Chapter 7 Notes: Chromosomes and Cell Division

Chromosomes

  • Definition: Chromosomes are structures made of DNA and proteins that help sequester DNA within the nucleus of the cell, ensuring it remains organized.

  • Human Chromosome Count: Humans typically possess 46 chromosomes, organized into 23 pairs. These pairs are called homologous chromosomes, where one set is inherited from each parent.

  • Types of Cells:

    • Diploid cells: Most somatic cells in humans with two sets of chromosomes (23 pairs).
    • Haploid cells: Gametes (sperm and egg cells) that contain only one set of chromosomes (23 total).

  • Variation Among Species: The number of chromosomes can vary widely across species and does not correlate directly with the organism's complexity. For example, humans have fewer chromosomes than rice plants.

Cell Division

  • Purpose: Cell division is essential for growth, tissue repair, and reproduction. It is fundamental to the cell theory that new cells arise only from pre-existing cells.

  • Types of Cell Division:

    • Binary Fission: Happens in prokaryotic cells (bacteria). DNA replication occurs, followed by the elongation of the cell and division into two identical daughter cells. This process is simpler due to the lack of a nucleus and the circular form of DNA.
    • Mitosis: Occurs in eukaryotic cells. Involves multiple phases where chromosomes are duplicated and divided to produce two identical daughter cells as well, but the process is more complex due to multiple chromosomes and the presence of the nucleus.
Chromosome Structure during Cell Division
  • Significance of Proper Number and Structure: Each organism requires a specific number of chromosomes to maintain normal development. Deviations can lead to genetic disorders or developmental issues.
  • Sex Chromosomes: One pair of chromosomes determines biological sex; XX is female and XY is male. The Y chromosome contains genes for male development.
Chromosome Abnormalities

  • Importance of Stability: Stability of chromosome number and structure during replication is critical for normal function. Abnormalities can occur:

    • Deletions: Removal of a chromosome segment, leading to loss of genes.
    • Duplications: Extra copies of chromosome segments, leading to additional genes that can disrupt normal function.
    • Inversions: Flipping segments of a chromosome, altering the reading frame of genes.
    • Insertions: Addition of genetic material from another chromosome.
    • Translocations: Exchange of segments between non-homologous chromosomes.

  • Example of Abnormality: Down syndrome, or trisomy 21, occurs when an individual has an extra copy of chromosome 21, leading to distinct developmental characteristics.