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61 Cards in this Set

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A virus is composed of an outer capsid made of protein and an inner core of DNA. The use of radioactive tracers showed that DNA, but not protein, enters bacteria and directs the formation of new viruses.
By the early 1950s, investigators had learned that genes are composed of DNA and that mutated genes result in errors of metabolism. Therefore, DNA in some way must control the cell.
Before Erwin Chargaff began his work, it was known that DNA contains four different types of nucleotides based on their nitrogen-containing bases
The bases adenine (A) and guanine (G) are purines with a double ring, and the bases thymine (T) and cytosine (C) are pyrimidines with a single ring.
In contrast to accepted beliefs, Chargaff found that each species has its own percentages of each type of nucleotide. For example, in a human cell, 31% of bases are adenine; 31% are thymine; 19% are guanine; and 19% are cytosine.
n all the species Chargaff studied, the amount of A always equaled the amount of T, and the amount of G always equaled the amount of C. These relationships are called
Chargaff’s rules:
The amount of A, T, G, and C in DNA varies from species to species.

In each species, the amount of A = T and the amount of G = C.
Rosalind Franklin was a researcher at King’s College in London in the early 1950s. She was studying the structure of DNA using X-ray crystallography.
When a crystal (a solid substance whose atoms are arranged in a definite manner) is X-rayed, the X-ray beam is diffracted (deflected), and the pattern that results shows how the atoms are arranged in the crystal.
First, Franklin made a concentrated, viscous solution of DNA and then saw that it could be separated into fibers. Under the right conditions, the fibers were enough like a crystal that, when they were X-rayed, a diffraction pattern resulted.
The X-ray diffraction pattern of DNA shows that DNA is a double helix. The helical shape is indicated by the crossed (X) pattern in the center of the photograph
In 1951, James Watson, having just earned a Ph.D., began an internship at the University of Cambridge, England. There, he met Francis Crick, a British physicist who was interested in molecular structures. Together, they set out to determine the structure of DNA and to build a model that would explain how DNA varies from species to species, replicates, stores information, and undergoes mutation.

Based on the available data, they knew the following:
DNA is a polymer of four types of nucleotides with the bases adenine (A), guanine (G), cytosine (C), and thymine (T).

Based on Chargaff’s rules, the amount of A = T, and the amount of G = C.

Based on Franklin’s X-ray diffraction photograph, DNA is a double helix with a repeating pattern.
DNA replication
refers to the process of making an identical copy of a DNA molecule.
During DNA replication, the two DNA strands, which are held together by hydrogen bonds, are separated and each old strand of the parent molecule serves as a
template for a new strand in a daughter molecule
This process is referred to as
semiconservative, since one of the two old strands is conserved, or present, in each daughter molecule.
then unwinding the helix structure using an enzyme called
helicase
The addition of the new strand is completed using an enzyme complex called
DNA polymerase.
Any breaks in the deoxyribose-phosphate backbone are sealed by the enzyme
DNA ligase.
RNA (ribonucleic acid)
is made up of nucleotides containing the sugar ribose, thus accounting for its name
he four nucleotides that make up an RNA molecule have the following bases:
adenine (A), uracil (U), cytosine (C), and guanine (G). Notice that, in RNA, uracil replaces the thymine in DNA
Messenger RNA (mRNA)
is produced in the nucleus of eukaryotes, and in the nucleoid of prokaryotes.
Transfer RNA (tRNA)
is also produced in the nucleus of eukaryotes, and a portion of DNA serves as a template for its production.
In eukaryotic cells, ribosomal RNA (rRNA)
is produced in the nucleolus of a nucleus, where a portion of DNA serves as a template for its formation. Ribosomal RNA joins with proteins made in the cytoplasm to form the subunits of ribosomes, one large and one small.
The central dogma of molecular biology
states that genetic information flows from DNA to RNA to protein
During transcription,
a portion of DNA serves as a template for mRNA formation.
During translation,
the sequence of mRNA bases (which are complementary to those in template DNA) determines the sequence of amino acids in a polypeptide.
But if the code were a triplet code,
the four bases could supply 64 different triplets, far more than needed to code for 20 different amino acids.
Each three-letter (nucleotide) unit of an mRNA molecule is called a codon
which codes for a single amino acid
mRNA Is Formed
Transcription begins when the enzyme RNA polymerase binds tightly to a promoter,
The resulting mRNA transcript
is a complementary copy of the sequence of bases in the template DNA strand.
mRNA Is ProcessedThe newly synthesized primary-mRNA
must be processed in order for it to be used properly. Processing occurs in the nucleus of eukaryotic cells.
Codon (mRNA) Anticodon (tRNA) Amino Acid (protein)
ACC UGG Threonine
After translation is complete, a protein contains the sequence of amino acids originally specified by DNA. This is the genetic information that DNA stores and passes on to each cell during the cell cycle, then to the next generation of individuals.
Translation Has Three Phases
During initiation, mRNA binds to the smaller of the two ribosomal subunits; then the larger subunit associates with the smaller one.

During an elongation cycle, a peptide lengthens one amino acid at a time. The growing peptide is transferred from the outgoing tRNA to the incoming tRNA–amino acid complex, and then the outgoing tRNA leaves. The ribosome then translates the next mRNA codon as it receives a new incoming tRNA–amino acid complex.

Termination occurs at any one of three special codons that mean “stop.” The ribosomal subunits and mRNA dissociate, and the completed polypeptide is released.
A small ribosomal subunit binds to mRNA; an initiator tRNA’s anticodon binds to its codon, and the large ribosomal subunit completes the ribosome.
The small ribosomal subunit attaches to the mRNA in the vicinity of the start codon (AUG).

The anticodon of the initiator tRNA–methionine complex pairs with this codon.

The large ribosomal subunit joins to the small subunit.
TerminationWhen a stop codon (see Fig. 11.10) appears at the A site of a ribosome, termination occurs
A protein called a release factor binds to the stop codon and cleaves the polypeptide from the last tRNA.
mRNA Is FormedTranscription begins when the enzyme RNA polymerase binds tightly to a
promoter, a region of DNA with a special nucleotide sequence that marks the beginning of a gene.
The resulting mRNA transcript is a complementary copy of the sequence of bases in the template DNA strand
Once transcription is completed, the mRNA is ready to be processed before it leaves the nucleus for the cytoplasm.
mRNA Is ProcessedThe newly synthesized primary-mRNA must be processed in order for it to be used properly.
rocessing occurs in the nucleus of eukaryotic cells. Three steps are required: capping, the addition of a poly-A tail, and splicing (Fig. 11.12). After processing, the mRNA is called a mature mRNA molecule.
Transfer RNA Brings Amino Acids to the RibosomesEach tRNA is a single-stranded nucleic acid that doubles back on itself such that complementary base pairing results in the cloverleaf-like shape shown in Figure 11.13. Th
There is at least one tRNA molecule for each of the 20 amino acids found in proteins. The amino acid binds to one end of the molecule. The opposite end of the molecule contains an anticodon, a group of three bases that is complementary to a specific codon of mRNA.
During translation, the order of codons in mRNA determines the order in which tRNAs bind at the ribosomes.
When a tRNA–amino acid complex comes to the ribosome, its anticodon pairs with an mRNA codon. For example, if the codon is ACC, what is the anticodon, and what amino acid will be attached to the tRNA molecule?
we can determine this:
Codon (mRNA) ACC Anticodon (tRNA) UGG Amino Acid (protein) Threonine
After translation is complete, a protein contains the sequence of amino acids originally specified by DNA.
Translation Has Three Phases

Polypeptide synthesis has three phases: initiation, an elongation cycle, and termination. Enzymes are required for each of the steps to occur, and energy is needed for the first two steps.
During initiation, mRNA binds to the smaller of the two ribosomal subunits; then the larger subunit associates with the smaller one.

During an elongation cycle, a peptide lengthens one amino acid at a time. The growing peptide is transferred from the outgoing tRNA to the incoming tRNA–amino acid complex, and then the outgoing tRNA leaves. The ribosome then translates the next mRNA codon as it receives a new incoming tRNA–amino acid complex.

Termination occurs at any one of three special codons that mean “stop.” The ribosomal subunits and mRNA dissociate, and the completed polypeptide is released.
A small ribosomal subunit binds to mRNA; an initiator tRNA’s anticodon binds to its codon, and the large ribosomal subunit completes the ribosome.
The small ribosomal subunit attaches to the mRNA in the vicinity of the start codon (AUG).

The anticodon of the initiator tRNA–methionine complex pairs with this codon.

The large ribosomal subunit joins to the small subunit.
Two tRNAs can be at a ribosome at one time. The tRNA at the P site passes its peptide to the tRNA at the A site. The tRNA at the P site leaves. The ribosome moves forward (translocation), and the tRNA-peptide complex is now at the P site. A new tRNA–amino acid complex comes to the A site.
tRNA at the P site contains the growing peptide chain. (See .)

This tRNA passes its peptide to tRNA–amino acid at the A site. The tRNA at the P site leaves. (See and .)

During translocation, the tRNA-peptide moves to the P site, and the codon at the A site is ready for the next tRNA–amino acid. (See .)
TerminationWhen a stop codon (see Fig. 11.10) appears at the A site of a ribosome, termination occurs.
A protein called a release factor binds to the stop codon and cleaves the polypeptide from the last tRNA. Then, the polypeptide and the assembled components that carried out protein synthesis are separated from one another.
A gene is a sequence of DNA bases that codes for a product, most often a protein.
A gene mutation is a change in the sequence of those bases. A mutation can increase the diversity of organisms by creating an entirely new product with a positive function to the organism; however, the results of a mutation can be negative.
A gene mutation can be caused by an error in replication, a transposon, or an environmental mutagen. Mutations due to DNA replication errors are rare: a frequency of 1 in 100 million per cell division on average in most eukaryotes
Transposons (Fig. 11.18) are specific DNA sequences that have the remarkable ability to move within and between chromosomes. Their movement often disrupts genes, rendering them nonfunctional.
Mutagens are environmental influences that cause mutations.
Different forms of radiation, such as radioactivity, X-rays, and ultraviolet (UV) light, and chemical mutagens, such as pesticides and compounds in cigarette smoke, may cause breaks or chemical changes in DNA.
Point mutations
involve a change in a single DNA nucleotide, and the severity of the results depends on the particular base change that occurs.
A frameshift mutation
is caused by extra or missing nucleotides in a DNA sequence. They are usually much more severe than point mutations because codons are read from a specific starting point. Therefore, all downstream codons are affected.
During genetic engineering, a cloned gene can be inserted into the genome of an organism, which is then called a
transgenic organism.
Recombinant DNA (rDNA)
ntains DNA from two or more different sources (Fig. 11.19). To make rDNA, a researcher needs a vector, a piece of DNA that acts as a carrier for the foreign DNA
Two enzymes are needed to introduce foreign DNA into plasmid DNA:
restriction enzymes that can cleave, or cut, DNA at specific places (for example, the restriction enzyme EcoRI always cuts DNA at the base sequence GAATTC) and (2) DNA ligase, which can seal the foreign DNA into an opening in a cut plasmid.
Cloned genes have many uses. A scientist may allow the
genetically modified bacterial cells to express the cloned gene and retrieve the protein. Or copies of the cloned gene may be removed from the bacterial cells and then introduced into another organism, such as a corn plant, to produce a transgenic organism.
The term biotechnology
refers to the use of natural biological systems to create a product or achieve some other end desired by human beings.
Recombinant DNA technology is used to produce transgenic bacteria, which are grown in huge vats called bioreactors.
The gene product is usually collected from the growth medium. Products now on the market that are produced by bacteria include insulin, human growth hormone, t-PA (tissue plasminogen activator, used to dissolve blood clots), and hepatitis B vaccine.
Foreign genes have been transferred to cotton, corn, and potato strains to make these plants resistant to pests by causing their cells to produce an insect toxin.
Similarly, soybeans have been made resistant to a common herbicide. Some corn and cotton plants are both pest- and herbicide-resistant.
Foreign genes have also been inserted into the eggs of animals, often to give them the gene for bovine growth hormone (bGH).
The procedure has resulted in larger fishes, cows, pigs, rabbits, and sheep. Gene “pharming,” the use of transgenic farm animals to produce pharmaceuticals, is being pursued by a number of firms
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. PCR is very sensitive—it amplifies (makes copies of) a targeted DNA sequence that can be less than one part in a million of the total DNA sample!
DNA fingerprinting
PCR is used to amplify the entire repeated region. The greater the number of repeats, the longer the segment of DNA that will result after PCR amplification is done. People can be heterozygous for the number of repeats
In the previous century, researchers discovered the structure of DNA, how DNA replicates, and how protein synthesis occurs. Genetics in the twenty-first century concerns
genomics, the study of genomes—our genes and intergenic DNA sequences, as well as those of other organisms.
Researchers are comparing the human genome with the genomes of other species for clues to our evolutionary origins.
One surprising discovery is that the genomes of all vertebrates are similar. Researchers were not surprised to find that the genes of chimpanzees and humans are 98% alike, but they did not expect to find that our sequence is also 85% like that of a mouse.
One study compared the human genome with that of chromosome 22 in chimpanzees.
Among the many genes that differed in sequence were several of particular interest: a gene for proper speech development, several for hearing, and several for smell.
he known sequence of bases in the human genome predicts that about 20,500 genes are translated into approximately 100,000 different proteins due to alternative mRNA splicing;
collectively, all of these proteins are referred to as the human proteome.
The field of proteomics
explores the structure and function of these cellular proteins and examines how they interact to contribute to the production of traits.
Bioinformatics
As a result of bioinformatics, scientists hope to find cause-and-effect relationships between an individual’s overall genetic makeup and resulting genetic disorders.