Genes are the blueprints for building the chemical machinery that keeps cells alive. That’s true for humans and all other forms of life. But did you know that with 20,000 genes, people have almost 11,000 fewer genes than water fleas? If the number of genes doesn’t predict complexity, what does?
The answer is that our genetic material contains much more than the units we call genes. Just as important are the switches that turn a gene on and off. And how cells read and interpret genetic instructions is far more complex in people than in those water fleas.
Genes and the switches that control them are made of DNA. That’s a long molecule resembling a spiral ladder. Its shape is known as a double helix. A total of three billion rungs connect the two outer strands — the upright supports — of this ladder. We call the rungs base pairs for the two chemicals (pair) from which they are made. Scientists refer to each chemical by its initial: A (adenine), C (cytosine), G (guanine) and T (thymine). A always pairs with T; C always pairs with G.
In human cells, the double-stranded DNA doesn’t exist as one gigantic molecule. It’s split into smaller chunks called chromosomes (KROH-moh-soams). These are packaged into 23 pairs per cell. That makes 46 chromosomes in total. Together, the 20,000 genes on our 46 chromosomes are referred to as the human genome.
The role of DNA is similar to the role of the alphabet. It has the potential to carry information, but only if the letters are combined in ways that make meaningful words. Stringing words together makes instructions, as in a recipe. So genes are instructions for the cell. Like instructions, genes have a “start.” Their string of base pairs must follow in a specific order until they reach some defined “end.”
If genes are like a basic recipe, alleles (Ah-LEE-uhls) are versions of that recipe. For instance, the alleles of the “eye color” gene give directions for making eyes blue, green, brown and so on. We inherit one allele, or gene version, from each of our parents. That means most of our cells contain two alleles, one per chromosome.
But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.
By combining two sets of 23 chromosomes — one set from the egg, one set from the sperm cell — that new person ends up with the usual two alleles and 46 chromosomes. And her unique combination of alleles will never arise in the exact same way again. It’s what makes each of us unique.But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.
A fertilized cell needs to multiply to make all of a baby’s organs and body parts. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell. Then the process repeats itself many times as one cell copies to become two. And two copy to become four. And so on.
To make organs and tissues, the cells use the instructions on their DNA to build tiny machines. They control reactions between chemicals in the cell that eventually produce organs and tissues. The tiny machines are proteins. When a cell reads a gene’s instructions, we call it gene expression. A fertilized cell needs to multiply to make all of a baby’s organs and body parts. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell. Then the process repeats itself many times as one cell copies to become two. And two copy to become four. And so on.
How does gene expression work?
Gene expression relies on helper molecules. These interpret a gene’s instructions to make the right types of proteins. One important group of those helpers is known as RNA. It’s chemically similar to DNA. One type of RNA is messenger RNA (mRNA). It’s a single-stranded copy of the double-stranded DNA.
Making mRNA from DNA is the first step in gene expression. That process is known as transcription and happens inside a cell’s core, or nucleus. The second step, called translation, takes place outside of the nucleus. It turns the mRNA message into a protein by assembling the appropriate chemical building blocks, known as amino (Ah-MEE-no) acids.
All human proteins are chains with different combinations of 20 amino acids. Some proteins control chemical reactions. Some carry messages. Still others function as building materials. All organisms need proteins so that their cells can live and grow.
To build a protein, molecules of another type of RNA — transfer RNA (tRNA) — line up along the mRNA strand. Each tRNA carries a three-letter sequence on one end and an amino acid on the other. For example, the sequence GCG always carries the amino acid alanine (AL-uh-neen). The tRNAs match up their sequence with the mRNA sequence, three letters at a time. Then, another helper molecule, known as a ribosome (RY-boh-soam), joins the amino acids on the other end to make the protein.
One gene, several proteins
Scientists first thought that each gene held the code to make one protein only. They were wrong. Using the RNA machinery and its helpers, our cells can make way more than 20,000 proteins from their 20,000 genes. Scientists don’t know exactly how many more. It could be a few hundred thousand — perhaps a million!
How can one gene make more than one type of protein? Only some stretches of a gene, known as exons, code for amino acids. The regions in between them are introns. Before the mRNA leaves a cell’s nucleus, helper molecules remove its introns and stitch together its exons. Scientists refer to this as mRNA splicing.
The same mRNA may be spliced in different ways. This often happens in different tissues (perhaps skin, the brain or the liver). It’s like the readers “speak” different languages and interpret the same DNA message in multiple ways. That’s one way the body can have more proteins than genes.
Here’s another way. Most genes have multiple switches. The switches determine where an mRNA begins to read a DNA sequence, and where it stops. Different start or end sites create different proteins, some longer and some shorter. Sometimes, transcription doesn’t start until several chemicals attach themselves to the DNA sequence. These DNA binding sites may be far away from the gene, but still influence when and how the cell reads its message.
Splicing variations and gene switches result in different mRNAs. And these are translated into different proteins. Proteins also may change after their building blocks have been assembled into a chain. For example, the cell may add chemicals to give a protein some new function.
DNA holds more than building instructions
Making proteins is far from DNA’s only role. In fact, only one percent of human DNA contains the exons that the cell translates into protein sequences. Estimates for the share of DNA that controls gene expression range from 25 to 80 percent. Scientists do not yet know the exact number because it’s harder to find these regulatory DNA regions. Some are gene switches. Others make RNA molecules that aren’t involved in building proteins.
Controlling gene expression is almost as complex as conducting a large symphony orchestra. Just consider what it takes for a single fertilized egg cell to develop into a baby within nine months.
So does it matter that water fleas have more protein-coding genes than people? Not really. Much of our complexity hides in the regulatory regions of our DNA. And decoding that part of our genome will keep scientists busy for many, many years.
allele One of two or more alternative versions of a gene.
amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic building blocks of proteins.
base pairs (in genetics) Sets of nucleotides that match up with each other on DNA or RNA. For DNA, adenine (A) matches up with thymine (T), and cytosine (C) matches up with guanine (G).
biology The study of living things. The scientists who study them are known as biologists.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemical reaction A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).
chromosome A single threadlike piece of coiled DNA found in a cell’s nucleus. A chromosome is generally X-shaped in animals and plants. Some segments of DNA in a chromosome are genes. Other segments of DNA in a chromosome are landing pads for proteins. The function of other segments of DNA in chromosomes is still not fully understood by scientists.
coding (in genetics) The instructions contained in DNA (or its genes) that allow a cells to know what proteins to make and when to make them. (in computing) A slang term for developing computer programming — or software — that performs a particular, desired computational task.
decoding Figuring out a message hidden in some code.
development (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape. (v. develop)
DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
egg The unfertilized reproductive cell made by females.
exon Part of a DNA or RNA molecule that holds the directions for creating part of a protein or peptide.
expression (in genetics) The process by which a cell uses the information coded in a gene to direct a cell to make a particular protein.
gene A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
genome The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.
guanine One of four substances that organisms need to produce DNA.
helix An object with a three-dimensional shape like that of a wire wound uniformly in a single layer around a cylinder or cone, as in a corkscrew or spiral staircase.
intron A section of DNA or RNA that does not carry the blueprints for making some protein.
messenger RNA (or mRNA) A type of genetic material that is copied from DNA. It carries the instructions for building a cell’s proteins.
molecule An electrically neutral group of atoms that represents the smallest possibleamount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nucleus Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information.
organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.
RNA A molecule that helps “read” the genetic information contained in DNA. A cell’s molecular machinery reads DNA to create RNA, and then reads RNA to create proteins.
sequence (in genetics) n. The precise order of the nucleotides within a gene. (v.)
To figure out the precise order of the nucleotides making up a gene.
sibling An offspring that shares the same parents (with its brother or sister).
sperm The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.
splice Originally meant to weave the ends of two pieces of rope together so that it becomes one longer piece of rope. It can now also mean to take two long things (movie film, pieces of lumber or pieces of DNA, for instance) and make them a single longer one.
stitch A length of thread that binds two or more fabrics together.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
translation (in genetics) The process of turning the mRNA message into a protein. A cell does this by assembling the appropriate chemical building blocks, known as amino (Ah-MEE-no) acids. Translation occurs outside of a cell’s inner core, or nucleus.
transcription (v. transcribe) To copy something down, word for word. (in genetics) The first step in gene expression. It's where an enzyme copies a selected piece of DNA into RNA (especially messenger RNA). Both DNA and RNA are made up of base pairs of nucleotides.
transfer RNA (tRNA) A type of RNA (ribonucleic acid) molecule that a cell uses to read a section of messenger RNA. This takes place during the production of a cellular protein.
unique Something that is unlike anything else; the only one of its kind.