: Cytokinesis follows each karyokinesis.
  • Simultaneous: Cytokinesis happens only after the completion of meiosis II.

Significance of Meiosis

• Gamete formation.
• Maintains a fixed chromosome number in sexually reproducing organisms.
• Crossing over produces new gene combinations (variation).
• Formation of spores in many organisms.

Comparison of Mitosis and Meiosis