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Genes and Gene Action
Genes and Proteins
Now that it has been shown that DNA is what makes up the genetic material, it is time to look more
closely at genes. What is a gene? Genes are really packages of information that tell a cell how to make
proteins. Proteins are polymers, or long chains, of amino acids. As you learned already, there are 20
different types of amino acids. The order in which the amino acids are joined determines which protein
is made. Every different protein has a unique sequence of amino acids. This sequence determines the
shape of a protein molecule. It is the shape of the protein that allows the molecule to do its work in the
cell.
Genes are specific sections of DNA molecules that are
made up of linear sequences of subunits. Proteins are
linear sequences of amino acids. How do cells use a
linear sequence of subunits in DNA to build a linear
sequence of amino acids for a protein? In all cells,
except for bacteria, DNA is stored in the nucleus. Yet
protein synthesis occurs outside the nuclear membrane,
at the ribosomes. These small organelles are distributed
throughout the cytoplasm. How does the genetic
information in DNA within the nucleus get to the
ribosomes? A third type of molecule, ribonucleic acid,
or RNA, works as a helper to transfer the information.
That is, the genetic information flows from the DNA to
the RNA to a protein. (See Figure 20-1.)
From DNA to RNA
Each gene is a portion of a chromosome, in
effect a portion of the DNA chain. An RNA
molecule called messenger RNA does the job
of moving the information in the base
sequence out to the ribosomes. DNA is copied
into RNA by a process that is similar to DNA
replication. The DNA double helix opens up
where a particular gene is located. Special
enzymes begin to match up RNA subunits
with the correct DNA subunits. The new RNA
molecule has the same base sequence as one strand of the original DNA. This RNA molecule then goes
out of the nucleus through pores in the nuclear membrane to ribosomes in the cytoplasm. (See Figure
20-2.)
From RNA to Protein
So far, the genetic information, stored as a base sequence, has moved from the nucleus to the
cytoplasm by using RNA .Another problem remains: how to use the nucleotide base sequence
Adapted from AMSCO Genetics and Molecular Biology Revised Edition
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in the RNA to build a protein with the correct sequence of amino acids. This problem involves a
change of “language,” from the base sequence language of RNA into the amino acid language of
proteins. This process is called translation, and it occurs at the ribosome. Built into every living cell in
the world is a genetic code. It is called the triplet code. Each different combination of three bases
makes up a word, called a codon. Each codon represents a specific amino acid. Each of the 20 amino
acids has at least one codon, and most have more than one. This genetic code is universal; in other
words, all organisms on Earth use the same genetic code. For example, the codon GCA stands for the
amino acid alanine in all life-forms, from bacteria to trees to humans. This similarity among living
things is good evidence that all organisms evolved from a common ancestral life-form in Earth’s
distant past. (See Figure 20-3.)
Mutations: A Closer Look
A mutation is defined as a change in the base sequence of a DNA molecule. The possible effects of a
mutation can now be explained in terms of what you know about protein synthesis.
The order of bases in DNA determines the order of amino acids in proteins. In certain cases, a mutation
in one subunit will change the triplet code, which in turn may make a change in an amino acid. If this
change occurs in a body cell, then all other cells in the organism’s body that reproduced (through
mitosis) from that cell will have the same change. It is more important, however, if the mutation occurs
in the DNA of a gamete. If that gamete fuses with another gamete in sexual reproduction, then the
mutation will be inherited. The change in the DNA will be passed on to succeeding generations. The
new organism will have the mutation, as will all offspring of that organism. This will be an inherited
condition. If the mutation is harmful, the individual and its offspring will have a genetic disease.
Adapted from AMSCO Genetics and Molecular Biology Revised Edition
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Gene Expression Cell Differentiation
Chromosomes contain
extremely long DNA
molecules. Many genes are
stretched out along these
molecules. For example, it
is estimated that there are
20,000 to 24,000 different
genes in human cells. After
fertilization, every cell of a
growing organism arises
from the mitotic cell
division of other cells.
Through mitosis, every cell
in our body has the same
46 chromosomes with the
same DNA as the original
fertilized egg cell.
There are different types of
cells in our bodies. We
have skin cells, muscle
cells, bone cells, nerve
cells, blood cells, and so
on. If all of these cells have
the same DNA, why are
they so different from each
other? The answer is that
only certain genes are used in certain cells. The use of specific information from a gene is called gene
expression. Proteins are synthesized only from genes that are being expressed, or “turned on.” All
other genes in the cell are kept silent, or “turned off.” This gives the cell its own structure, enzymes,
functions, and physical characteristics. A muscle cell contracts, a nerve cell transmits an impulse, and a
skin cell helps form a flat, protective layer. The process by which special types of cells are formed
through controlled gene expression is called cell differentiation. This is an essential process of life.
Without cell differentiation, we could not survive, because our bodies would be made up of only one
type of cell. While the exact process is not known for certain, it is thought that environmental factors both outside and inside each cell - influence gene expression. (See Figure 20-4.)
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Review Questions
1. Genes can best be described as
A. directions for making DNA
B. directions for making proteins
C. subunits of proteins
D. molecules that transfer information out of the nucleus
2. Which path correctly describes the flow of information in cells?
A. DNA→ RNA→ protein
B. protein → RNA→ DNA
C. protein → DNA→ RNA
D. RNA→ DNA→ protein
3. The kinds of genes that an organism has is determined by the
A. type of amino acids in its cells
B. size of simple sugar molecules in its organs
C. sequence of the subunits A, T, C, and G in its DNA
D. shape of the protein molecules in its organelles
4. A change in the order of DNA bases that code for a respiratory protein will most likely cause
A. the production of a starch that has a similar function
B. a change in the sequence of amino acids determined by the gene
C. the digestion of the altered gene by enzymes
D. the release of antibodies by certain cells to correct the error
5. The role of messenger RNA is to
A. prevent mutations during DNA replications
B. match ribose-containing subunits to subunits of DNA
C. move the information in a base sequence out to the ribosomes
D. translate the base sequence at the ribosomes
6. RNA receives information from DNA by
A. binding with a double helix as a third strand
B. matching with subunits of a single strand of DNA
C. making an exact copy of the DNA molecule
D. accepting proteins through pores in the nuclear membrane
7. What happens at the ribosome?
A. The DNA strands separate.
B. RNA matches up with DNA strands.
C. Genetic information is mutated.
D. RNA is translated into amino acids.
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8. The diagram below represents a process that occurs within a cell in the human pancreas. This
process is known as
A.
B.
C.
D.
digestion by enzymes
energy production
protein synthesis
replication of DNA
9. How many bases make up a codon?
A. one
B. two
C. three
D. four
10. What does a codon represent?
A. a specific amino acid
B. a specific base
C. an RNA molecule
D. an enzyme
11. The genetic code is
A. different for every organism
B. the same for all organisms
C. constantly changing
D. impossible to identify
12. The sequence of amino acids in a protein is determined by the
A. speed at which translation occurs
B. size of the cell involved
C. number of ribosomes in a cell
D. order of bases in the DNA
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13. The diagram below provides some information concerning proteins. Which phrase does the
letter A represent?
A.
B.
C.
D.
Sequence of amino acids
Sequence of starch molecules
Sequence of simple sugars
Sequence of ATP molecules
14. A mutation is inherited if it
A. occurs in a gamete used in sexual reproduction
B. occurs in a cell that undergoes mitosis
C. gives the organism a better chance for survival
D. endangers the organism’s chance for survival
15. People with cystic fibrosis inherit defective genetic information and cannot produce normal
CFTR proteins. Scientists have used gene therapy to insert normal DNA segments that code for
the missing CFTR protein into the lung cells of people with cystic fibrosis. Which statement
does not describe a result of this therapy?
A. Altered lung cells can produce the normal CFTR protein.
B. The normal CFTR gene may be expressed in altered lung cells.
C. Altered lung cells can divide to produce other lung cells with the normal CFTR gene.
D. Offspring of someone with altered lung cells will inherit the normal CFTR gene.
16. About how many genes are contained on the 46 chromosomes in each human body cell?
A. 200–1000
B. 5000–10,000
C. 20,000–24,000
D. 50,000–100,000
17. The cells that make up a person’s skin have some functions that are different from those of the
cells that make up the person’s liver. This is because
A. all types of cells have a common ancestor
B. environment and past history have no influence on cell function
C. different cell types have completely different genetic material
D. different cell types use different parts of the genetic instructions
18. Gene expression means that
A. different genes are found in different cells
B. genes are passed through the nuclear membrane
C. only some genes are turned on in each type of cell
D. some cells have genes and some cells do not
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19. Scientific studies have shown that identical twins who were separated at birth and raised in
different homes may vary in height, weight, and intelligence. The most probable explanation
for these differences is that
A. the original genes of each twin increased in number as they developed
B. the environments in which they were raised were different enough to affect the
expression of their genes
C. one twin received genes only from the mother while the other twin received genes only
from the father
D. the environments in which they were raised were different enough to change the genetic
makeup of both individuals
20. After a series of cell divisions, an embryo develops different types of body cells, such as
muscle cells, nerve cells, and blood cells. This development occurs because
A. the genetic code changes as the cells divide
B. different genetic instructions are created to meet the needs of new types of cells
C. different segments of the genetic instructions are used to produce different cell types
D. some sections of the genetic material are lost as a result of
fertilization
21. The letters in the diagram at right represent genes on a particular
chromosome. Gene B contains the code for an enzyme that cannot be
synthesized unless gene A is also active. Which statement best explains
why this can occur?
A. A hereditary trait can be determined by more than one gene.
B. All the genes on a chromosome act to produce a single trait.
C. Genes are made up of double stranded segments of DNA.
D. The first gene on each chromosome controls all the other genes.
22. Why is the location of DNA within the nucleus a potential problem for protein synthesis? How
does RNA solve this problem for the cell? ______________________________________
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23. Arrange the following structures from the largest to the smallest: DNA molecule; chromosome;
nucleus; gene. ____________________________________________________________
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Base your answers to questions 24 & 25 on the table below, which provides the DNA codes for
several amino acids.
24. A certain DNA strand has the following base sequence: TACACACAAACGGGG. What is the
sequence of amino acids that would be synthesized from this code (if it is read from left to
right)?
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25. Suppose the DNA sequence undergoes the following change: TACACACAAACGGGG →
TACACCCAAACGGGG. How would the sequence of amino acids be changed as a result of
this mutation?
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26. Explain why the genetic code is called universal. What is the evolutionary significance of this
fact?
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Adapted from AMSCO Genetics and Molecular Biology Revised Edition
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