Every cell in the body stems from just one cell: the fertilized egg.
Every cell contains the same library of instructions, but the cells read and use different genetic chapters.
Reading Chapters from the Genome
Every cell in the body contains all of the DNA sequence, but the composition of each cell depends on which sections of the DNA are used. We know that each cell reads only those chapters from the library of instructions that it needs. The selective reading process creates many different kinds of cells, such as skin, muscle, neural, and bone cells, all of which develop from the many cells of the embryo produced by the growth and division of one cell: the fertilized egg. Studies of the fruit fly, Drosophila melanogaster, have been useful in revealing how organisms develop these cell types, with each cell knowing what chapters to read according to its position in the developing embryo.
Grassroots Cooperation
The human body has between 50 and 100 trillion cells and no single cell is in charge. Throughout a lifetime, each cell interacts with many other cells to determine which instructions to use at a particular time and place.
Monday, June 7, 2010
DNA Is Like A Library Of Instructions
The genome is like a library of instructions.
A gene is a sequence of A’s, T’s, G’s, and C’s that usually provides the instructions for a single protein component of an organism.
The letters of the genetic alphabet – A, T, G, and C – are meaningless on their own, but they are combined into useful instructions in genes. Some genes carry enough information for one complete characteristic of an organism, but most characteristics result from combinations of genes. Genes are like chapters in the books that fill the library of the genome.
DNA Sequencing
The sequence of letters within a gene is like the letters in a book of instructions. Deciphering the enormously long sequence of A’s, T’s, G’s, and C’s in an organism’s genome reveals useful information. For example, finding a difference in a gene sequence that governs muscle structure raises questions. Could the difference affect health? Just as changing one letter in a word can change its meaning – for example, mice to rice to nice – so changing one DNA letter can sometimes cause illness.
Not all of the sequences in the genes of two humans are identical. For example, because your face is unique, the precise set of sequences in the large group of genes that control the shape of your face are presumably unique too. Some special parts of the DNA sequence vary from person to person with unusually high frequency. As you will see, finding sequences in DNA samples can be used to identify individuals and help solve crimes, even when there are no eyewitnesses.
A gene is a sequence of A’s, T’s, G’s, and C’s that usually provides the instructions for a single protein component of an organism.
The letters of the genetic alphabet – A, T, G, and C – are meaningless on their own, but they are combined into useful instructions in genes. Some genes carry enough information for one complete characteristic of an organism, but most characteristics result from combinations of genes. Genes are like chapters in the books that fill the library of the genome.
DNA Sequencing
The sequence of letters within a gene is like the letters in a book of instructions. Deciphering the enormously long sequence of A’s, T’s, G’s, and C’s in an organism’s genome reveals useful information. For example, finding a difference in a gene sequence that governs muscle structure raises questions. Could the difference affect health? Just as changing one letter in a word can change its meaning – for example, mice to rice to nice – so changing one DNA letter can sometimes cause illness.
Not all of the sequences in the genes of two humans are identical. For example, because your face is unique, the precise set of sequences in the large group of genes that control the shape of your face are presumably unique too. Some special parts of the DNA sequence vary from person to person with unusually high frequency. As you will see, finding sequences in DNA samples can be used to identify individuals and help solve crimes, even when there are no eyewitnesses.
Reading the Sequences
The sequence of nucleotides in a gene gives it meaning by storing the instructions for building the other molecules necessary for life. These instructions are read as a string of A’s, T’s, G’s, and C’s, such as ACGGTAACT. In the sense that there are 26 letters in the English alphabet, there are four letters in the alphabet of DNA.
The four letters in the DNA alphabet are actually abbreviations for the chemicals that make up the library of instructions.
A for Adenine
T for Thymine
G for Guanine
C for Cytosine
The four letters in the DNA alphabet are actually abbreviations for the chemicals that make up the library of instructions.
A for Adenine
T for Thymine
G for Guanine
C for Cytosine
Unzipping DNA
The Sequence On One Strand
The DNA molecule is composed of two very long strands of A’s, T’s, G’s and C’s, which are tightly paired with each other. An A on one strand is always paired with a T on the other strand, and a G is always paired with a C. This means that if the sequence of nucleotides on one strand is known, the sequence of the other strand will be automatically known as well.
One strand of DNA is like a photographic negative of the other strand. A negative can be used to make many copies of a photograph because it contains all of the information that is part of that photograph. Similarly, to read the sequence of A’s, T’s, G’s, and C’s in a genome, it is only necessary to read one strand of DNA to be able to deduce the sequence of the other strand.
The DNA molecule is composed of two very long strands of A’s, T’s, G’s and C’s, which are tightly paired with each other. An A on one strand is always paired with a T on the other strand, and a G is always paired with a C. This means that if the sequence of nucleotides on one strand is known, the sequence of the other strand will be automatically known as well.
One strand of DNA is like a photographic negative of the other strand. A negative can be used to make many copies of a photograph because it contains all of the information that is part of that photograph. Similarly, to read the sequence of A’s, T’s, G’s, and C’s in a genome, it is only necessary to read one strand of DNA to be able to deduce the sequence of the other strand.
 | One Strand of DNA Is Like a Photographic Negative to the Other An adenine (A) on one strand is always paired with a thymine (T) on the other strand, and a guanine (G) is always paired with a cytosine (C). If the sequence of nucleotides on one strand is known, the sequence of the other strand will be automatically known as well. |
The DNA Sequence
The extremely long DNA molecule is actually made of a long string of chemical building blocks called “nucleotides.” There are four different nucleotides, which are labeled adenine (A), thymine (T), guanine (G), and cytosine (C). The human genome is made of a sequence of roughly three billion of these nucleotides, and it is about the same size as the genome of a chimpanzee or a mouse. In contrast, a fruit fly has 180 million, a yeast has 12 million, and the flowering weed “thale cress” has 100 million nucleotides of DNA in its genome.
Learn more about how to read the DNA sequence and probe the sequence for matches in the following sections:
Learn more about how to read the DNA sequence and probe the sequence for matches in the following sections:
Inheritance
Two Copies of the Genome
You Inherit One Copy From Each Parent
A person has two copies of every gene sequence, one inherited from the mother, and the other inherited from the father. A child thus inherits one copy from each parent's own pair, and this copy is selected from the parent randomly. In order to fully understand a person's DNA sequences, both inherited copies of a gene need to be examined.
These sequences are often not identical. Gene sequences for so-called "dominant" traits are expressed over "recessive" traits. Do you have a cleft chin? It is an indentation at the tip of your chin. If so, you have at least one copy of the dominant gene for cleft chin. But if your chin has no indentation, you have two copies of the recessive gene for this trait.
You Inherit One Copy From Each Parent
A person has two copies of every gene sequence, one inherited from the mother, and the other inherited from the father. A child thus inherits one copy from each parent's own pair, and this copy is selected from the parent randomly. In order to fully understand a person's DNA sequences, both inherited copies of a gene need to be examined.
These sequences are often not identical. Gene sequences for so-called "dominant" traits are expressed over "recessive" traits. Do you have a cleft chin? It is an indentation at the tip of your chin. If so, you have at least one copy of the dominant gene for cleft chin. But if your chin has no indentation, you have two copies of the recessive gene for this trait.
 | Two Copies of Each Chromosome You inherit one copy of every chromosome from each parent. Therefore you have two copies of every gene sequence. |
Tracing Similarities And Differences In Our DNA
What percent of their genes match yours?
Why Were Genes Used In This Comparison, and How Do They Relate To DNA?
Genes are the fundamental units of DNA function. In DNA terms, genes are discrete sections of the DNA sequence that are part of much longer DNA molecules. They provide the biochemical instructions for producing all of the components of biological organisms. Some genes specify visible physical traits, while others govern metabolic processes. Most traits, such as the shape of your face, require the actions of many genes.
Why Are We So Similar?
The DNA of these species is so similar because the basic organization of life is widely shared, with the largest differences found between plants and animals, or between tiny single-celled organisms like yeast and large multicellular organisms like ourselves. The similarities reflect a common ancestry that appears to be shared by all life on Earth.
Are People Really Identical?
Even though humans share 100% of the same genes, the instructions contained within the genes are not entirely identical. Each person is unique. People have different hair colors, facial structures, and other traits. These differences between individuals result from very small differences in their DNA sequences. DNA also contains many so-called "housekeeping genes" that control important metabolic processes. As you will see, some of the differences in these genes can cause illness.
Although the DNA of any two people on Earth is, in fact, 99.9% identical, even a tiny difference can have a big effect if this difference is located in a critical gene.
| Another human? | 100% - All humans have the same genes, but some of these genes contain sequence differences that make each person unique. |
| A chimpanzee? | 98% - Chimpanzees are the closest living species to humans. |
| A mouse? | 92% - All mammals are quite similar genetically. |
| A fruit fly? | 44% - Studies of fruit flies have shown how shared genes govern the growth and structure of both insects and mammals. |
| Yeast? | 26% - Yeasts are single-celled organisms, but they have many housekeeping genes that are the same as the genes in humans, such as those that enable energy to be derived from the breakdown of sugars. |
| A weed (thale cress)? | 18% - Plants have many metabolic differences from humans. For example, they use sunlight to convert carbon dioxide gas to sugars. But they also have similarities in their housekeeping genes. |
Genes are the fundamental units of DNA function. In DNA terms, genes are discrete sections of the DNA sequence that are part of much longer DNA molecules. They provide the biochemical instructions for producing all of the components of biological organisms. Some genes specify visible physical traits, while others govern metabolic processes. Most traits, such as the shape of your face, require the actions of many genes.
Why Are We So Similar?
The DNA of these species is so similar because the basic organization of life is widely shared, with the largest differences found between plants and animals, or between tiny single-celled organisms like yeast and large multicellular organisms like ourselves. The similarities reflect a common ancestry that appears to be shared by all life on Earth.
Are People Really Identical?
Even though humans share 100% of the same genes, the instructions contained within the genes are not entirely identical. Each person is unique. People have different hair colors, facial structures, and other traits. These differences between individuals result from very small differences in their DNA sequences. DNA also contains many so-called "housekeeping genes" that control important metabolic processes. As you will see, some of the differences in these genes can cause illness.
Although the DNA of any two people on Earth is, in fact, 99.9% identical, even a tiny difference can have a big effect if this difference is located in a critical gene.