Grade 7 Reading — Science

Climate Change and Living Systems

Earth's climate has changed throughout its long history, but scientists today observe a rapid warming trend linked to human activities. When people burn coal, oil, and natural gas, they release carbon dioxide and other greenhouse gases into the atmosphere. These gases trap heat, much like a blanket wrapped around the planet. Since the late nineteenth century, average global temperatures have risen more than one degree Celsius, and the pace has accelerated in recent decades. Living systems respond to shifting conditions in complex ways. Many species migrate toward cooler regions or higher elevations as habitats warm. Coral reefs bleach when ocean temperatures stay high for weeks, weakening the partnerships between coral animals and the algae that feed them. Arctic sea ice melts earlier each spring, reducing hunting platforms for polar bears and changing travel routes for Indigenous commu…

DNA: The Instruction Manual Inside Cells

Every living organism—from a towering redwood to a single bacterium—depends on a molecule called DNA to store biological instructions. DNA stands for deoxyribonucleic acid. Its structure resembles a twisted ladder, a shape James Watson and Francis Crick described in 1953 with critical contributions from Rosalind Franklin's X-ray images. The rungs of the ladder are pairs of chemical bases, and their order spells out genes. Genes are segments of DNA that code for proteins. Proteins build structures, carry signals, and speed up chemical reactions inside cells. When a body makes a new cell, it must copy its DNA accurately. Enzymes unzip the double helix and assemble matching strands. Occasionally a copying error, called a mutation, changes a base pair. Some mutations have little effect; others alter traits or increase disease risk. Inheritance explains why children resemble their parents y…