Cell Cycle and Cell Division

Overview of Cell Cycle and Cell Division

• Cell growth and division are coordinated processes that ensure genomic integrity.
• These processes occur in a specific sequence and are genetically controlled by DNA.

Cell Growth and DNA Synthesis

• Cell growth involves an increase in cytoplasm, which is a continuous process.
• DNA synthesis occurs only during the S phase of the cell cycle.

Duration of Cell Cycle

• A typical human cell cycle takes 24 hours.
• In yeast, the cell cycle is completed in approximately 90 minutes.

Phases of the Cell Cycle

• The cell cycle consists of two main phases: Interphase and M phase (Mitosis).
• Interphase is the preparation phase, accounting for 95% of the cell cycle duration.
  • In humans, interphase lasts about 23 hours, while the actual division (M phase) takes only 1 hour.
  • In yeast, interphase takes 85 minutes, and the division phase lasts 5 minutes.

Sub-Phases of Interphase

• Interphase is divided into three phases: G1, S, and G2.
  • G1 phase: The cell is metabolically active and grows in size but does not replicate DNA.
  • S phase: DNA replication occurs where the amount of DNA per cell doubles; centrioles also duplicate during this phase.
  • G2 phase: The cell prepares for division, synthesizes tubulin protein, and cellular organelles like mitochondria and Golgi bodies duplicate.

M Phase: Cell Division

• M phase involves two processes: karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).

G1 Phase in Detail

• Cells are metabolically active during G1 phase, which is the longest phase of the cell cycle.
• The cell grows, synthesizes proteins and RNAs, and duplicates most organelles.
• Semi-autonomous organelles such as mitochondria, chloroplasts, and centrioles do not duplicate during G1.

S Phase in Detail

• Replication occurs during the S phase, doubling the amount of DNA per cell.
• Before this, a cell has 46 single-stranded chromosomes. After, it has double-stranded DNA, but chromosomes number remains at 46.
• The amount of DNA content doubles, indicated as 2C before S phase and 4C after S phase.
• Centrioles duplicate, and histone proteins are synthesized during the S phase.

G2 Phase in Detail

• The G2 phase involves the synthesis of tubulin proteins and duplication of remaining organelles such as mitochondria, Golgi apparatus, and chloroplasts.

G0 Phase: Quiescent Stage

• Some cells exit the cell cycle from the G1 phase and enter an inactive stage called the quiescent stage (G0 phase).
  • Heart cells are an example of cells that enter the G0 phase and do not divide.
• G0 phase isn't exiting the cell cycle, rather suspending it.
• Cells in the G0 phase are metabolically active but do not divide unless called back.
  • Parenchyma cells in plants, derived from meristematic cells, can enter the G0 phase but can be called back for division.

M Phase: Mitosis and its Stages

• M phase includes karyokinesis (nuclear division), further divided into prophase, metaphase, anaphase, and telophase (PMAT).
  • Prophase: Chromosomes condense, and the centrosomes move to opposite poles.
  • Compact chromosomes with two chromatids become attached to the spindle fibers.
  • The Golgi complex, ER, and nucleolus disintegrate.
  • Metaphase: Condensation of chromosomes completes, and morphology can be studied.
  • Chromosomes align at the equatorial plate, with spindle fibers attached to the kinetochore of the centromere.
  • There is only one metaphase plate, and the number of spindle fibers is twice the number of chromosomes (92 in humans).
• Spindle fibers attach to kinetochores, and chromosomes align at the equatorial plate.

Anaphase

• Centromere splitting occurs in anaphase.
• The chromosomes split, with 46 single-stranded chromosomes moving to opposite poles.
• Centromeres lead, and the chromosome tails trail behind during movement.

Telophase

• Telophase reverses prophase; nuclear membranes reform around chromosomes at each pole.
• Nucleolus, ER, and Golgi complex reappear.
• The chromosomes decondense, losing their individual identity.

Cytokinesis

• Cytokinesis happens after karyokinesis and involves the physical division of the cytoplasm, resulting in two daughter cells.
• In animal cells, cytokinesis occurs through cell furrow formation, from outside to inside (centripetal).
• In plant cells, cytokinesis involves cell plate formation, from the center outwards (centrifugal).

Significance of Mitosis

• Mitosis is essential for repair and growth.
• In plants, it facilitates the growth of meristematic cells and stem cells.
• Enables the formation of multicellular organisms with millions of cells from a single cell.

Overview of Meiosis: Reduction Division

• Meiosis is a reductional division that results in gamete formation and gametogenesis.
• In meiosis, one cell divides into four cells.
• Different from mitosis, meiosis involves halving the chromosome number.

Meiosis vs. Mitosis

• Mitosis is an equational division because the chromosome number remains the same from parent to daughter cells and occurs in diploid animal cells and haploid cells in algae/bryophytes.
• Meiosis leads to the formation of gamete.

Stages of Meiosis

• Meiosis includes interphase followed by Meiosis I and Meiosis II, separated by interkinesis.
• Interkinesis is a phase where RNA and proteins are synthesized, but DNA replication does not occur.

Meiosis I

• The prophase I of meiosis 1 is longer, and has 5 phases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis
  • Leptotene: Chromosomes start to condense.
  • Zygotene: Pairing of homologous chromosomes (synapsis) initiates. The synapsed structure is the bivalent.
  • Pachytene: The structure is now a tetrad. Crossing over event that involves the exchange of genetic material between non-sister chromatids of the homologous chromosomes. Key enzyme is recombinase.
  • Diplotene: The synaptonemal complex dissolves and the homologous chromosomes separate from each other except a the sites of chiasmata. In oocytes from same vertebrates some can enter the diplotene stage and suspend itself. This can last for years.
  • Diakinesis: Terminalization of chiasmata. The nucleolus disappears and the nuclear envelope disintegrates.

Meiosis II

Metaphase I

• Homologous chromosomes align on the metaphase plate.

Anaphase I

• Homologous chromosomes separate (reduction in chromosome number).
• Results in 23 chromosomes moving to each pole, still with two strands each.

Telophase I

• Nuclear envelope reforms around each set of chromosomes.
• Results in cells with 23 chromosomes each, with each chromosome having two strands.

Interkinesis

• A brief stage between meiosis I and meiosis II, with no DNA replication.

Meiosis II Details

• Very similar to mitosis, it is an equational division.

Anaphase II

• The splitting of the centromere
• 23 single stranded chromosomes move to the top, and 23 single stranded chromosomes move to the bottom

Overall Outcome of Meiosis

• The chromosome number is halved.
• 46 (2C) parent structures are divided giving four cells. Each cell has 23 chromosomes.
• DNA content becomes ("C").

Significance of Meiosis

• Creates genetic variation
• Maintains chromosome number between generations.