CELLS ARE US

FROM GENES TO PROTEINS

NAVIGATION TABLE

From Genes to Proteins

Pre-Test

• Google assessment

From Genes to Proteins

Introduction

INFORMATION CENTRAL

From Genes to Proteins

Introduction

"Information Central!" The control center of the cell. Do you know where it is in the cell? How it works?

This unit explains "Information Central:” where it is and how cells preserve and use information to accomplish all the things that cells must do to stay alive, to thrive, and to reproduce.

Image source: Crabtree + Company. Credit: National Institute of General Medical Sciences. https://images.nigms.nih.gov/pages/DetailPage.aspx?imageid2=2540

From Genes to Proteins

Objectives

After completing this lesson, the student should be able to: 

• Define heredity as the passage of genetic instructions from one generation to the next generation.
• Recognize that inherited traits of individuals are governed in the genetic material found in the genes within chromosomes in the nucleus.
• Understand Mendel's experiments and findings.
• Explain the difference between dominant and recessive genes.
• Explain the purpose and process of transcription and translation in making proteins for the body.

This spiral staircase is very similar in structure to DNA. The steps are like the nucleotide base pairs and the railing resembles the sugar-phosphate backbone.

From Genes to Proteins

Why It Matters

From Genes to Proteins

Why It Matters

Cells are very busy - sort of like a factory that is making lots of different things at the same time. As in a factory, the activities inside a cell take a lot of instructions so that the right things get done at the right time.

Why does it matter how a cell stores information?

Experiments by Gregor Mendel, in what is now Slovakia in Eastern Europe, revolutionized scientific thinking (see "Story Time" in this unit). Mendel is considered the “Father of Genetics” because he did experiments that show that organisms can pass on traits in predictable ways from parent to offspring. This is called “heredity.” However, scientists in that time (the 1880s) did not appreciate the importance of Mendel's experiments. Later scientists took a new look at Mendel's data and realized that his ideas helped to explain:

• Darwin's theory of evolution.
• The results of centuries of selective breeding of animal and plant populations.

WHY DOES IT MATTER HOW A CELL STORES INFORMATION?

From Genes to Proteins

Why It Matters

The principles advocated by Mendel caused later scientists to look for the physical origin of traits, which we now know are found in genes that are located on chromosomes. These genes are made of DNA (deoxyribonucleic acid) that carries the codes for inheritable traits. Because the chromosomes are located in the nucleus of the cell, the nucleus is considered “information central” or the “brain” of the cell. It directs the building of proteins that form the very traits that make you.

Chromosome structure. DNA is tightly wound around proteins called histones and packaged into cells’ nuclei in the form of chromosomes. Genes are sections of DNA that, under the right circumstances, can be transcribed into proteins.

THREE MAIN FIELDS USE THE INFORMATION ABOUT THE CODE IN DNA IN IMPORTANT WAYS

From Genes to Proteins

Why It Matters

Criminal Justice

Everybody's DNA is slightly different from that of everybody else. Thus, a sample of blood, or saliva that contains tongue or cheek cells, can be used to identify a person. Police use this technique to catch criminals who leave traces of some of their cells at crime scenes. Click here for more.

THREE MAIN FIELDS USE THE INFORMATION ABOUT THE CODE IN DNA IN IMPORTANT WAYS

From Genes to Proteins

Why It Matters

Medicine

Doctors use DNA samples to detect the presence of inheritable diseases. Scientists are working to develop gene therapies in which correctly working genes can be inserted into cells to direct normal function. DNA directs the production of the body's proteins, and proteins create most of the structure of cells and control most of the functions. If you have a faulty or mutated gene that causes a disease or disorder, you can have a faulty protein. If we can manipulate that gene into a working gene, we have a treatment. Or maybe, we can find ways to counteract the faulty protein. Click here for more.

THREE MAIN FIELDS USE THE INFORMATION ABOUT THE CODE IN DNA IN IMPORTANT WAYS

From Genes to Proteins

Why It Matters

Agriculture

A complete list of GMO crops grown and sold in the United States:

Scientists can insert or delete genes in plants or animals to improve their growth, drought tolerance, disease resistance, or other functions. This is known as genetically modified organisms, or GMO. Animals can be cloned, so that exact copies can be made of one animal that has very desirable genetic characteristics. Click here for more.

IS GENETICALLY MODIFIED FOOD AN ENVIRONMENTAL HAZARD?

From Genes to Proteins

Why It Matters

Scientists can insert genes into food plants in order to make them grow more quickly or be resistant to certain diseases or environmental conditions. A GMO (genetically modified organism) is a plant, animal, or microorganism that has had its genetic material (DNA) changed using technology that generally involves the specific modification of DNA, including the transfer of specific DNA from one organism to another. Scientists often refer to this process as genetic engineering. Some people are afraid of such genetically modified plants, because they believe these new genes might get into animals or humans or the ecosystem and cause unexpected adverse or harmful effects.

Many European countries ban the importing of such genetically modified plants. This causes a problem for U.S. farmers, because many of them rely on genetically modified plants and many export markets are closed to them.

ARE GENETICALLY MODIFIED FOODS SAFE? WHAT IS THE EVIDENCE?

From Genes to Proteins

Why It Matters

A comprehensive study has been completed by the National Academy of Sciences on GMO (also called GE for “genetically engineered) foods and scientists have found that they are safe for humans. National Geographic Magazine published a summary of this report.

They reported:

• GE crops are safe to eat. There is no evidence of harm to people.
• The GE crops in our food system don’t improve on the crops’ potential yields. They have, however, helped farmer protect yields from insects and weeds.
• Both herbicide-tolerant crops and crops with the organic pesticide Bt built in have decreased pesticide use, although those decreases came early on, and some have not been sustained.
• Increased use of glyphosate, the herbicide GE crops tolerate, has been responsible for a widespread and expensive problem of glyphosate-resistant weeds.
• The report found no adverse affects on biodiversity or danger from interbreeding between GE crops and wild relatives.
• Although both the use of GE crops and the employment of farming techniques that reduce tilling have been on the rise, the report finds no cause-and-effect relationship.
• The economic benefits to farmers have been well-documented, although individual results vary.
• Small-scale farmers may have trouble seeing those economic gains because of the price of seed and lack of access to credit.
• Appropriate regulation is imperative, and that regulation should be based on the characteristics of the crop rather than the technique used to develop it, whether GE or non-GE.
• Ongoing public conversations about GE crops and related issues should be characterized by transparency and public participation.

For more information on GE crops, click here.

ARE GENETICALLY MODIFIED FOODS SAFE? WHAT IS THE EVIDENCE?

From Genes to Proteins

Why It Matters

Research has been done on the safety of GE crops on insects, particularly honeybees and butterflies. A pest-resistant plant can make a protein that is toxic only to certain insects, not to other insects, animals, or humans. GE crops do not harm honeybees or butterflies. They actually may reduce the need for insecticides that do them harm. For more information, click here.

Still, even after studies have shown these foods are safe, the public is still very skeptical. It is up to each individual to read the evidence and make logical decisions about their food consumption.

From Genes to Proteins

How We Know

HEREDITY

From Genes to Proteins

How We Know

How do we find out how bodily traits are inherited?

Read the Story Time about Gregor Mendel. This monk, who had the job of tending the monastery garden, made a most profound discovery. Between 1856 and 1863, he cultivated and tested at least 28,000 pea plants, carefully analyzing seven pairs of seed and plant characteristics (see table below). He saw that many of these traits are inherited in a very clear and simple way. Because of this, he is sometimes called “the father of heredity.” Heredity is simply the passing of traits from parent to offspring. It seems like a simple idea to us now, however, what seems simple today was not so simple for Mendel because he had no prior information about how things worked. He had only his observations of what happened in the offspring when he bred one kind of plant with another kind of plant. The seven pairs of traits that Mendel Studied in the peas were:

Remember that Mendel worked almost 150 years ago when nobody knew about genes or even the structures (chromosomes) that carry genes. Mendel and his neighbors did know about the role of heredity in farming. If a farmer breeds all his cows to a large bull, most of the offspring will be larger than if a small bull had been used.

HEREDITY

From Genes to Proteins

How We Know

How did Mendel discover the principles of heredity?

Mendel's success depended on several things:

• He picked a simple system (pea plants) that reproduced rapidly. 
• He kept records on the traits of each plant and then took pollen from one plant to fertilize the flowers from another plant and saw predictable patterns in the offspring plants. The pollen and the ovule (egg) of the flower are the sex cells of plants. The pollen is the male sex cell and the ovule is the female sex cell.

HEREDITY

From Genes to Proteins

How We Know

What did Mendel discover?

Try to see these results as Mendel did, not knowing what is happening to cause the results. For example, Mendel first grew purebred lines of green peas and yellow peas. Then he took pollen from a green pea plant, smeared it on the flowers of a yellow pea plant, and saw that the new peas were always all green.

What happened to the yellow trait? Did the traits that control yellow somehow disappear? How about the possibility that they were still there in the offspring but their influence was hidden by the green trait (see table on next slide)? Mendel suspected that the trait for green was “stronger” and covered the yellow because some of the following generation's peas were yellow! And this occurred in a consistent ratio of three green pea plants for every yellow one. We now call traits that are “stronger” and cover other traits “dominant” traits. In genetics, we designate dominant traits with a capital letter. The traits that are covered, in this case, the yellow color, are called “recessive” traits. They are designated with a lower-case letter. It’s important to note that, although dominant traits are “stronger” genetically, they may not lead to a physically stronger or more desirable trait in the offspring. Also, recessive traits many not be physically weaker or less desirable. They just get suppressed by the dominant trait in genetics.

HEREDITY

From Genes to Proteins

How We Know

Why did this happen?

Things only made sense to Mendel if he assumed that each trait had TWO "controllers," and that the sex cells of the plant, the male pollen and the female ovule each only carried one of the two controllers. Assuming that, Mendel could make a chart like the one below. In this table, the “X” means a cross or breeding of two plants. You will see the word “cross” used often to designate a breeding of two individuals. In this case, this means the pollen from one plant was transferred to the flower of the other plant.

HEREDITY

From Genes to Proteins

How We Know

How did Mendel explain this result?

We don't know exactly how Mendel figured it out, but he probably tried several ideas until he found the one that made sense. You can do it too. Suppose that each plant has TWO trait controllers. He realized that something in the plant controlled for pea color and that there must be two controllers in the peas of each plant. In any plant, if either one of the two controllers is for green, the peas will be green because green is a dominant trait in peas. Using a Punnett Square, which is a basic chart to show all the possible combinations of traits when two organisms are crossed, the FIRST cross Mendel did would look something like this:

In this square, the capital letter “G” stands for the dominant trait, green. The lower case “g” stands for the recessive trait, yellow. The pea pod on the left has two capital “G’s,” noting that both of its traits are dominant for green, or GG. The yellow pod above has two lower case “g’s” showing both of its traits are recessive or gg. This cross would be shown as GG X gg. In a Punnett Square, the two traits for the first organism are listed vertically on the left side of the square.

HEREDITY

From Genes to Proteins

How We Know

In a Punnett Square, the two traits for the first organism are listed vertically on the left side of the square.(Left)

The two traits for the other individual are listed horizontally across the top.(Right)

HEREDITY

From Genes to Proteins

How We Know

The letters of the traits are combined in the square to show all the possible outcomes of the cross. The letters to the left of the square and the top of the square are combined inside the square to show the possible offspring.

So ALL of these offspring, or baby peas, would be green, BUT each would have a trait for green and a trait for yellow. Their traits are Gg. Why would they be all green and not green and yellow? Because remember, in this case, green is dominant over yellow. Any G trait would cover any g trait. So the green would “cover” the yellow. Again, remember that this doesn’t mean that green is “better” than yellow. It just means that the green trait is dominant genetically over the yellow trait.

HEREDITY

From Genes to Proteins

How We Know

So how could you get a 3 to 1 ratio of green peas to yellow peas like Mendel did in his experiment? You saw above that the offspring have one version of the gene for green (G) and another version for yellow (g). Their traits are Gg. Setting up a Punnett Square again shows how easy it is to see all the possible combinations. This would be for two plants from the cross above were bred to each other by putting the pollen of one on the flower of the other. This cross is Gg x Gg. Both of these plants would have the two traits G and g.

Three green peas and one yellow pea are predicted in this cross! The combination possibilities predict a 3:1 ratio! Remember, anytime one of the two traits is for green (G), the peas will be green because green is dominant over yellow. The only way peas can be yellow is if BOTH traits control for yellow (gg).

HEREDITY

From Genes to Proteins

How We Know

Mendel’s experiments led to his basic laws of heredity. Now we know that the traits are directed by genes on the chromosomes. The genes are made of DNA. There is a gene for the color of the pea pod in Mendel’s experiment. There can be two possibilities for that gene, G and g. Mendel called those “controllers.” We now call those two choices alleles. Alleles are one of two or more alternative forms of a gene that are found at the same place on a chromosome.

Mendel's Laws of Heredity are generally stated as:

• The Law of Segregation: Each inherited trait is defined by a pair of alleles for the same gene. Parental alleles are randomly separated to the sex cells so that sex cells contain only one version of the gene or one allele. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization. 
• The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another. For example, the color of the pea pod is independent from the texture of the peas. 
• The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.

HEREDITY

From Genes to Proteins

How We Know

The genetic experiments Mendel did with pea plants took him eight years (1856-1863), and he published his results in 1865. During this time, Mendel grew over 28,000 pea plants, keeping track of progeny number and type. Mendel's work and his Laws of Inheritance were not appreciated in his time. It wasn't until 1900, after the rediscovery of his Laws, that his experimental results were understood.

To learn about the history of genetics, from Mendel to present, click here for a self-paced primer.

HOW DID WE FIND OUT WHAT A GENE IS?

From Genes to Proteins

How We Know

We now know that the traits Mendel investigated are, in fact, genes. The word "gene" originally meant a piece of DNA in chromosomes that controls a given bodily trait. 

The Basic Structure of DNA

DNA is a chemical structure that contains molecules called bases. Information is carried in the pairing and ordering of bases in the structure. There are four bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). The base adenine pairs with the base thymine (A-T) and cytosine pairs with guanine (C-G). Billions of such pairs line up in a specific sequence to create DNA. When a cell divides, the DNA splits where the dashed lines are in the diagram. Because C can only pair with G and A only pairs with T, the two new cells can create an identical copy of DNA using the half strand of DNA they have received.

HOW DID WE FIND OUT WHAT A GENE IS?

From Genes to Proteins

How We Know

This structure was discovered in a famous analysis of data that other people were generating at the time by Francis Crick and James Watson. The most useful data came from x-ray crystallography taken by Rosalind Franklin and Raymond Gosling, which captures on film the x-ray scattering produced by crystals of DNA. The patterns, plus what others had learned about A, T, C, and G, led them to conclude that these molecules occurred in pairs and that the pair was held together by a weak bonds involving hydrogen. Each pair is called a "base pair.” The bases are also connected by a sugar phosphate “backbone.” The whole structure forms a double helix shape.

So now, the question is:

If the genetic code is nothing more than the chain of A-Ts and C-Gs, how do we know what a gene is? That is, where does one segment (gene) end and another begin?

DISCOVERING THE SEQUENCE OF GENES

From Genes to Proteins

How We Know

As scientists learned of the structure of DNA, they began “reading’ the genes of organisms and mapping those genes on the chromosomes. Each DNA sequence that contains instructions to make a protein is known as a gene. The first gene maps were done in fruit flies, because they have a small genome (set of all of their genes together) and because they were so often used in genetic experiments because of their small size and easy-to-observe traits. Today, complete genome maps exist for humans (about 3 billion base pairs!), along with many other organisms from platypus to mosquitoes.

DISCOVERING THE SEQUENCE OF GENES

From Genes to Proteins

How We Know

But how do they know which base pair begins a gene and which ends a gene? The size of a gene may vary greatly, ranging from about 1,000 bases to 1 million bases in humans. The key is to know the sequence of base pairs that make a functioning protein. A combination of three approaches is used, and computers keep track of the information.

• Certain short chains of base pairs are known to be markers for the beginning and ending of base-pair sequences that make a functioning protein.
• A large series of enzymes exist that can cut DNA into smaller pieces. By splitting the two strands, one of the strands can be probed with a single strand from a known gene, for example a bacterial gene that makes a protein with a known function. The base sequence that matches the probe sequence is then identified as a gene. 
• Insert an unknown piece of DNA into a bacterium or cell, and test for any new protein product that appears.

DISCOVERING THE SEQUENCE OF GENES

From Genes to Proteins

How We Know

The results of such sequencing approaches led to the conclusion that the human may have only about 25,000 genes, which amount to only about 1 percent of the total genome (DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made). That is only about twice the number that fruit flies have. Does that make sense? We are so much larger and more complex than a fruit fly. How come we only have twice the genes? Some species, like a type of mouse and a species of rice, have more genes than people do! Possible explanations include:

• the proteins humans make are more complex than those usually made by fruit flies. 
• humans have many more "regulatory genes," which are genes that turn on or turn off other genes.
• many genes may work together to make a protein and each combination can make a different protein.

Humans apparently have a lot of genetic "junk." Huge fractions of our genome don't seem to do anything. Some seem to be residues from viral and bacterial infections in our ancestors. So does this "junk" do anything? One obvious possibility is that it is a source of mutations.

It seems clear that our understanding is only just beginning. Even if we were sure what a gene is, we still don't know what many of them do, nor what turns them on or off. We have only begun to consider the possibility that several genes are involved in making a single protein, as is the case with the workings of the body's immune system.

MAPPING THE HUMAN GENOME

From Genes to Proteins

How We Know

The Human Genome Project (HGP) was the international collaborative research program whose goal was the complete mapping and understanding of all the genes of human beings (https://www.genome.gov/human-genome-project/What). Remember, all of our genes together are known as our "genome." The contents of the human genome are now completely determined, although we don't know nearly as much about the proteins that most genes make.

MAPPING THE HUMAN GENOME

From Genes to Proteins

How We Know

Your genes differ from those of a monkey or chimp by only about 1 or 2%. You differ from other people by far less than that. How then, can we be so different from chimpanzees?

The key lies in knowing that what is important is not so much that we share so many of the same genes, but that we differ so greatly in which genes GET EXPRESSED. That is, a gene does nothing if it is not "turned on" so that it can be used to make protein. 

Recent research has discovered a small set of genes that make proteins that control expression of other genes. These gene-expression control genes make proteins that stick to the DNA helix and determine which segments can be used to make proteins. The gene-expression control genes are called "zinc finger" genes, because the proteins that they make have little "fingers" of amino acids, held in that shape by electrical interaction of certain amino acids with zinc atoms. These fingers insert themselves into the DNA and determine which genes can get expressed and which are shut down.

Comparison of zinc-finger genes across species indicates that it is THIS PORTION of the genome that varies most greatly among species. Thus, these zinc-finger genes seem to be more susceptible to natural selection forces than the rest of the genome.

MAPPING THE HUMAN GENOME

From Genes to Proteins

How We Know

Some facts about human genes

Although the complete sequence of the A-T and C-G pairs have been determined, it is still somewhat of a guess as to how many genes there actually are. The smallest human chromosome, number 21, has about:

• 48 million base pairs
• 225 protein-coding genes
• Over 130 “fake” genes (sequences like active genes but which don't seem to make any protein)
• Duplications (one 93-base sequence has 10 copies and a six-base sequence that repeats 17 times)

Segments of DNA that don't do anything may be left over from our primitive ancestors. But many segments of DNA could be active but just don't have clear signs saying "I am a gene!" Another thing to remember is that many body structures and functions are controlled by more than one gene. Yet another point to consider is that genes may be present but turned off only temporarily.

Chromosome 21 was among the first to be sequenced because it contains genes that cause conditions such as Down's syndrome, Alzheimer's disease, and leukemia. Sequencing this chromosome was only the first step to discovering how it is involved in these conditions.

From Genes to Proteins

What We Know

WHAT DOES DNA DO?

From Genes to Proteins

What We Know

Over a hundred years ago, there was a raging debate over whether we could inherit things that happen to us in our environment. For example, if your parents had lifted weights and became super strong, would they have passed that on to you when you were born? The answer is a definite NO. What can be passed on to children is how much capability you will have for responding to the lifting of weights.

As we have seen, in cells, the instructions begin in the genes, which are found in the structures called chromosomes inside the cell nucleus. Chromosomes contain the DNA, and specific sequences of base pairs code for specific amino acids, which are the building blocks of proteins. Because the instructions for building proteins are contained in the nucleus, the nucleus is considered “information central” or the “brain” of the cell.

NATURE OF GENETIC CODING

From Genes to Proteins

What We Know

Have you ever played with "secret decoder rings" that you sometimes get in cereal boxes? Or perhaps you have played games where the object of the game is to figure out a code - to discover what the various symbols or letters represent. In order to discover the genetic code in humans, scientists first had to learn about the structures that contain the genetic code, the chromosomes. Humans have 23 pairs of chromosomes in most of their cells, 46 chromosomes total per cell. The reproductive cells have only 23 chromosomes. Can you think of a reason why a sperm or egg have only 23 and not 46? That’s right! It’s because they come together at fertilization to make a total of 46. So you know that you get half of your chromosomes from your father through the sperm, and half from your mother through the egg.

The sperm and egg contain 23 chromosomes, which combine to create an offspring with 46 chromosomes. In this image, only one chromosome of the 23 is shown for simplicity of the image. This shows one of the chromosomes coming from the father and the other from the mother. The child has two chromosomes. This image is from https://www.genomicseducation.hee.nhs.uk/education/core-concepts/where-does-our-genome-come-from/. This website has a more in-depth explanation of this process if you would like to learn more.

NATURE OF GENETIC CODING

From Genes to Proteins

What We Know

This image shows a complete set of chromosomes for a male and female human! Notice the 23^rd pair of chromosomes is different in the male and female. This set is the chromosomes that determine whether the individual is genetically male or female. If the individual has two “x” chromosomes, they are genetically female, if they have an “x” and a “y” then they are genetically male. Different species have different numbers of chromosomes. As you might notice from the image below, number of chromosomes, does not correlate with complexity of a species. 

GENES

From Genes to Proteins

What We Know

As shown in the previous section of this unit, the coding for inheritance originates in the sequence of molecules in the DNA of chromosomes. "DNA" stands for deoxyribonucleic acid. This name comes from the fact that the compound is acidic and found in the cell nucleus ("nucleic acid") and is built up of a 5-carbon sugar, called ribose, that lost an oxygen atom when it was generated ("deoxyribose"). A "gene" is that length of DNA that is needed to code for a complete protein. There are only four coding molecules in DNA:

• adenine (A) 
• thymine (T)
• cytosine (C) 
• guanine (G)

They occur in bonded pairs, either A-T or C-G, called base pairs. The base pairs are held together like a ladder by hydrogen bonds (dashed lines in diagram above). A "gene map" shows the sequence of pairs, for example in the diagram to the right: C-G, A-T, G-C, and T-A.

We have said that genes provide the code for making proteins. A gene is a section of the chain of A-T and C-G sequences that codes for a particular protein. Proteins are made as a string of compounds called amino acids (see the unit on proteins). It takes three base pairs to code for a given amino acid. Thus, the first three pairs code for one amino acid, the next three pairs code for another amino acid, and so on, until the complete sequence of a protein is accounted for.

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

What is DNA replication?

When a cell divides, the DNA must be duplicated, so that each of the two daughter cells has a complete set of DNA. To do this, the pairs pull apart (think of it like unzipping a zipper). Each half of the unzipped chain is used to form a duplicate. Because A only binds with T and C with G, each newly formed half becomes an automatically correct copy. That way, the cell that is dividing has two sets of DNA for the two new daughter cells.

How does DNA code for proteins?

To make proteins, the code in the DNA has to be translated to a language code that can make amino acids line up in the proper order. The process is a little more complicated than it might seem. DNA does not make proteins directly. There are two major in-between steps.

Image source: Crabtree + Company. Credit: National Institute of General Medical Sciences. https://images.nigms.nih.gov/pages/DetailPage.aspx?imageid2=2543

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

Step 1: DNA Transcription

The first step is to transcribe, or copy, the code in DNA to another compound called "messenger RNA". You can think of DNA like a zipper that unzips when it is time to read the code. For an informational animated video that shows the difference between DNA and RNA, click here. This is similar to changing a computer program (DNA code) into a written statement (messenger RNA). Remember, DNA does nothing unless the code is read.

Messenger RNA (mRNA) is a small molecule (green in diagram to the right) that copies the message of the DNA code. The message can be copied because mRNA has base pairs that preserve the DNA code. mRNA is just like DNA except that a base called Uracil is substituted for Thymine. Uracil performs the same function of matching with adenine.

When a gene is actively making protein, it "unzips" partially so that base pairs separate, and each member can match specific base pairs in the mRNA. Thus, mRNA preserves the code in DNA. The key point is that RNA chains of bases are smaller than DNA and they can move out of the nucleus into other parts of the cell.

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

Step 2: RNA Translation

The second step in coding is to translate the code in mRNA to protein-code language. The "language" of proteins is the sequence of amino acids.

In stringing together different amino acids to make a protein (called a polypeptide), the RNA code must be able to deal with the 20 known amino acids. If the code were based on only one base compound (A or U or C or G), then the code could only translate for four amino acids. If a PAIR of bases compounds served as the code, then more amino acids could be specified, but still not enough to account for all 20 amino acids that appear in proteins. Thus, mathematically we know that it must take at least three nucleic acids to code 20 amino acids. Experimentally, that has been demonstrated, and a Nobel Prize was awarded for that discovery.

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

To understand how we can go from these three nucleotides to a protein, we must bring in another player: tRNA. Transfer RNA or tRNA is a complex made of three RNA bases and an amino acid. These three bases complement mRNA codons (sequences of three base pairs that code for an amino acid). tRNAs match up their sequence to the mRNA, which allows the amino acids to bind together in a specific order; the order determined by the mRNA. This allows for a new amino acid to be added to the chain based on the mRNA code.

Image source: Crabtree + Company. Credit: National Institute of General Medical Sciences. https://images.nigms.nih.gov/pages/DetailPage.aspx?imageid2=2549

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

This process of translation happens inside of an organelle called a ribosome. This organelle is made up of proteins and ribosomal RNA (rRNA). Ribosomes can be found floating around in the cytosol or attached to a membranous organelle called the Rough Endoplasmic Reticulum or RER. It’s “rough” because of the ribosomes attached to the membrane. Proteins made in the cytosol usually stay inside the cell whereas proteins synthesized in the RER eventually go to either the cell membrane or are secreted outside the cell.

The final protein product is typically moved into another membrane system, an organelle called Golgi body, which "fine tunes" the proteins by adding certain sugar residues. It’s these proteins that build the cell and the organism and build the traits that make each organism individual. All of this starts in the tiny part of the cell, the nucleus. It’s easy to see now why it’s the ‘information central” of the cell.

Click here to see a fun animated video that shows the whole process of protein synthesis, including transcription and translation. A great site that explains these things in a different way can be found by clicking here.

Image source: By Gabi Slizewska

DNA REPLICATION, TRANSCRIPTION, AND TRANSLATION

From Genes to Proteins

What We Know

As you can now see, this unit covered a lot of ground. From Mendel’s first experiments in heredity to Nobel prizes awarded for discovering the structure of DNA and the process of transcription, we have come a long way in learning about how our parents pass on their traits to us. An interesting thought, though, is how much we still have to learn about genetics! There is still so much more to explore in this field.

To summarize this unit, you should now be able to:

• Define heredity as the passage of genetic instructions from one generation to the next generation.
• Recognize that inherited traits of individuals are governed in the genetic material found in the genes within chromosomes in the nucleus.
• Understand Mendel's experiments and findings.
• Explain the difference between dominant and recessive genes.
• Explain the purpose and process of transcription and translation in making proteins for the body.

GREGOR MENDEL (1822-1884)

From Genes to Proteins

Story Time

Many scientists call him the "father of genetics." But others know of him as a sickly child who became a priest because he could not stand the rigors of a life in the world outside of a monastery. There is good reason for both views.

CHILDHOOD

From Genes to Proteins

Story Time

His childhood name was Johann, which he changed to Gregor when he became a priest. Johann was born in 1822 to peasant farmers in the Czech Republic part of Eastern Europe. The place of birth was a village called Heinzendorf, which is now called Hyncice. The area borders where Germany, Poland, and the Czech Republic come together.

The town of Heinzendorf occupies both sides of a stream that carves its way through the rolling countryside. Most of the houses had two stories with slate roofs, as they still do today. The people live in the town and work the farms in the countryside. At the time when Johann was born the town had 102 families.

Johann was an only son and had four sisters. He was born in 1822.

Not much is known about Johann's childhood. Johann himself did not say much about it in his short autobiography, which was written in the third person (see the book by Olby).

Another thing that influenced his later life was his puzzlement about why he had a mixture of his parents' traits, but his sisters did not. Johann had the short stocky build of his father and the cheerfulness and language skills of his mother. His elder sister was more like the father, both in appearance and disposition, while a younger sister was more like the mother.

We do know that Johann worked with his father a lot in the family orchard, which stimulated his interest in the things of nature. He learned to love growing plants and gardening, a love that he brought to his scientific research when he grew up.

MENDEL’S EDUCATION

From Genes to Proteins

Story Time

Up until about 20 years before Johann's birth, his village had no school. Most children were unable to read or write. But a school was there for Johann. He and the 80 other children in the school learned their "3 Rs," plus the essentials of fruit growing and bee keeping.

The only teacher in the school, Thomas Makitta, was quick to realize that Johann had special ability, being what we would call today "gifted and talented." Johann had heard about a more exciting school at Leipnik, a town about 13 miles away. It was much like today's middle schools. Two boys in Heinzendorf were going to this school, and on their vacations they impressed Johann and the other local children with all the new things they were learning.

Johann and his teacher pleaded with the parents to send him off to this school. Johann's father wanted him to stay on the farm and be prepared to run it upon inheritance. But his father also knew that education was the only way Johann would escape from the narrow and hard life of a peasant. In those times, the peasants were partial slaves, being required to work three days a week for the Lord of the Manor.

So Johann did go to this larger school, and he quickly achieved top-of-the-class standing.

MENDEL’S EDUCATION

From Genes to Proteins

Story Time

In order to go to college, Johann had to work to pay the bills. He tried to get work as a private tutor but failed because of lack of friends and contacts with people who could give him recommendations. He became so stressed that he became sick and had to go back home to recuperate. It took a year before he was well enough to go back to college. Even after he went back, his health broke down again and again. He had to drop out after the first two years. His college professor, Friederick Franz, had taught at the Augustinian monastery in Brno and had been asked to become a scout for promising candidates. He was impressed with Johann, whom he described as "a young man of very solid character. In my own branch (of science) he is almost the best."

When he was 21, Johann decided to become a monk and dedicate his life to religious contemplation and scientific research. Like many young men of his time, Johann (now Gregor) found security and happiness in the life of a monastery. Four years later, he was ordained a priest. He attended the University of Vienna to study science and mathematics before returning to the monastery. His duties at first included ministering to dying people in the hospital, but this was so stressful for Gregor that his superiors assigned him to teaching and research.

MENDEL’S EDUCATION

From Genes to Proteins

Story Time

He took a leave of absence in 1851 to attend the University of Vienna, where he wanted to study to become a teacher. However, he apparently flunked the elementary teacher's examination. But he returned to the monastery and became a teacher anyway, teaching math and natural science.

As a teacher, Gregor became an immediate success and was very popular with both staff and students. As a researcher, Gregor puttered around in a garden, and as we will see, Gregor more or less invented the science of genetics.

But Gregor's goal was not to found genetics. In college, his botany teachers impressed him with Darwin's recently announced the theory of evolution. This set Gregor to thinking more deeply about how things worked in the selective breeding of plants and animals that was practiced by farmers in his home village and throughout the region. In particular, he wanted to know more about if and how new species might be created from mating parents of different characteristics. He never created any new species, but he did create many different strains of the same species by cross breeding various kinds of peas in his monastery garden.

MENDEL AS A “HIRED GUN” FOR THE LOCAL ECONOMY

From Genes to Proteins

Story Time

Many cities in America compete with each other to lure high-tech industries to stimulate the local economy. Especially popular are the biotech companies that develop genetically engineered animals, plants, and medicines. Would you believe that a similar circumstance helped create the whole science of genetics over 135 years ago?

Many people think that Mendel stumbled on basic genetic principles by puttering around in his garden. That is the way the story is usually told. But Gregor was specifically recruited to the monastery because he was scientifically trained and because the people of the countryside had a long tradition in selective breeding of livestock and plants. Of special interest to the farmers and the city fathers was the breeding programs for sheep, because wool had to be imported from Spain. The Brno civic leaders had founded several scientific societies for the purposes of improving the town's prestige and economy. So, in some ways, Gregor was a "hired gun," who was expected to do something for the local economy. Although Gregor worked in a monastery, today we would call his work environment an incubator focused on promoting scientific progress in agricultural biotechnology.

MENDEL’S LEGACY

From Genes to Proteins

Story Time

Mendel published a now-famous research report in 1865 in the Journal of the Brünn Society of Natural Science. This was not a mainstream scientific journal, and the paper went generally unnoticed and unappreciated by the scientific world.

Another thing that kept Mendel's discoveries from being appreciated was that at that time, nobody knew anything about the physical basis for heredity. Mendel was only able to document some of the main principles of heredity with abstract mathematics.

A copy of the original paper is at http://www.mendelweb.org/MWpaptoc.html.

Gregor knew he had discovered fundamental principles and was most depressed because he could not persuade others of his conclusions. And he never received recognition is his lifetime. He died in obscurity in 1884. It took 35 years before three competing botanists had discovered Gregor's manuscripts and realized that their own research had to be interpreted in the light of Gregor's data and conclusions. In 1900, they christened Gregor's conclusions as "Mendel's Laws."

MENDEL’S LEGACY

From Genes to Proteins

Story Time

Today, a university in Brno is named in honor of Gregor (http://mendelu.cz/en/). There is not much of a memorial today to Gregor's pea garden in the courtyard of the monastery. All that remains is a small museum, the stone foundation of the garden hothouse, a grass yard, and a lone sycamore tree. But a group of researchers has drafted ambitious plans to build a major modern genetics center and research institute in Brno.

What a fitting tribute this would be to Gregor. This would also overcome the stigma that was imposed on the science of Czechoslovakia by the Russian Communists who banned the teaching and practice of Mendelian genetics. The Communists, believing that all humans were perfectible from experience and teaching, insisted that personal traits were acquired only from the environment and personal experience. Scientists who talked about genes and Mendel's laws were often banished to prisons in Siberia.

Today, all the world knows that Gregor was right. Not only that, but all the miracles of modern genetic engineering that are now unfolding would not be possible had it not been for those simple experiments that Gregor conducted in his pea garden.

Modern day research still uses some of Mendel's original approaches. Here, U.S. Department of Agriculture scientists are pollinating sunflowers to selective breed plants that produce certain kinds of oils.

REFERENCES

From Genes to Proteins

Story Time

Iltis, H. 1966. Life of Mendel. Hafner Publishing Co., New York, N.Y.

Olby, Robert C., The Origins of Mendelism. 2d ed. University of Chicago Press. 1985.

https://www1.villanova.edu/villanova/president/university_events/mendelmedal/aboutmendel.html

From Genes to Proteins

Common Hazards

A TOXIC SUBSTANCE - AFLATOXIN

From Genes to Proteins

Common Hazards

Aflatoxins are a family of toxic substances (toxins) produced by certain types of fungi that are found in crops such as corn, peanuts, cottonseed, and nuts. The main fungi that produce aflatoxins are Aspergillus flavus and Aspergillus parasiticus, which are abundant in warm and humid regions of the world. Aflatoxin-producing fungi can contaminate crops in the field, at harvest, and during storage. Aflatoxin causes liver cancer because it causes chemical bonds to form on pieces of DNA strands that trigger cancerous DNA replication and cell division. Aflatoxins have been a health problem for a long time. The Chinese knew and wrote about mycotoxins (toxins produced by fungi) thousands of years ago.

A TOXIC SUBSTANCE - AFLATOXIN

From Genes to Proteins

Common Hazards

How are we exposed to aflatoxins?

People get exposed to aflatoxins from the foods they eat. Fungi can grow on grain and food at any stage, from pre-harvest to the time it is eaten, without being detected. The fungi can leave behind these aflatoxins, which we cannot see, and aflatoxins grow as the feed is being processed for animals and humans to eat. It is hard to know if a food contains these substances because we can't taste them or, sometimes, even see them. No level of aflatoxin has been shown to be completely safe.

Of the many mycotoxins (toxins produced by fungi) that occur as natural products in foods, aflatoxins are the only ones that are currently regulated in the United States. We know that these aflatoxins are "unavoidable contaminants" in our food. So, the U.S. Food and Drug Administration (FDA) regulates aflatoxins by setting a limit on how much contamination is allowed. For example, if corn has more than 20 parts per billion of aflatoxin, then it is considered unsafe and cannot be sold to be processed into food. Aflatoxins are found in grains such as corn and are especially a problem during extended periods of drought.

Four types of the aflatoxins have been reported to occur naturally and one, aflatoxin B₁, is the strongest toxin and can cause cancer. The young of all species of animals are most affected by aflatoxins and can have a wide range of problems following ingestion, including digestive distress, anemia, jaundice, reduced appetite and decreased growth.

A TOXIC SUBSTANCE - AFLATOXIN

From Genes to Proteins

Common Hazards

Why are aflatoxins a concern?

Aflatoxin has attracted a lot of concern due to its frequent occurrence in human food and its ability to cause cancer by triggering faulty DNA replication. A significant percentage of the world's grain and oil seed supply is contaminated with aflatoxins. 

Aflatoxins can survive heat and a variety of processing procedures. They occur as "unavoidable" contaminants of corn, peanuts, peanut butter, breakfast cereals, cornmeal, tortillas, and milk. Dairy cows can have some of these toxins in their milk following the ingestion of feed containing aflatoxins.

A TOXIC SUBSTANCE - AFLATOXIN

From Genes to Proteins

Common Hazards

How can you avoid exposure to aflatoxins?

• Throw away all food contaminated with mold.
• Do not leave foods uncovered for long periods of time. Store them in a cool, dry place.
• Buy only major brands of nuts and peanut butters.
• Discard any nuts that look moldy, discolored or shriveled.

Other kinds of mycotoxins

Some kinds of mold make toxins that are carried in the air. Old, unclean, and moist buildings are especially like to have such mold toxins. This situation is often referred to as "sick building syndrome" and can be a major hazard to public health. Click here for more information.

From Genes to Proteins

Activities

CLICK TO VIEW AN ACTIVITY

From Genes to Proteins

Activities

• Student Activity Sheet #1 - Secret Codon Write a Message in DNA!
• Student Activity #5 - Squishy Strawberry Science

From Genes to Proteins

Self-Study Game

CLICK BELOW TO PLAY A GAME!

From Genes to Proteins

Self-Study Game

• Quizizz

From Genes to Proteins

Post-Test

• Google assessment

From Genes to Proteins

Glossary

Allele - one of two or more versions of a gene that arises out of mutation found in the same place on a chromosome. Example, there were two alleles (green and yellow) for the gene controlling pea color in Mendel’s experiments. Return to How We Know

Genome - the complete set of genes or genetic material in an organism. Includes coding and non-coding DNA. Return to How We Know

Heredity - the passage of genetic information from one generation to the next. Return to How We Know

Inherited - something passed down from parent to offspring through sexual or asexual reproduction. Return to How We Know

Transcribe - process by which enzymes use a fragment of DNA to make a complementary strand of mRNA which is used in protein production. Return to What

We Know

Translate - process that takes place in ribosomes where mRNA strands are paired with complementary tRNA segments to code for and create a polypeptide strand. Return to What We Know