Cell Reproduction and Differentiation

Introduction to Cell Division

Cell division is a fundamental process in multicellular organisms, enabling growth from a single fertilized egg to a complex individual. In humans, this process transforms one fertilized egg into approximately 10 trillion cells. After cell division, cells undergo differentiation, becoming specialized and distinct from their parent cells.

• Cell division allows for organismal growth and tissue repair.

• Differentiation results in cells with unique structures and functions.

• Example: Muscle cells, nerve cells, and blood cells all originate from the same fertilized egg but become specialized through differentiation.

The Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of two major phases: Interphase and the Mitotic phase.

• Interphase: The period between cell divisions, including three subphases:

  • G1 (first gap): Primary growth phase, very active growth.

  • S (synthesis): DNA is replicated in preparation for cell division.

  • G2 (second gap): Final growth phase before division.

• Mitotic phase: Includes Mitosis (nuclear division) and Cytokinesis (cytoplasmic division).

  • Mitosis: Duplicated DNA is distributed between two daughter nuclei.

  • Cytokinesis: Cytoplasm divides, forming two new daughter cells.

• Duration: The complete cell cycle takes 18–24 hours; mitosis and cytokinesis take less than one hour.

• G0 phase: Many cells enter a nondividing state, either temporarily or permanently (e.g., neurons, osteocytes after adolescence).

Diagram: The Cell Cycle

Figure 17.1 illustrates the cell cycle, showing the progression through interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

DNA Structure and Function

Human DNA is organized into 46 chromosomes, containing approximately 3 billion base pairs. Chromosomes are composed of DNA and histone proteins. During most of the cell cycle, DNA exists as chromatin (loose and diffuse). During mitosis, DNA condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere.

• Gene: A short segment of DNA that contains the code for a protein.

• Humans have about 20,000–21,000 genes on their chromosomes.

• Chromatin: The relaxed form of DNA and protein found in the nucleus during interphase.

• Chromosome: The condensed, visible form of DNA during cell division.

Genetic Questions and Mechanisms

• How many genes are in the human genome? Approximately 20,000–21,000.

• How many proteins do humans make? Far more than the number of genes, due to mechanisms like alternate splicing and alternate reading frames.

• Alternate splicing: The process by which different combinations of exons are joined to produce multiple protein variants from a single gene.

• Alternate reading frames: Different ways of reading the nucleotide sequence, resulting in different proteins.

Summary Table: Key Terms and Concepts

Additional info:

• Alternate splicing and reading frames allow for protein diversity from a limited number of genes.

• Cell cycle regulation is crucial for normal growth and development; errors can lead to diseases such as cancer.