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A hush falls over the courtroom. A lawyer points dramatically at a man sitting 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, or cytology. Even many people who are not sure what a cell is have heard of DNA evidence and DNA testing. Police today routinely catch criminals by using DNA tests. Your DNA is as individual as your fingerprint pattern. Just a bit of hair, saliva, or blood can provide enough DNA to solve an otherwise unsolvable mystery.
DNA is found in the cell’s nucleus. DNA is 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 eventually curls up into a small ball, just the way DNA becomes “supercoiled.”
HISTONE
Histones are protein molecules. They act as spools around which DNA winds, and they play a role in gene regulation.
▲ The nucleus of each human cell contains approximately 25,000 genes. Genes, the most basic unit of heredity, carry the traits we inherit from our parents. 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 that 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 believe that eukaryotes form from prokaryotes that fuse together to create new, more complex life forms. A prokaryote might have one small strand of DNA, but a eukaryote can have up to 1,000 times that. Today, prokaryotes are mainly one-celled bacteria, while most other types of life are eukaryotes.
DOUBLE-STRANDED DNA
If you stretched out one DNA strand, it would look like a spiral ladder, a shape scientists call the “double helix.” Running down the center of the ladder are steps 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 which base pairs follow which 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 the first cheetahs born by means of embryo transfer, at the Columbus Zoo, in Ohio. 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 so it can grow and develop until it is ready to be born.
This process is artificial, of course, because it involves humans. Nevertheless, it marks an important step toward keeping cheetahs from going extinct. Because of habitat destruction, hunting, illegal pet trade, and other unfortunate interactions with humans, the cheetah population is in danger. Cheetahs have become extinct in more than 13 countries. Only about 7,500 remain in the wild. As a result, the success of embryo transfer can help maintain the cheetah population and its important role in the biodiversity of our planet. (Donated by Grahm Jones/Columbus Zoo)
