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Unit 2: DNA
Mader
Inquiry into Life, Thirteenth Edition
Chapter Outline
CHAPTER 25: DNA STRUCTURE AND CONTROL
OF GENE EXPRESSION
25.1 DNA Structure and Replication
DNA makes up our genes. Hershey and Chase performed two experiments that showed that DNA
is the genetic material.
Structure of DNA
The structure of DNA was determined by Watson and Crick. DNA is a double helix,
each strand of which is composed of nucleotides. The strands are held together by
hydrogen bonding.
Replication of DNA
The process of copying one DNA double helix into two identical double helices is called
DNA replication. DNA replication is termed semiconservative because a new double
helix has one conserved old strand and one new strand.
25.2 RNA Structure and Function
RNA is made up of nucleotides containing the sugar ribose and contains four bases: adenine,
uracil, cytosine, and guanine.
Messenger RNA
DNA serves as the template for messenger RNA (mRNA), which carries the information
from DNA to the ribosomes in the cytoplasm.
Transfer RNA
Transfer RNA (tRNA) transfers amino acids to the ribosomes to form proteins.
Ribosomal RNA
Ribosomal RNA (rRNA) joins with proteins made in the cytoplasm to form the subunits
of the ribosomes.
25.3 Gene Expression
Gene expression requires two processes called transcription and translation.
Transcription
During transcription, a segment of the DNA serves as a template for the production of an
RNA molecule.
Messenger RNA
Transcription begins when the enzyme RNA polymerase binds tightly to a
promoter. The RNA polymerase joins the RNA nucleotides to form an mRNA
molecule.
Processing of mRNA
After the mRNA is transcribed in eukaryotic cells, it must be processed
before entering the cytoplasm. Introns are removed and the ends are
modified.
Translation
Translation is the second step by which gene expression leads to protein synthesis.
The Genetic Code
The genetic code is a triplet code. The genetic code is just about universal.
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Transfer RNA
Transfer RNA molecules bring amino acids to the ribosomes, the site of protein
synthesis. Each tRNA contains an amino acid on one end and an anticodon
complementary to the codon on the other.
Ribosomes and Ribosomal RNA
Ribosomes are composed of many proteins and several rRNAs. A ribosome
moves down the mRNA molecule producing a protein.
Translation Requires Three Steps
The codons of an mRNA base-pair with the anticodons of tRNA molecules carrying
specific amino acids. The process of translation must be extremely orderly.
Initiation
Initiation is the step that brings all the translation components together.
Elongation
Elongation is the protein synthesis step in which a polypeptide increases in length
one amino acid at a time.
Termination
During termination, the polypeptide and the assembled components that carried
out protein synthesis are separated from one another.
Review of Gene Expression
Genes are made up of DNA in the nucleus that contains a triplet code. Gene expression
involves transcription and translation.
25.4 Control of Gene Expression
Differences in gene expression account for the specialization of the various types of cells. Genes
are turned on and off at different times and in different cells. Many steps are required for gene
expression and regulation can occur at any of these steps.
Control of Gene Expression in Prokaryotes
An operon is a cluster of genes usually coding for proteins related to a particular
metabolic pathway, along with the short DNA sequences that control their transcription.
The parts of an operon include a repressor and a regulator gene as well as the structural
genes.
Gene Expression in Eukaryotes
So-called “housekeeping genes” are not finely regulated.
Levels of Gene Control
Eukaryotic genes exhibit control of gene expression at five different levels.
Pretranscriptional Control
Eukaryotes use chromatin packing as a way to keep genes turned off.
Transcriptional Control
The transcriptional level is the most important level of gene control.
Transcriptional control is dependent on the interaction of proteins with
particular DNA sequences.
Posttranscriptional Control
The processing of mRNA before it leaves the nucleus as well as the
speed of transport of mRNA from the nucleus are all areas where gene
expression control can be exerted.
Translational Control
The longer an mRNA remains in the cytoplasm before it is broken down,
the more gene product can be translated.
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Posttranslational Control
Some proteins are not active immediately after synthesis. Folding and
cleavage may be required for activation. Other modifications can also
affect the activity of a protein.
25.5 Gene Mutations
A gene mutation is a change in the sequence of bases within a gene. Gene mutations can lead to
malfunctioning proteins in cells.
Causes of Mutations
Three causes of mutations are errors in replication, mutagens, and transposons.
Errors in Replication
DNA replication errors are a rare source of mutations.
Mutagens
Environmental influences such as radiation and certain organic chemicals
cause mutations in humans.
Transposons
Transposons are specific DNA sequences that have the ability to move
within and between chromosomes.
Effects of Mutations on Protein Activity
Changes in the DNA sequence may result in changes in the amino acid sequence.
Frameshift and point mutations can alter the protein.
Nonfunctional Proteins
A single, nonfunctioning protein can have a dramatic effect on phenotype.
Mutations Can Cause Cancer
The development of cancer involves a series of accumulating mutations that can be
different for each type of cancer. Mutations in tumor suppressor or proto-oncogenes often
lead to cancer. Tumor suppressor genes encode proteins that inhibit the cell cycle and
promote apoptosis. Proto-oncogenes encode proteins that promote the cell cycle and
prevent apoptosis.
Characteristics of Cancer Cells
Cancer cells are genetically unstable. They do not correctly regulate the cell cycle.
Cancer cells escape the signals for cell death. Cancer cells can survive and proliferate
elsewhere in the body.
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CHAPTER 26: BIOTECHNOLOGY AND GENOMICS
26.1 DNA Cloning
Knowledge of DNA biology has led to our ability to manipulate the genes of organisms.
The Cloning of a Gene
Gene cloning is the production of many identical copies of a single gene.
Recombinant DNA Technology
Recombinant DNA contains DNA from two or more different sources. To make
recombinant DNA, a researcher needs a vector in order to add foreign DNA to it.
The Polymerase Chain Reaction
The polymerase chain reaction (PCR) can create millions of copies of a segment
of DNA very quickly in a test tube without the use of a vector or a host cell.
DNA Analysis
DNA fingerprinting, STR profiling, and other applications of PCR can be used to
determine the differing sequences of DNA nucleotides that exist between
individuals. These techniques have many uses, including identification in
forensics.
26.2 Biotechnology Products
The field of biotechnology uses natural biological systems to create a product or to achieve a goal
desired by humans. Organisms that have had a foreign gene inserted into their DNA are called
transgenic organisms.
Transgenic Bacteria
Recombinant DNA technology is used to produce transgenic bacteria, which
express a cloned gene. The gene product is often collected from the medium the
bacteria are grown in.
Transgenic Plants
Foreign genes have been introduced into plant cells. Foreign genes transferred to
cotton, corn, and potato strains have made these plants resistant to pests.
Transgenic Animals
Techniques have been developed to insert genes into the eggs of animals. Animal
pharming, the use of transgenic farm animals to produce pharmaceuticals, is being
pursued by a number of firms.
26.3 Gene Therapy
Testing for Genetic Disorders
The testing method depends on the particular genetic disorder. In some instances it is
appropriate to test for a specific protein, and in others to test for the mutated gene.
Testing the DNA
Genetic Markers
Testing for a genetic marker is similar to the traditional procedure for
DNA fingerprinting.
DNA Microarrays
A DNA probe is a single-stranded piece of DNA that will bind to
complementary DNA. For the purpose of genetic testing, the DNA probe
bears a genetic mutation of interest. A DNA microarray, or “chip” is a
new technology that consists of a very small glass square containing
several rows of DNA probes. The chip allows testing for many genetic
disorders at one time.
Gene Therapy
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Gene therapy is the insertion of genetic material into human cells for the treatment of
genetic disorders and various other human illnesses.
Ex Vivo Gene Therapy
In one form of gene therapy, bone marrow stem cells are removed from the blood
and infected with an RNA retrovirus that carries a normal gene for a missing
enzyme. The cells are then returned to the patient.
In Vivo Gene Therapy
Genes needed to cure diseases are injected into the body.
26.4 Genomics and Bioinformatics
Genomics is the study of genomes—our genes and the genes of other organisms.
Sequencing the Genome
We now have the sequence of our genome due to the Human Genome Project.
Genome Architecture
Nearly 99% of the human genome is DNA that does not directly code for amino acid
sequences. Nearly half of the human genome is made up of repetitive elements.
What Is a Gene?
The modern definition of a gene is that a gene is a DNA or RNA sequence that directly
encodes a functional product, either an RNA or a protein.
Functional and Comparative Genomics
The genomes of many other organisms are also complete. Researchers can
compare the human genome with the genomes of other organisms.
Proteomics
Proteomics is the study of the structure, function, and interaction of cellular proteins.
Bioinformatics
Bioinformatics is the application of computer technologies to the study of the genome.
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