Biology Chapter on Somatic and Gametic Cells

Somatic Cells

• Somatic cells are body cells produced by mitosis.
• Somatic cells are diploid, containing 46 chromosomes (23 pairs).

Haploid Cells

• Haploid cells contain half the DNA count (n).
• They are produced by meiosis.

Diploid Cells

• Diploid cells contain the full DNA count (2n).
• They are found in somatic cells.

Gametes

• Haploid gametes are sex cells (sperm and egg) produced by meiosis to prevent doubling of chromosome numbers in offspring.

Meiosis

• Meiosis is a special cell division that produces gametes.
• It occurs in reproductive organs: ovaries and testes.
• Meiosis involves two cell divisions, producing 4 cells from one parent cell.
• Each daughter cell has 23 individual chromosomes, halving the chromosome number from diploid to haploid.
• Chromosome duplication precedes meiosis, but the cell divides twice to produce four daughter cells.

Genetic Variation in Meiosis

• Meiosis generates genetic variation through two mechanisms:
  • Crossing Over: Homologous chromosomes exchange DNA segments during prophase I.
  • Independent Assortment: Pairs of chromosomes assort independently of each other in metaphase I.
• The number of genetically distinct gametes produced from a diploid cell is 2ⁿ, where n is the number of chromosome pairs.

Errors in Meiosis

• Chromosome Number Errors:
  • Trisomy: An extra chromosome (2n+1).
  • Monosomy: A missing chromosome (2n-1).
  • Nondisjunction: Homologous chromosomes fail to separate during anaphase I or sister chromatids fail to separate during anaphase II.
• Chromosome Structure Errors:
  • Deletion: Portion of a chromosome is deleted.
  • Inversion: A section of a chromosome is flipped.
  • Duplication: A section of a chromosome is duplicated.
  • Translocation: A section of one chromosome attaches to a different chromosome.

Errors in Chromosome Number and Syndromes

• Trisomy:
  • Down Syndrome: Trisomy 21
  • Patau Syndrome: Trisomy 13
  • Edward's Syndrome: Trisomy 18
• Sex Chromosome Nondisjunction:
  • Klinefelter Syndrome: 47, XXY
  • Jacobs Syndrome: 47 XYY
  • Trisomy X: 47XXX
  • Turner's Syndrome: Monosomy X

Gametogenesis

• Gametogenesis is the formation of haploid gametes during meiosis.
• It combines with fertilization to complete the life cycle.

Spermatogenesis

• Occurs in the testes.
• Begins with a diploid spermatocyte that undergoes mitotic divisions before meiosis.
• Produces 1 billion sperm cells daily.
• Cytoplasmic division is equal after meiosis I, resulting in 4 sperm cells.
• Sperm lose cytoplasm and become streamlined, forming a head and tail for locomotion.
• This process ensures a high number of viable sperm, increasing the chance of fertilization.

Oogenesis

• Occurs in the ovaries.
• Begins with a diploid oocyte that becomes dormant in prophase I until puberty.
• One ootid (egg cell) is produced each month.
• Cytoplasmic division is unequal after meiosis I, with one daughter cell receiving most of the cytoplasm.
• The other daughter cell becomes a polar body, which dies and is absorbed by the body.
• This unequal division ensures the ovum (egg cell) has enough nutrients to support the developing zygote.

Explore the differences between somatic cells, haploid and diploid cells, and the process of meiosis. This quiz delves into how genetic variation occurs during meiosis and the importance of gametes in reproduction. Test your understanding of these foundational concepts in cell biology.