Chromosome Transmission During Cell Division and Sexual Reproduction

Types of Animal Cells

Animal cells can be classified based on their role in reproduction and chromosome content.

• Somatic cells: These are body cells other than gametes (e.g., blood cells). They are typically diploid, containing two sets of chromosomes.

• Germ cells: These are gametes (sperm and egg cells). They are haploid, containing a single set of chromosomes.

In humans, somatic cells have a total of 46 chromosomes (23 pairs), while gametes have 23 chromosomes.

Eukaryotic Chromosomes Are Inherited in Sets

Chromosomes in eukaryotic species are organized in sets. Each member of a pair is called a homolog. Homologous chromosomes:

• Are nearly identical in size

• Have the same banding pattern

• Share the same centromere location

• Contain the same genes, but not necessarily the same alleles

Homologous chromosomes form a homologous pair during cell division.

Homologous Chromosomes Have the Same Loci

The locus (plural: loci) is the physical location of a gene on a chromosome. Homologous chromosomes have the same loci for genes, but may carry different alleles.

• Genotype types:

  • Homozygous for the dominant allele

  • Heterozygous

  • Homozygous for the recessive allele

Cell Division

Cell division is essential for both asexual reproduction and multicellularity. In multicellular organisms, all cells are derived from a single cell that has undergone repeated cell divisions. For example, humans start as a single fertilized egg and develop into adults with trillions of cells.

The Eukaryotic Cell Cycle

Eukaryotic cells destined to divide progress through a series of stages known as the cell cycle:

• G1 phase: Cell grows and prepares for DNA replication.

• S phase: DNA is replicated, resulting in sister chromatids joined at the centromere.

• G2 phase: Cell prepares for mitosis.

• M phase (Mitosis): Division of the nucleus and cytoplasm (cytokinesis) occurs.

Key Point: The cell cycle ensures that genetic material is accurately duplicated and distributed to daughter cells.

Diagram: The Eukaryotic Cell Cycle

Additional info: The diagram typically shows the cyclical progression through G1, S, G2, and M phases, with checkpoints ensuring proper completion of each stage before proceeding.

Summary Table: Diploid vs. Haploid Cells