Cell Cycle Overview and Division Types

Cell Cycle Overview

• Cell division is the basis of life's continuity, distinguishing living from non-living matter.
• Cell division enables single-celled organisms to reproduce (e.g., amoeba). Multicellular organisms use it for development and repair (e.g., embryos, wound healing).
• Cell division involves copying DNA, separating DNA copies, and dividing the cytoplasm to create two complete cells.

Cell Division Types

• Asexual Reproduction (Genetically identical cells):
  • Mitosis is the eukaryotic process.
  • Binary fission is the prokaryotic process.
• Sexual Reproduction (Genetically non-identical cells):
  • Meiosis creates gametes (sperm/egg).
  • Prokaryotic variation: transformation (DNA uptake), transduction (virus-mediated DNA transfer), conjugation (DNA transfer via sex pilus).

Cellular Organization Of Genetic Material

• A cell's genome encompasses all its DNA, holding the cell's genetic information.
• Prokaryotic cells contain a single DNA molecule, while eukaryotic cells have multiple.
• DNA is packaged into chromosomes within the cell.
• When a cell isn't dividing, its chromosomes exist as a long, thin chromatin fiber.

Chromosomes and Cell Types

• Every eukaryotic species has a specific number of chromosomes per cell nucleus. Examples include humans (46), Drosophila (8), potatoes (48), and horsetails (>200).
• Somatic cells (non-reproductive) have two sets of chromosomes, and mitosis generates new somatic cells.
• Gametic cells (reproductive; sperm/egg) have half as many chromosomes as somatic cells, and meiosis generates gametes.

Chromosome Distribution During Eukaryotic Cell Division

• DNA replicates before cell division.
• Each replicated chromosome is composed of two sister chromatids.
• Sister chromatids are joined together at the centromere (a constricted region on the chromosome).
• The centromere is the narrow "waist" of the chromosome where sister chromatids are most closely attached.
• In eukaryotic cell division, sister chromatids separate and move to opposite poles during division.

Histone Structure

• Histones are the building blocks of chromosomes, though this is now superseded by research.
• Condensin rings promote DNA interactions to shape chromosomes.
• Histones aid in chromosome compaction and protection.

Mitotic Phase and Interphase

• Mitotic phase: mitosis (nuclear division) and cytokinesis (cytoplasm division).
• Interphase: cell growth and chromosome copying. Interphase is the longest phase of the cell cycle, lasting for about 90% of the total cycle time.
• Interphase is further divided into G1, S, and G2 phases.
  • G1 ("first gap"): cell grows and performs normal functions.
  • S ("synthesis"): DNA replicates.
  • G2 ("second gap"): cell continues to grow, duplicates organelles and prepares for division.

Mitotic Spindle

• The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis.
• It's composed of centrosomes, spindle microtubules, and asters.
• Centrosomes act as microtubule-organizing centers (MTOCs).
• Replication of centrosomes leads to formation of two centrosomes to opposite ends of the cell as spindle microtubules grow.
• Asters (astral microtubules) are short radial arrays extending from each centrosome.
• Centrosome is located at each pole of the cell in mitosis.
• Microtubules from each pole are attached to the chromosomes via the kinetochores.
• Kinetochore microtubules capture sister chromatids and pull them to the opposite poles; non-kinetochore microtubules elongate the cell.

Phases of Mitosis

• Prophase: the nucleolus disappears; chromatin condenses into chromosomes.
• Prometaphase: the nuclear envelope breaks down; microtubules attach to kinetochores.
• Metaphase: chromosomes align at the metaphase plate.
• Anaphase: sister chromatids separate and move to opposite poles.
• Telophase: chromosomes decondense; nuclear envelope reforms; nucleoli reappear.

Cytokinesis

• Animal cells form a cleavage furrow (a shallow groove in the cell surface) to separate into two cells.
• Plant cells form a cell plate to separate into two cells.

Binary Fission

• The asexual reproduction process for prokaryotes.
• Cells elongate, DNA is replicated, cross-walls begin to divide, then the cells are separated.

Cell Cycle Regulation

• The frequency of cell division varies among cell types, controlled by a molecular control system.
• The cell cycle is driven by signals in the cytoplasm.
• The cell cycle contains checkpoints, and some molecular components, like cyclins and cyclin-dependent kinases.
• These checkpoints ensure that the cell cycle proceeds through each phase only if the previous phase has been completed successfully. The G1 checkpoint being the most important.

.

Explore the essential concepts of the cell cycle and the various methods of cell division, including mitosis and meiosis. Understand how these processes contribute to life's continuity and the organization of genetic material. This quiz will enhance your knowledge about both asexual and sexual reproduction in living organisms.