Peering Into the Microscope
THE cell has been called the fundamental unit of life. Indeed, living things—including plants, insects, animals, and humans—are made up of cells. Over the years, scientists have peered into the inner workings of the cell and have unlocked many of the secrets of molecular biology and genetics. Let us take a closer look at cells and consider what science has discovered about these fascinating microscopic units of life.
A Peek at the Microscopic
Cells vary in shape. Some are rectangular; others are square. There are round cells, egg-shaped cells, and some that simply look like blobs. Consider the amoeba, a one-celled organism that has no defined shape at all. Instead, it changes its form as it moves. Interestingly, the function of a cell is often suggested by its shape. For example, some muscle cells are long and thin and contract as they perform their work. Nerve cells—which relay messages throughout the body—have long branches.
Cells also differ in size. Most, though, are too small to be seen by the naked eye. To illustrate the size of an average cell, look at the period at the end of this sentence. About 500 average-size cells could fit within that little dot! If that seems tiny, consider that some bacterial cells are about 50 times smaller. The largest cell? That designation belongs to the yolk of an ostrich egg—a one-celled “giant,” which is about the size of a baseball or a cricket ball!
Since most cells cannot be seen with the naked eye, scientists employ instruments, such as the microscope, to study them.* Even then, some intricate details of a cell cannot be fully discerned. Consider this: An electron microscope can magnify a cell some 200,000 times—an enlargement that would make an ant appear more than half a mile [0.8 km] long. Yet, even at this magnification, some of the cell’s detail is missed!
Scientists have thus found the cell to be amazingly intricate. In his book The Fifth Miracle, physicist Paul Davies states: “Each cell is packed with tiny structures that might have come straight from an engineer’s manual. Minuscule tweezers, scissors, pumps, motors, levers, valves, pipes, chains, and even vehicles abound. But of course the cell is more than just a bag of gadgets. The various components fit together to form a smoothly functioning whole, like an elaborate factory production line.”
DNA—The Molecule of Heredity
Humans as well as multicelled plants and animals start as a single cell. After that cell reaches a certain size, it divides and forms two cells. Then these two cells divide and form four cells. As the cells continue to divide, they specialize—that is, they differentiate, becoming muscle cells, nerve cells, skin cells, and so forth. As the process continues, many of the cells group together to form tissues. Muscle cells, for example, join forces and form muscle tissue. Different types of tissues form organs, such as the heart, the lungs, and the eyes.
Underneath the thin covering of each cell lies a jellylike fluid called cytoplasm. Beyond that is the nucleus, which is separated from the cytoplasm by a thin membrane. The nucleus has been called the cell’s control center because it directs nearly all the cell’s activities. Inside the nucleus lies the cell’s genetic program, written in deoxyribonucleic acid—DNA, for short.
DNA molecules lie tightly coiled in the chromosomes of the cell. Your genes, which are sections of the DNA molecules, contain all the information necessary to make you what you are. “The genetic program carried in DNA makes every living thing different from all other living things,” explains The World Book Encyclopedia. “This program makes a dog different from a fish, a zebra different from a rose, and a willow different from a wasp. It makes you different from every other person on the earth.”
The amount of information contained within the DNA of just one of your cells is staggering. It could occupy about a million pages this size! Since DNA is responsible for passing on hereditary information from one generation of cells to the next, it has been called the master plan of all life. But what does DNA look like?
DNA is made up of two strands wound around each other and takes on a shape like that of a spiral staircase or a twisted ladder with rungs. The two strands are connected by combinations of four compounds called bases. Each base of one strand is paired with a base on the other strand. These base pairs form the rungs of the twisting DNA ladder. The exact order of the bases in the DNA molecule is what determines the genetic information it carries. Simply put, this sequence determines virtually everything about you, from the color of your hair to the shape of your nose.
DNA, RNA, and Protein
Proteins are the most abundant macromolecules found in cells. It has been estimated that they account for more than half the dry weight of most organisms! Proteins are made up of smaller building blocks called amino acids. Some of these are made by your body; others must be obtained from your diet.
Proteins have many functions. For example, there is hemoglobin, a protein found in red blood cells, which transports oxygen throughout your body. Then there are antibodies, which help your body to ward off disease. Other proteins, such as insulin, help you to metabolize foods as well as regulate various cellular functions. In all, there may be thousands of different kinds of proteins in your body. There may be hundreds within just a single cell!
Each protein carries out a specific function that is determined by its DNA gene. But how is the genetic information in a DNA gene decoded so that a particular protein is made? As shown in the accompanying diagram “How Proteins Are Made,” the genetic information stored in the DNA must first be transferred from the nucleus of the cell into the cytoplasm, where the ribosomes, or protein-producing factories, are located. This transfer is accomplished by means of an intermediary called ribonucleic acid (RNA). The ribosomes in the cytoplasm “read” the RNA instructions and assemble the proper sequence of amino acids to form a particular protein. Thus, there exists an interdependent relationship between DNA, RNA, and the formation of proteins.
Where Did It Begin?
The study of genetics and molecular biology has intrigued scientists for decades. Physicist Paul Davies is skeptical that a Creator could be behind it all. Still, he acknowledges: “Each molecule has a specified function and a designated place in the overall scheme so that the correct objects are manufactured. There is much commuting going on. Molecules have to travel across the cell to meet others at the right place and the right time in order to carry out their jobs properly. This all happens without a boss to order the molecules around and steer them to their appropriate locations. No overseer supervises their activities. Molecules simply do what molecules have to do: bang around blindly, knock into each other, rebound, embrace. . . . Somehow, collectively, these unthinking atoms get it together and perform the dance of life with exquisite precision.”
With good reason, many who have studied the inner workings of the cell have concluded that there must be an intelligent force responsible for its creation. Let us consider why.
To study the chemical content and characteristics of cells, scientists also use a centrifuge, an instrument that separates their components.
[Box/Diagram on page 5]
A Look Inside the Cell
Inside each cell is a nucleus—the cell’s command center. Contained within the nucleus are chromosomes, which consist of tightly coiled DNA molecules and protein. Our genes are located on these DNA molecules. Ribosomes, the protein-producing factories, are located in the cell’s cytoplasm, which is outside the nucleus.
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DNA—the ladder of life
[Diagram on page 7]
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How DNA Replicates
For the sake of visual simplicity, the twisted DNA helix has been flattened
1 Before cells divide to produce the next generation of cells, they must replicate (make a copy of) the DNA. First, proteins help to unzip sections of the double-stranded DNA
2 Then, following strict base-pairing rules, free (available) bases in the cell are linked together with their matching bases on the two original strands
3 Finally, two duplicate codebooks are made. So when the cell divides, each new cell gets an identical DNA codebook
The DNA base-pair rule:
A always with T
A T Thymine
T A Adenine
C always with G
C G Guanine
G C Cytosine
[Diagram on page 8, 9]
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How Proteins Are Made
For simplicity, we illustrate a protein made of 10 amino acids. Most proteins have more than 100
1 A special protein zips open a section of the DNA strands
2 Free RNA bases link up with the exposed DNA bases on one strand only, thus forming a strand of messenger RNA
Free RNA bases
3 The newly made messenger RNA peels off and moves away to the ribosomes
4 A transfer RNA picks up an amino acid and brings it to the ribosome
5 As the ribosome sweeps across the messenger RNA, a chain of amino acids is linked together
6 As it is being formed, the protein chain begins to fold into the shape needed to function properly. Then the chain is released by the ribosome
Transfer RNA has two important ends:
One recognizes the messenger RNA code
The other carries the correct amino acid
RNA bases use U rather than T, so U pairs with A
A U Uracil
U A Adenine