- Identify the structure and functions of RNA.
- Describe the genetic code and how to read it.
- Explain how proteins are made.
- List causes and effects of mutations.
- genetic code
- protein synthesis
- RNA (ribonucleic acid)
Blueprints, like those pictured in Figure below, contain the instructions for building a house. Your cells also contain “blueprints.” They are encoded in the DNA of your chromosomes.
Blueprints for a house
DNA, RNA, and Proteins
DNA and RNA are nucleic acids. DNA stores genetic information. RNA helps build proteins. Proteins, in turn, determine the structure and function of all your cells. Proteins consist of chains of amino acids. A protein’s structure and function depends on the sequence of its amino acids. Instructions for this sequence are encoded in DNA.
In eukaryotic cells, chromosomes are contained within the nucleus. But proteins are made in the cytoplasm at structures called ribosomes. How do the instructions in DNA reach the ribosomes in the cytoplasm? RNA is needed for this task.
Comparing RNA with DNA
RNA stands for ribonucleic acid. RNA is smaller than DNA. It can squeeze through pores in the membrane that encloses the nucleus. It copies instructions in DNA and carries them to a ribosome in the cytoplasm. Then it helps build the protein.
RNA is not only smaller than DNA. It differs from DNA in other ways as well. It consists of one nucleotide chain rather than two chains as in DNA. It also contains the nitrogen base uracil (U) instead of thymine (T). In addition, it contains the sugar ribose instead of deoxyribose. You can see these differences in Figure below.
Comparison of RNA and DNA
Types of RNA
There are three different types of RNA. All three types are needed to make proteins.
- Messenger RNA (mRNA) copies genetic instructions from DNA in the nucleus. Then it carries the instructions to a ribosome in the cytoplasm.
- Ribosomal RNA (rRNA) helps form a ribosome. This is where the protein is made.
- Transfer RNA (tRNA) brings amino acids to the ribosome. The amino acids are then joined together to make the protein.
The Genetic Code
How is the information for making proteins encoded in DNA? The answer is the genetic code. The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the “letters” of the code. Groups of three bases each make up code “words.” These three-letter code words are called codons. Each codon stands for one amino acid or else for a start or stop signal.
There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals. You can see how to translate the genetic code in Figure below. Start at the center of the chart for the first base of each three-base codon. Then work your way out from the center for the second and third bases.
Translating the genetic code
Find the codon AUG in Figure above. It codes for the amino acid methionine. It also codes for the start signal. After an AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. You can see how this works in Figure below. The figure shows the bases in a molecule of RNA. The codons are read in sequence until a stop codon is reached. UAG, UGA, and UAA are all stop codons. They don’t code for any amino acids.
How the genetic code is read
Characteristics of the Genetic Code
The genetic code has three other important characteristics.
- The genetic code is the same in all living things. This shows that all organisms are related by descent from a common ancestor.
- Each codon codes for just one amino acid (or start or stop). This is necessary so the correct amino acid is always selected.
- Most amino acids are encoded by more than one codon. This is helpful. It reduces the risk of the wrong amino acid being selected if there is a mistake in the code.
The process in which proteins are made is called protein synthesis. It occurs in two main steps. The steps are transcription and translation. Watch this video for a good introduction to both steps of protein synthesis: http://www.youtube.com/watch?v=h5mJbP23Buo.
Transcription: DNA → RNA
Transcription is the first step in protein synthesis. It takes place in the nucleus. During transcription, a strand of DNA is copied to make a strand of mRNA. How does this happen? It occurs by the following steps, as shown in Figure below.
- An enzyme binds to the DNA. It signals the DNA to unwind.
- After the DNA unwinds, the enzyme can read the bases in one of the DNA strands.
- Using this strand of DNA as a template, nucleotides are joined together to make a complementary strand of mRNA. The mRNA contains bases that are complementary to the bases in the DNA strand.
Transcription step of protein synthesis
Translation is the second step in protein synthesis. It is shown in Figure below. Translation takes place at a ribosome in the cytoplasm. During translation, the genetic code in mRNA is read to make a protein. Here’s how it works:
- The molecule of mRNA leaves the nucleus and moves to a ribosome.
- The ribosome consists of rRNA and proteins. It reads the sequence of codons in mRNA.
- Molecules of tRNA bring amino acids to the ribosome in the correct sequence.
- At the ribosome, the amino acids are joined together to form a chain of amino acids.
- The chain of amino acids keeps growing until a stop codon is reached. Then the chain is released from the ribosome.
Translation step of protein synthesis
Causes of Mutations
Mutations have many possible causes. Some mutations occur when a mistake is made during DNA replication or transcription. Other mutations occur because of environmental factors. Anything in the environment that causes a mutation is known as a mutagen. Examples of mutagens are shown in Figure below. They include ultraviolet rays in sunlight, chemicals in cigarette smoke, and certain viruses and bacteria.
Examples of mutagens
Effects of Mutations
Many mutations have no effect on the proteins they encode. These mutations are considered neutral. Occasionally, a mutation may make a protein even better than it was before. Or the protein might help the organism adapt to a new environment. These mutations are considered beneficial. An example is a mutation that helps bacteria resist antibiotics. Bacteria with the mutation increase in numbers, so the mutation becomes more common. Other mutations are harmful. They may even be deadly. Harmful mutations often result in a protein that no longer can do its job. Some harmful mutations cause cancer or other genetic disorders.
Mutations also vary in their effects depending on whether they occur in gametes or in other cells of the body.
- Mutations that occur in gametes can be passed on to offspring. An offspring that inherits a mutation in a gamete will have the mutation in all of its cells.
- Mutations that occur in body cells cannot be passed on to offspring. They are confined to just one cell and its daughter cells. These mutations may have little effect on an organism.
Types of Mutations
The effect of a mutation is likely to depend as well on the type of mutation that occurs.
- A mutation that changes all or a large part of a chromosome is called a chromosomal mutation. This type of mutation tends to be very serious. Sometimes chromosomes are missing or extra copies are present. An example is the mutation that causes Down syndrome. In this case, there is an extra copy of one of the chromosomes.
- Deleting or inserting a nitrogen base causes a frameshift mutation. All of the codons following the mutation are misread. This may be disastrous. To see why, consider this English-language analogy. Take the sentence “The big dog ate the red cat.” If the second letter of “big” is deleted, then the sentence becomes: “The bgd oga tet her edc at.” Deleting a single letter makes the rest of the sentence impossible to read.
- Some mutations change just one or a few bases in DNA. A change in just one base is called a point mutation. Table below compares different types of point mutations and their effects.
Types of point mutations
mutated codon codes for the same amino acid
CAA (glutamine) → CAG (glutamine)
mutated codon codes for a different amino acid
CAA (glutamine) → CCA (proline)
mutated codon is a premature stop codon
CAA (glutamine) → UAA (stop)
- DNA encodes instructions for proteins. RNA copies the genetic code in DNA and carries it to a ribosome. There, amino acids are joined together in the correct sequence to make a protein.
- The genetic code is based on the sequence of nitrogen bases in DNA. A code “word,” or codon, consists of three bases. Each codon codes for one amino acid or for a *Protein synthesis is the process in which proteins are made. In the first step, called transcription, the genetic code in DNA is copied by RNA. In the second step, called translation, the genetic code in RNA is read to make a protein.
- A mutation is a change in the base sequence of DNA or RNA. Environmental causes of mutations are called mutagens. The effects of a mutation depend on the type of mutation and whether it occurs in a gamete or body cell.
Lesson Review Questions
- What are three types of RNA? What role does each type play in protein synthesis?
- Describe the genetic code and its characteristics.
- Give an overview of the transcription step of protein synthesis. Where does it take place?
- What is a mutation? What are some causes of mutations?
- Use Figure above to translate the following sequence of RNA bases into a chain of amino acids: AUGUACCCCACAGACUAA.
- Compare and contrast RNA and DNA.
- Explain what happens during the translation step of protein synthesis.
- Why is a single base insertion or deletion likely to drastically change how the rest of the genetic code is read?
Points to Consider
Offspring generally resemble their parents. This is true even when the offspring are not genetically identical to the parents.
- Can you apply your knowledge of reproduction and protein synthesis to explain why offspring and parents have similar traits?