Chromosomes, Genes, and Cell Division: Mitosis vs. Meiosis

Fundamentals of Genetics and Chromosomes

In humans, there are 46 chromosomes, occurring in 23 pairs. The diploid number refers to the total number of chromosomes in each body cell of a plant or animal.

Key Genetic Terms

• Gene: A length of DNA that is the code for making a specific protein molecule. Genes are always located in the same position on a chromosome.
• Allele: Different versions of the same gene.
  • A dominant allele is expressed and affects the appearance (phenotype) of the individual.
  • A recessive allele is only expressed if two copies are present or if the dominant allele is absent.
• Genotype: The specific combination of alleles an organism possesses for a trait.
  • Homozygous: Having two identical alleles for a trait (e.g., TT or tt).
  • Heterozygous: Having two different alleles for a trait (e.g., Tt).
• Phenotype: The observable characteristics or traits of an individual, resulting from how the alleles are expressed.

During sperm production, meiosis ensures that half of the sperm cells receive a Y chromosome and half receive an X chromosome. All egg cells contain an X chromosome.

Mitosis: Somatic Cell Division

Mitosis is a type of cell division that occurs in somatic cells (body cells). Its purpose includes the growth of tissues like muscle or bone in animals, and root, leaf, stem, or fruit development in plants.

Steps in Mitosis

1. Just before the cell divides, chromosomes become visible in the nucleus.
2. The chromosomes condense and become thicker.
3. Each chromosome is now seen to consist of two identical sister chromatids joined together.
4. The nuclear membrane disappears. Spindle fibers attach to the chromatids and pull them apart towards opposite ends (poles) of the cell.
5. A nuclear membrane reforms around each set of separated chromatids (now considered individual chromosomes). The cell begins to divide.
6. Cell division is completed, resulting in two daughter cells, each containing the same diploid number of chromosomes as the parent cell.

Meiosis: Gamete Formation

Meiosis is a type of cell division involved in the production of gametes (sperm and egg cells).

Steps in Meiosis

1. Chromosomes appear in the nucleus (e.g., red inherited from the organism's mother and blue from the father).
2. Homologous chromosomes (pairs of corresponding chromosomes) lie alongside each other (synapsis).
3. The nuclear membrane disappears. Homologous chromosomes separate and move to opposite ends of the cell.
4. By now, each chromosome consists of two sister chromatids.
5. A second division takes place to separate the sister chromatids.
6. Four gametes (daughter cells) are formed. Each contains only half the original number of chromosomes (haploid number).

Comparing Mitosis and Meiosis

• Mitosis:
  • Occurs during the division of somatic cells.
  • A full set of chromosomes (diploid number) is passed on to each daughter cell.
  • Produces two genetically identical daughter cells.
  • If new organisms are produced by mitosis (asexual reproduction), they will all resemble each other and their parent(s). They are said to form a 'clone'.
• Meiosis:
  • Occurs in the final stages of cell division leading to the production of gametes.
  • Only half the number of chromosomes (haploid number) is passed on to each daughter cell.
  • Homologous chromosomes and their genes are randomly assorted between the gametes, leading to genetic variation.
  • Produces four genetically different daughter cells.
  • New organisms produced via meiosis (sexual reproduction) will show variations from each other and from their parents.