Explain how cells with the same DNA become specialized through differential gene expression, and describe the role of stem cells in development and repair (NYSSLS LS1, structure and function; cause and effect).

What this topic is asking

NYSSLS LS1 asks one of biology's most striking questions: how can cells with identical DNA become so different? The Life Science: Biology Regents wants you to explain differentiation through gene expression, and to describe stem cells. The crosscutting concepts are cause and effect and structure and function.

The puzzle: same DNA, different cells

A nerve cell, a muscle cell and a skin cell in the same person all contain the same DNA, because they all descended by mitosis from one zygote. Yet they look and behave completely differently. The resolution is gene expression: although every cell has the whole genome, each cell type uses only part of it, switching on the genes it needs and leaving the rest off. Because the genes a cell expresses determine the proteins it makes (see protein synthesis and gene expression), different patterns of expression produce different cells.

Differentiation

Specialized cells are worked examples of structure fitting function: a nerve cell has long extensions to carry signals over distances; a red blood cell is packed with haemoglobin and has no nucleus, maximizing oxygen carriage; a root hair cell has a large surface area for absorbing water. In each case, the structure follows from the proteins the cell makes, which follow from the genes it expresses.

Stem cells

Stem cells matter for two reasons the exam tests: they build the body during development and they maintain and repair it throughout life. Their ability to become many cell types also makes them important in medical research and treatment.

Why specialization helps

Specialization allows a division of labor: instead of every cell doing everything, different cells carry out different tasks efficiently, and the organism coordinates them (linking to levels of biological organization). This is why a complex multicellular organism can do more than a single cell could, but it also means cells depend on one another and on systems that supply and coordinate them.

Try this

Q1. Explain how two cells with identical DNA can become different cell types. [2]

• Cue. They express different genes, so they make different proteins, even though their DNA is the same.

Q2. State what a stem cell is and one way it is useful. [2]

• Cue. An unspecialized cell that can divide and differentiate into different cell types; useful in building the body during development or in repairing and replacing damaged cells.