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A hush falls over the court. A lawyer points at a man behind a table and says, “DNA evidence proves that this man is the killer.”
Moments like that—on TV and in real courtrooms—have made DNA the rock star of cell science. Another word for cell science is cytology. Even people who don’t know what a cell is have heard of DNA evidence and DNA testing. Police today often catch criminals by testing for DNA. It is as individual as a fingerprint. Just a bit of hair, saliva, or blood contains enough DNA to identify a criminal. That’s how it helps solve crimes.
DNA is found in the cell’s nucleus. Why is DNA a big deal? Because it carries the instructions the body needs 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 space, 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. DNA winds around them like they’re spools. 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 make sure that skin grows and blood circulates. One gene might produce a single trait. For example, it might make you 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). Those are cells with no nucleus. They appeared about 4 billion years ago. Cells with a nucleus are called eukaryotes (you-KAHR-ee-oats). They first appeared about 1.6 billion years ago. Scientists believe that eukaryotes form from prokaryotes that join 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 that shape the “double helix.” Running down the center of the ladder are rungs that hold it together. These are made of pairs of molecules called bases. ▶
BASE (LETTERS) OF THE GENETIC CODE
The code of life determines things like whether you have blue eyes or brown. It depends on which base pairs follow which along the ladder. There are four different possible base pairs. Any number of different patterns can be made with them. These patterns form the codes for an individual. 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. The cubs are special because they are the first ones born by means of embryo transfer. An embryo is an unborn offspring. It has its parents’ DNA. 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. They are 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.
