Cell Division and Reproduction

Introduction to Cell Division

Cell division is a fundamental biological process that enables organisms to grow, repair tissues, and reproduce. All cells arise from pre-existing cells through division, ensuring the continuity of life.

• Cell Division: The process by which a parent cell divides to form two or more daughter cells.

• Asexual Reproduction: Involves a single parent and produces offspring that are genetically identical to each other and to the parent.

• Sexual Reproduction: Involves the combination of genetic material from two parents, resulting in genetically unique offspring.

• Example: Bacteria reproduce asexually by binary fission, while animals and plants reproduce sexually.

Cell Division in Prokaryotes

Binary Fission

Prokaryotic cells, such as bacteria, reproduce asexually through a process called binary fission. This process is simpler than cell division in eukaryotes due to the presence of a single, circular DNA molecule.

• Binary Fission: Means "dividing in half"; a method of asexual reproduction in prokaryotes.

• Most prokaryotes have a single, circular chromosome composed of DNA and associated proteins.

• Steps of Binary Fission:

  1. The chromosome is replicated.

  2. The two copies move apart as the cell elongates.

  3. The plasma membrane pinches inward, and a new cell wall forms, dividing the cell into two genetically identical daughter cells.

• Example: Escherichia coli (E. coli) bacteria divide by binary fission approximately every 20 minutes under optimal conditions.

The Cell Cycle in Eukaryotes

Phases of the Eukaryotic Cell Cycle

The eukaryotic cell cycle is a series of ordered events that lead to cell growth and division. It consists of interphase and the mitotic (M) phase.

• Interphase: The period of cell growth and DNA replication, comprising three subphases:

  • G1 (First Gap): Cell grows and carries out normal functions.

  • S (Synthesis): DNA is replicated.

  • G2 (Second Gap): Cell prepares for division.

• Mitotic (M) Phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

• Example: Human skin cells undergo the cell cycle to replace dead or damaged cells.

Eukaryotic Chromosomes and Chromatin

Structure and Organization

Eukaryotic cells contain multiple linear chromosomes located in the nucleus. Each chromosome consists of a long DNA molecule and associated proteins, forming chromatin.

• Chromatin: The complex of DNA and proteins that makes up chromosomes; appears as thin fibers when the cell is not dividing.

• Chromosomes: Become visible under a light microscope only during cell division, when chromatin condenses.

• Sister Chromatids: Identical copies of a chromosome, joined together at a region called the centromere after DNA replication.

• During cell division, sister chromatids are separated and distributed to daughter cells, ensuring each receives a complete set of chromosomes.

• Example: Human somatic cells have 46 chromosomes (23 pairs).

Distribution of Chromatids During Mitosis

The Role of the Mitotic Spindle

Mitosis ensures the equal distribution of duplicated chromosomes into two daughter nuclei. The mitotic spindle, composed of microtubules, is essential for this process.

• Mitotic Spindle: A structure made of microtubules that organizes and separates chromosomes during mitosis.

• Chromosomes align at the center of the cell, and spindle fibers attach to the centromeres.

• Sister chromatids are pulled apart to opposite poles of the cell, resulting in two genetically identical nuclei.

• Example: In animal cells, the spindle apparatus forms from centrosomes located at opposite poles of the cell.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cell Division

Key Terms and Definitions

• Cell Cycle: The ordered sequence of events in the life of a cell, from its origin to its own division into two daughter cells.

• Chromosome: A threadlike structure of nucleic acids and proteins carrying genetic information.

• Chromatin: The material of which chromosomes are composed; consists of protein, RNA, and DNA.

• Sister Chromatids: Two identical copies of a single replicated chromosome, connected by a centromere.

• Mitotic Spindle: The structure that separates sister chromatids during mitosis.

Additional info: Later sections of the original notes (not shown in these images) likely cover the details of mitosis, meiosis, and their roles in genetic variation and human health.