Final answer:
Multicellular organisms develop from a zygote through mitosis and cell differentiation. DNA replication ensures each cell has the same genetic code, while gene expression guides the specialization of cells into tissues and organs.
Step-by-step explanation:
Development of Multicellular Organisms from a Single Cell
All multicellular organisms, including humans, start life as a single cell known as a fertilized egg or zygote. The journey from a single cell to a being composed of trillions of cells involves several critical biological processes.
First, the zygote grows through a process called mitosis, where the cell divides to form two identical daughter cells. This process includes DNA replication, where the DNA within the cell's nucleus is duplicated to ensure each new cell has the complete set of genetic information. Mitosis continues, resulting in an increasing number of cells.
As the number of cells increase, a process known as cell differentiation occurs, whereby cells become specialized in their shape and function. This differentiation is guided by gene expression, which involves certain genes in the DNA being turned 'on' or 'off,' resulting in specific proteins that give the cell its characteristics. Despite the specialization, each cell retains the same DNA in its nucleus, the same as that of the original zygote.
Through countless cycles of mitosis and differentiation, cells form tissues and organs, each with a unique function, eventually resulting in the development of a complex multicellular organism. This intricate dance of cell growth and specialization is regulated through gene expression, ensuring that every cell performs its role while maintaining the original genetic blueprint.