Download learning objectives

13 HOW GENES WORK
CHAPTER OUTLINE
LEARNING OBJECTIVES












Understand the experiments that led to the discovery that DNA is the genetic material.
Describe the double helix structure of DNA in which base pairing occurs.
Explain what is meant by semiconservative.
List the steps of DNA replication.
Describe how mRNA is formed from the transcription of DNA.
Explain how translation proceeds, and know the roles of ribosomes and tRNA in protein synthesis.
Understand how the DNA molecule indicates where to start transcription at the promoter site.
Realize that gene expression is carefully controlled.
Describe exons, introns, and transposons.
Know that genes exist in multiple copies called multigene families.
Explain how repressor, activator, and enhancer proteins operate to regulate gene expression.
Explain how mutagens may lead to mutations in DNA.
From Gene To Protein (p. 228)
13.1
13.2
Transcription (p. 228; Fig. 13.1)
A. Transcription is the process whereby a messenger RNA (mRNA) molecule is synthesized
from a portion of the DNA molecule in the nucleus, and is the first step in gene expression.
B. The second step, called translation, occurs when the mRNA leaves the nucleus of the cell and
directs the production of a protein molecule.
C. The Transcription Process
1. Transcription uses an enzyme called RNA polymerase that binds to the DNA molecule at
a specific site called the promoter and then moves along the DNA molecule.
2. A strand of mRNA is produced whose nucleotide sequence is complementary to that of
the DNA.
Translation (p. 229; Figs. 13.2, 13.3, 13.4, 13.5, 13.6)
A. The Genetic Code
1. The genetic code is written such that a three-nucleotide sequence codes for a given
amino acid, the building blocks of proteins.
2. The mRNA sequence that corresponds to the three-nucleotide sequence on DNA is
called a codon.
3. There are 64 different possible codons in the genetic code dictionary, and the same
genetic code is employed, for the most part, by every living creature.
B. Translating the RNA Message into Proteins
1. In translation, organelles called ribosomes use the mRNA transcript to direct the
synthesis of a protein.
C. The Protein-Making Factory
1. Translation occurs in the cytoplasm in conjunction with ribosomes, which are made up
of proteins and ribosomal RNA (rRNA).
2. Ribosomes hold the mRNA in position so translation of the code can occur.
48
13.3
D. The Key Role of tRNA
1. A third type of RNA, called transfer RNA (tRNA) has on one end an anticodon, which is
a sequence of three nucleotides complementary to an mRNA codon.
2. On the other end of the tRNA molecule, is the amino acid that corresponds with the
codon of the mRNA.
E. Making the Protein
1. The role of tRNA is to bring the appropriate amino acid into position along the mRNA
molecule held by the ribosome.
2. As the ribosome proceeds along the mRNA, the next amino acid is added to the growing
peptide chain.
3. When the process is finished, the ribosome complex falls apart, and the completed
protein is released into the cell.
Architecture of the Gene (p. 232; Figs. 13.7, 13.8)
A. Introns
1. Prokaryotic DNA is made up of a continuous sequence of genes with no interruptions.
2. Eukaryotic DNA is constructed differently because it possesses gene sequences that
code for amino acids, called exons, plus intervening, nonusable sequences of
nucleotides, called introns.
3. Intron sequences must be removed from mRNA before translation can occur.
B. Gene Families
1. A number of other interesting discoveries about the nature of genes have been made in
recent years.
2. Multigene families arise when genes in cells exist in multiple copies.
C. Transposons: Jumping Genes
1. Transposons are genes that are able to jump from one position to another on a
chromosome, perhaps preventing the expression of a portion of a particular gene
sequence.
Regulating Gene Expression (p. 234)
13.4
Turning Genes Off and On (p. 234; Figs. 13.9, 13.10, 13.11)
A. Cells must also have the ability to regulate which genes will be expressed and how often
expression occurs.
B. Repressors
1. In some cases, a regulatory protein, called a repressor, is joined to its regulatory site,
known as the operator, which prevents the gene from being transcribed.
2. When the gene needs to be transcribed, a signal molecule binds to the repressor causing
it to change shape so that it can no longer prevent gene expression.
C. Activators
1. In other instances, a regulatory protein known as an activator has to help the DNA
unwind prior to transcription.
2. No repressor blocks transcription in this case, but the activator must be present for it to
proceed.
D. Enhancers
1. A third type of control over gene expression is called an enhancer.
2. Enhancers are located on the DNA molecule and help the RNA polymerase locate and
bind to the promoter site.
KEY TERMS




transcription (p. 228) The process of “reading” the DNA molecule and assembling a complementary
strand of mRNA.
mRNA (p. 228) Messenger RNA.
RNA polymerase (p. 228) A sophisticated enzyme that transcribes DNA into RNA.
genetic code (p. 229) Each three-nucleotide block in a gene corresponds to a specific amino acid.
49












codon (p. 229) The three-nucleotide sequence on mRNA that corresponds to an amino acid.
translation (p. 229 Ribosomes use mRNA to direct the synthesis of a protein.
ribosome (p. 230)
tRNA (p. 230) Transfer RNA.
anticodon (p. 230) The three-nucleotide sequence on the tRNA molecule that is complementary to the
mRNA codon.
intron (p. 232) Extra nucleotide sequences in DNA that code for nothing.
multigene family (p. 232) Genes exist in multiple copies, called multigene families.
transposons (p. 232) Jumping genes.
promoter (p. 234) The site on the DNA to which the RNA polymerase binds.
repressor (p. 234) The regulatory protein.
operon (p. 234) A cluster of genes that is transcribed as a unit.
enhancer (p. 235) Enhancers help RNA polymerase find its binding site.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
Tumor Viruses. Share with students the following information about tumor viruses:
Some forms of cancer are caused by viruses that trigger the formation of tumors. These can be either
RNA- or DNA-based viruses. A target gene is the proto-oncogene that normally functions to regulate
how cells differentiate and to produce growth factors that regulate cell division. These viruses induce
changes in proto-oncogenes, triggering them to become cancer-causing genes, or oncogenes. When
oncogenes are expressed, cells divide more rapidly than normal and do not differentiate and mature as
usual. Thus, a cancerous tumor has been produced. Scientists now believe that viruses are involved in
90–95% of all cervical cancers, and that these viruses produce other types of genital tumors. The
human papillomaviruses that cause genital warts (some of the lesions of which are flat and go
completely unnoticed) are the likely culprits that also trigger these forms of cancer. How can students
keep from acquiring or passing on such viruses?
CRITICAL THINKING QUESTIONS
1.
2.
Think of an appropriate analogy to describe the process of protein synthesis from transcription to
translation. (Hint: A factory that manufactures a product from instructions given in a control room.)
In what manner, or why, could introns have evolved in DNA?
50