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A hush falls over the courtroom. A lawyer points dramatically at a man behind a table and thunders, “DNA evidence proves that this man is the killer.”
Moments like that—both on TV and in real courtrooms—have made DNA the rock star of cell science. Another word for cell science is cytology. Even many people who are not sure what a cell is have heard of DNA evidence and DNA testing. Police today often catch criminals by using DNA tests. DNA is as individual as a fingerprint. Just a bit of hair, saliva, or blood contains enough DNA to identify a criminal and solve a crime.
DNA is found in the cell’s nucleus. Why is DNA a big deal? Because it carries the instructions the body needs in order to function. ▶
CHROMOSOME
Chromosomes are made up of tightly wound strands of DNA. They are found in the cell’s nucleus.
SUPERCOIL
A lot of DNA has to fit into a small package, so it is tightly coiled. If you keep twisting a rubber band, it will curl up into a small ball. DNA becomes “supercoiled” in a similar way.
HISTONE
Histones are protein molecules. They act as spools around which DNA winds. They help regulate genes.
▲ The nucleus of each human cell has around 25,000 genes. Genes are the most basic unit of heredity. They carry the traits we inherit from mom and dad. One gene is just a section of DNA on a chromosome that tells the cells to make a particular protein. Proteins determine such things as hair texture and foot size—and whether we have hair or feet at all. They also ensure that skin grows and blood circulates. One gene might produce a single trait, like being able to roll your tongue. But genes usually work together.
Check It Out!
What type of cell in your body has no nucleus?
The red blood cells in most mammals.
◀ Not every cell has a nucleus. Fossils show that Earth’s first life forms were prokaryotes (pro-KAHR-ee-oats)—cells with no nucleus. They appeared about 4 billion years ago. Cells with a nucleus—called eukaryotes (you-KAHR-ee-oats)—appeared about 1.6 billion years ago. Scientists now believe that eukaryotes form from prokaryotes joining together to make new, more complex life forms. A prokaryote might have one small strand of DNA. Eukaryotes can have up to 1,000 times that. Today, prokaryotes are mainly one-celled bacteria. Most other types of life are eukaryotes.
DOUBLE-STRANDED DNA
If you stretched out one DNA strand, it would look like a spiral ladder. Scientists call this shape the “double helix.” Running down the center of the ladder are rungs that hold it together. These rungs are made of pairs of molecules called bases. ▶
BASE (LETTERS) OF THE GENETIC CODE
The code of life—like whether you have blue eyes or brown—depends on the order of the base pairs along the ladder. There are four different possible base pairs, and any number of different patterns can be made with them. These patterns form the codes for an individual, and we call them genes. ▶
▲ February 19, 2020. Whether you’re a biologist, a conservationist, a naturalist, or a lover of cute baby animals, this is an important date. Why? Because it’s the birthday of two special cheetahs born at the Columbus Zoo, in Ohio. The cubs are special because they are the first ones born by means of embryo transfer. An embryo, which has DNA from its parents, is an unborn offspring at an early stage of development. Embryo transfer takes place when an embryo is placed in the mother’s body. The embryo develops there until it is ready to be born.
This process is artificial, of course, because it involves humans. Even so, it is an important step in keeping cheetahs from going extinct. Right now, cheetahs are in danger. Their habitats are being destroyed, they are being hunted, and they are being captured and sold as pets illegally. As a result, too many are dying. Cheetahs have already become extinct in more than 13 countries. Only about 7,500 remain in the wild. If embryo transfer is a success, it can help maintain the cheetah population. And that's good. Like all animals, cheetahs have an important role in the biodiversity of our planet.
