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biology final exam (text four)
Terms in this set (59)
Genetics and Heredity
Heredity- the transmission of traits from one generation to the next.
Genetics- the scientific study of heredity.
Gregor Mendel worked in the 1860s and argued that
parents pass on to their offspring discrete genes
(which he termed "heritable factors"),
genes are responsible for inherited traits
-genes retain their individual identities generation after generation, no matter how they are mixed up or temporarily masked.
In an Abbey Garden
is a heritable feature that varies among individuals.
is a variant of a character. (can also be called a character state)
-Each of the characters Mendel studied occurred in two distinct traits.
created purebred varieties of plants and
crossed two different purebred varieties.
Hybrids are the offspring of two different purebred varieties.
-The cross-fertilization itself is referred to as a genetic cross.
-The parental plants are the P generation.
-Their hybrid offspring are the F1 generation.
-A cross of the F1 plants forms the F2 generation.
Mendel performed many experiments in which he tracked the inheritance of characters, such as flower color, that occur as two alternative traits.
The results led him to formulate several hypotheses about inheritance.
Mendel performed a
between purebred parent plants that differ in only one character and found that the F1 plants all had purple flowers.
-Was the factor responsible for inheritance of white flowers now lost as a result of the cross?
-By mating the F1 plants with each other, Mendel found the answer to this question to be no.
Mendel figured out that the gene for white flowers did not disappear in the F1 plants but was somehow hidden or masked when the purple-flower factor was present.
He also deduced that the F1 plants must have carried two factors for the flower-color character,
-one for purple and
-one for white.
From these results and others, Mendel developed four hypotheses:
1. There are alternative versions of genes that account for variations in inherited characters.
The alternative versions of genes are called alleles
2. For each inherited character, an organism inherits two alleles, one from each parent.
-An organism that has two identical alleles for a gene is said to be homozygous for that gene.
-An organism that has two different alleles for a gene is said to be heterozygous for that gene.
3. If the two alleles of an inherited pair differ, then one determines the organism's appearance and is called the dominant allele, and the other has no noticeable effect on the organism's appearance and is called the recessive allele.
-Geneticists use uppercase italic letters (such as P) to represent dominant alleles and lowercase italic letters (such as p) to represent recessive alleles.
4. ***A sperm or egg carries only one allele for each inherited character because the two alleles for a character segregate (separate) from each other during the production of gametes.
-This statement is called the
law of segregation
-When sperm and egg unite at fertilization, each contributes its alleles, restoring the paired condition in the offspring.
-the four possible combinations of gametes and
-the resulting four possible offspring in the F2 generation.
Each square represents an equally probable product of fertilization.
Geneticists distinguish between an organism's
-physical appearance, its
-genetic makeup, its
Mendel found that each of the seven characters he studied had the same inheritance pattern: A parental trait disappeared in the F1 generation, only to reappear in one-fourth of the F2 offspring.
-The underlying mechanism is explained by Mendel's law of segregation:
-Pairs of alleles segregate during gamete
-the fusion of gametes at fertilization creates
allele pairs again.
Genetic Alleles and Homologous Chromosomes
Homologous chromosomes—chromosomes that carry alleles of the same genes.
locus* is a specific location of a gene along a chromosome.
(alternative versions) of a gene reside at the same locus on homologous chromosomes.
-However, the two chromosomes may bear either identical alleles or different ones at any one locus.
Mendel's Law of Independent Assortment
is the mating of parental varieties differing in two characters.
What would result from a dihybrid cross?
If the genes for the two characters were inherited together, then
-the F1 hybrids would produce only the same two kinds of gametes that they received from their parents, and
-the F2 generation would show a 3:1 phenotypic ratio.
If, however, the two seed characters sorted independently, then
-the F1 generation would produce four gamete genotypes (RY, rY, Ry, and ry) in equal quantities, and
-the F2 generation would have nine different genotypes producing four different phenotypes in a ratio of 9:3:3:1.
Mendel's dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation.
-Thus, the inheritance of one character has no effect on the inheritance of another.
This is called Mendel's law of independent assortment.
Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between an individual of dominant phenotype (but unknown genotype) and a
The Rules of Probability
Mendel's strong background in mathematics helped him understand patterns of inheritance.
-For instance, he understood that genetic crosses obey the rules of probability—the same rules that apply when tossing coins, rolling dice, or drawing cards.
rule of multiplication
states that the probability of a compound event is the product of the separate probabilities of the independent events.
Mendel's principles apply to the inheritance of many human traits.
A trait that is dominant does not imply that it is either normal or more common than a recessive phenotype.
(those seen most often in nature) are not necessarily specified by dominant alleles.
How can human genetics be studied?
-analyze the results of matings that have already occurred and
-assemble this information into a family tree, called a
Mendel's laws enable us to deduce the genotypes for most of the people in the pedigree.
Most human genetic disorders are recessive.
Individuals who have the recessive allele but appear normal are
of the disorder.
Using Mendel's laws, we can predict the fraction of affected offspring that is likely to result from a marriage between two carriers.
A number of human disorders are caused by dominant alleles.
Dominant alleles that cause lethal disorders are much less common than lethal recessive alleles.
One example is the allele that causes Huntington's disease, a degeneration of the nervous system that usually does not begin until middle age.
-Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal.
-Because the allele for Huntington's disease is dominant, any child born to a parent with the allele has a 50% chance of inheriting the allele and the disorder.
Today many tests can detect the presence of disease-causing alleles.
Most genetic tests are performed during pregnancy if the prospective parents are aware that they have an increased risk of having a baby with a genetic disease.
-In amniocentesis, a physician uses a needle to extract about 2 teaspoons of the fluid that bathes the developing fetus.
-In chorionic villus sampling, a physician inserts a narrow, flexible tube through the mother's vagina and into her uterus, removing some placental tissue.
-Newer genetic screening procedures involve isolating tiny amounts of fetal cells or DNA released into the mother's bloodstream. These newer technologies are gradually replacing more invasive screening methods because they
-are more accurate and
-can be performed earlier than other tests.
Once cells are obtained, they can be screened for genetic diseases.
Patients seeking genetic testing should receive counseling both before and after to explain the test and to help them cope with the results.
Variations on Mendel's Laws
Mendel's two laws explain inheritance in terms of genes that are passed along from generation to generation according to simple rules of probability.
-These laws are valid for all sexually reproducing organisms.
-But Mendel's laws stop short of explaining some patterns of genetic inheritance.
-In fact, for most sexually reproducing organisms, cases in which Mendel's rules can strictly account for the patterns of inheritance are relatively rare.
-More often, the observed inheritance patterns are more complex.
Incomplete Dominance in Plants and People
**In incomplete dominance, F1 hybrids have an appearance between the phenotypes of the two parents.
**Codominant- both alleles are expressed in heterozygous individuals
Structure/Function: Pleiotropy and Sickle-Cell Disease
is when one gene influences several characters.
-results in abnormal hemoglobin proteins, and
-causes disk-shaped red blood cells to deform into a sickle shape with jagged edges.
In most cases, only people who are homozygous for the sickle-cell allele have sickle-cell disease.
-the additive effects of two or more genes on a single phenotypic character (or when one trait is controlled by two or more genes) and
-the logical opposite of pleiotropy, in which one gene affects several characters.
There is evidence that height in people is controlled by several genes that are inherited separately. (Actually, human height is probably affected by a great number of genes, but we'll simplify here.)
Epigenetics and the Role of Environment
Many phenotypic characters result from a combination of heredity and environment.
-Whether human characters are more influenced by genes or by the environment—nature or nurture—is a very old and hotly contested issue.
-Spending time with identical twins will convince anyone that environment, and not just genes, affects a person's traits.
In general, only genetic influences are inherited and effects of the environment are not passed to the next generation.
In recent years, however, biologists have begun to recognize the importance of
- the transmission of traits by mechanisms not directly involving DNA sequence.
For example, components of chromosomes can be chemically modified by adding or removing chemical groups on the DNA and/or protein components of chromosomes.
Over a lifetime, the environment plays a role in these changes, which may explain how one identical twin can suffer from a genetically based disease whereas the other twin does not, despite their identical genomes.
—and the changes in gene activity that result—may even be carried on to the next generation.
Unlike alterations to the DNA sequence, chemical changes to the chromosomes can be reversed.
The Chromosomal Basis of Inheritance
chromosome theory of inheritance
-genes are located at specific positions (loci) on chromosomes and
-the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.
It is chromosomes that
-undergo segregation and independent assortment during meiosis and
-account for Mendel's laws.
-are located near each other on the same chromosome and
-tend to travel together during meiosis and fertilization.
Such genes are often inherited as a set and therefore often do not follow Mendel's law of independent assortment.
A gene located on a sex chromosome is called a sex-linked gene.
-Most sex-linked genes are found on the X chromosome.
A number of human conditions, including red-green colorblindness, hemophilia, and a type of muscular dystrophy, result from sex-linked recessive alleles.
Because they are located on the sex chromosomes, sex-linked genes exhibit unusual inheritance patterns.
DNA and RNA Structure
DNA and RNA are nucleic acids.
-They consist of long chains (polymers) of chemical units (monomers) called
-A nucleotide polymer is a
-Polynucleotides can be very long and may
have any sequence of the four different
types of nucleotides (abbreviated A, C, T,
Nucleotides are joined by covalent bonds between the sugar of one nucleotide and the phosphate of the next in a repeating pattern of sugar-phosphate-sugar-phosphate, which is known as a
The nitrogenous bases are arranged like ribs that project from this backbone.
Each nucleotide consists of three components:
1. a nitrogenous base,
2. a sugar, and
3. a phosphate group.
The sugar is called deoxyribose because, compared with the sugar ribose, it is missing an oxygen atom.
The full name for DNA is deoxyribonucleic acid, with nucleic referring to DNA's location in the nuclei of eukaryotic cells.
The four nucleotides found in DNA differ in their nitrogenous bases. The bases can be divided into two types.
1. Thymine (T) and cytosine (C) are single-ring structures.
2. Adenine (A) and guanine (G) are larger, double- ring structures.
RNA and DNA polynucleotides have the same chemical structure except that
-instead of thymine, RNA has a similar base called uracil (U), and
-RNA contains a slightly different sugar than DNA (ribose instead of deoxyribose, accounting for the names RNA vs. DNA).
Structure/Function: DNA Replication
When a cell reproduces, it must duplicate this information, providing one copy to the new offspring cell while keeping one copy for itself.
Watson and Crick's model of DNA suggests that each DNA strand serves as a mold, or template, to guide reproduction of the other strand.
-are enzymes that make the covalent bonds between the nucleotides of a new DNA strand and
-can help repair DNA that has been damaged by toxic chemicals or high-energy radiation, such as X-rays and ultraviolet light.
The whole process of DNA replication requires the cooperation of many enzymes.
-begins on a double helix at specific sites, called
origins of replication
, and then
-proceeds in both directions, creating what are called
-ensures that all the body cells in a multicellular organism carry the same genetic information and
-is also the means by which genetic information is passed along to offspring.
How an Organism's Genotype Determines Its Phenotype
An organism's genotype, its genetic makeup, is the heritable information contained in the sequence of nucleotide bases in its DNA.
The phenotype, the organism's physical traits, arises from the actions of a wide variety of proteins.
DNA specifies the synthesis of proteins in two stages:
1. transcription, the transfer of genetic information from DNA into an RNA molecule, and
2. translation, the transfer of information from RNA into a polypeptide (protein strand).
From Nucleotides to Amino Acids: An Overview
Genetic information in DNA is transcribed into RNA, then translated into polypeptides, which then fold into proteins.
Translation occurs in ribosomes (cytoplasm)
What is the language of nucleic acids?
-In DNA, it is the linear sequence of nucleotide bases.
-A typical gene consists of thousands of nucleotides in a specific sequence.
When a segment of DNA is transcribed, the result is an RNA molecule.
The sequence of nucleotides of the RNA molecule dictates the sequence of amino acids of the polypeptide.
Experiments have verified that the flow of information from gene to protein is based on a triplet code.
The genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of three-base words called
-Three-base codons in the DNA are transcribed into complementary three-base codons in the RNA.
-Then the RNA codons are translated into amino acids that form a polypeptide.
The Genetic Code
is the set of rules that convert a nucleotide sequence in RNA to an amino acid sequence.
Of the 64 triplets,
-61 code for amino acids and
-3 are stop codons, instructing the ribosomes to end the polypeptide.
A given RNA triplet always specifies a given amino acid.
The genetic code is nearly universal, shared by organisms from the simplest bacteria to the most complex plants and animals.
-Because diverse organisms share a common genetic code, it is possible to program one species to produce a protein from another species by transplanting DNA.
-This allows scientists to mix and match genes from various species—a procedure with many useful genetic engineering applications in agriculture, medicine, and research.
Transcription: From DNA to RNA
Transcription is the transfer of genetic information from DNA to RNA.
-As with DNA replication, two DNA strands must first separate at the place where the process will start.
-In transcription, however, only one of the DNA strands serves as a template for the newly forming RNA molecule; the other strand is unused.
-The nucleotides that make up the new RNA molecule take their place one at a time along the DNA template strand by forming hydrogen bonds with the nucleotide bases there.
Notice that the RNA nucleotides follow the usual base-pairing rules, except that U, rather than T, pairs with A.
The RNA nucleotides are linked by the transcription enzyme RNA polymerase.
Initiation of Transcription
The "start transcribing" signal is a nucleotide sequence called a
, which is
-located in the DNA at the beginning of the gene and
-a specific place where RNA polymerase attaches.
The first phase of transcription, called initiation, is
-the attachment of RNA polymerase to the promoter and
-the start of RNA synthesis.
During the second phase of transcription, called elongation,
-the RNA grows longer and
-the RNA strand peels away from its DNA template, allowing the two separated DNA strands to come back together in the region already transcribed.
Termination of Transcription
During the third phase of transcription, called termination,
-RNA polymerase reaches a special sequence of bases in the DNA template called a
, signaling the end of the gene,
-polymerase detaches from the RNA and the gene, and
-the DNA strands rejoin.
The Processing of Eukaryotic RNA
In the cells of prokaryotes, which lack nuclei, the RNA transcribed from a gene immediately functions as messenger RNA (mRNA), the molecule that is translated into protein.
The eukaryotic cell
-localizes transcription in the nucleus and
-modifies, or processes, the RNA transcripts in the nucleus before they move to the cytoplasm for translation by ribosomes.
One kind of RNA processing is the addition of extra nucleotides to the ends of the RNA transcript.
These additions, called the
cap and tail
, protect the RNA from attack by cellular enzymes and help ribosomes recognize the RNA as mRNA.
Another type of RNA processing is made necessary in eukaryotes by noncoding stretches of nucleotides that interrupt the nucleotides that actually code for amino acids.
-Most genes of plants and animals include such internal noncoding regions, which are called
-The coding regions—the parts of a gene that are expressed—are called
Both exons and introns are transcribed from DNA into RNA.
However, before the RNA leaves the nucleus,
-the introns are removed and
-the exons are joined to produce an mRNA molecule with a continuous coding sequence.
This process is called
Translation: The Players
Translation is a conversion between different languages, from the nucleic acid language to the protein language, and involves more elaborate machinery than transcription.
Messenger RNA (mRNA)
The first important ingredient required for translation is the mRNA produced by transcription.
Once it is present, the machinery used to translate mRNA requires enzymes and sources of chemical energy, such as ATP.
In addition, translation requires two other important components:
1. ribosomes and
2. a kind of RNA called transfer RNA.
Transfer RNA (tRNA)
Translation of the genetic message carried in mRNA into the amino acid language of proteins requires an interpreter to convert the three-letter words (codons) of nucleic acids to the amino acid words of proteins.
A cell uses a molecular interpreter, a type of RNA called transfer RNA (tRNA), to match amino acids to the appropriate codons to form the new polypeptide.
To perform this task, tRNA molecules must carry out two distinct functions:
1. pick up the appropriate amino acids and
2. recognize the appropriate codons in the mRNA.
-The unique structure of tRNA molecules enables them to perform both tasks.
A tRNA molecule is made of a single strand of RNA—one polynucleotide chain—consisting of about 80 nucleotides.
-The chain twists and folds upon itself, forming several double-stranded regions in which short stretches of RNA base-pair with other stretches.
At one end of the tRNA is a special triplet of bases called an
-The anticodon triplet is complementary to a codon triplet on mRNA.
During translation, the anticodon on the tRNA recognizes a particular codon on the mRNA by using base-pairing rules.
At the other end of the tRNA molecule is a site where one specific kind of amino acid attaches.
Although all tRNA molecules are similar, there are slightly different versions of tRNA for each amino acid.
Ribosomes are the organelles in the cytoplasm that
-coordinate the functioning of mRNA and tRNA and
-actually make polypeptides.
A ribosome consists of two subunits. Each subunit is made up of
-a considerable amount of another kind of RNA,
A fully assembled ribosome has
-a binding site for mRNA on its small subunit and
-binding sites for tRNA on its large subunit.
Translation: The Process
Translation is divided into the same three phases as transcription:
2. elongation, and
Initiation brings together
-the first amino acid with its attached tRNA, and
-two subunits of the ribosome.
The mRNA molecule has a cap and tail that help the mRNA bind to the ribosome.
Initiation occurs in two steps.
1. An mRNA molecule binds to a small ribosomal subunit, then a special initiator tRNA binds to the
, where translation is to begin on the mRNA.
2. A large ribosomal subunit binds to the small one, creating a functional ribosome.
Once initiation is complete, amino acids are added one by one to the first amino acid. Each addition occurs in the three-step elongation process.
Step 1: Codon recognition. The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome.
Step 2: Peptide bond formation.
-The polypeptide leaves the tRNA in the P site and attaches to the amino acid on the tRNA in the A site.
-The ribosome creates a new peptide bond.
-Now the chain has one more amino acid.
Step 3: Translocation.
-The P site tRNA now leaves the ribosome, and the ribosome moves the remaining tRNA, carrying the growing polypeptide, to the P site.
-The mRNA and tRNA move as a unit.
-This movement brings into the A site the next mRNA codon to be translated, and the process can start again with step 1.
Elongation continues until
-a stop codon reaches the ribosome's A site,
-the completed polypeptide is freed, and
-the ribosome splits back into its subunits.
Review: DNA→ RNA→ Protein
The flow of genetic information in the cell moves from DNA to RNA to protein.
In eukaryotic cells, transcription (DNA → RNA) occurs in the nucleus, and the RNA is processed before it enters the cytoplasm.
-Translation (RNA → protein) is rapid. A single ribosome can make an average-sized polypeptide in less than a minute.
-As it is made, a polypeptide coils and folds, assuming its final three-dimensional shape.
Transcription and translation are the processes whereby genes control the structures and activities of cells.
-The flow of information originates with the specific sequence of nucleotides in a DNA gene.
-The gene dictates the transcription of a complementary sequence of nucleotides in mRNA.
-In turn, the information within the mRNA specifies the sequence of amino acids in a polypeptide.
-Finally, the proteins that form from the polypeptides determine the appearance and capabilities of the cell and organism.
For decades, the DNA → RNA → protein pathway was believed to be the sole means by which genetic information controls traits.
In recent years, however, this notion has been challenged by discoveries that point to more complex roles for RNA.
Any change in the nucleotide sequence of a cell's DNA is called a mutation.
Mutations can involve
-large regions of a chromosome or
-just a single nucleotide pair, as occurs in sickle-cell disease.
Types of Mutations
Mutations within a gene can be divided into two general categories:
1. nucleotide substitutions and
2. nucleotide deletions or insertions.
Missense mutations involve a single nucleotide and change the amino acid coding.
Nonsense mutations change an amino acid codon into a stop codon.
Mutations involving the deletion or insertion of one or more nucleotides in a gene, called frameshift mutations, often have disastrous effects.
-Adding or subtracting nucleotides may alter the triplet grouping of the genetic message.
-A frameshift mutation most often produces a non-functioning polypeptide.
Mutations can occur in a number of ways.
-Spontaneous mutations result from random errors during DNA replication or recombination.
-Other sources of mutation are physical and chemical agents called
-The most common physical mutagen is
high-energy radiation, such as X-rays and
ultraviolet (UV) light.
-Chemical mutagens are of various types.
-Because many mutagens can act as
carcinogens, agents that cause cancer,
you should avoid them as much as
Although mutations are often harmful, they can also be beneficial, both in nature and in the laboratory.
Mutations are one source of the rich diversity of genes in the living world, a diversity that makes evolution by natural selection possible.
Viruses and Other Noncellular Infectious Agents
Viruses share some of the characteristics of living organisms, such as having genetic material in the form of nucleic acid packaged within a highly organized structure.
***A virus is generally not considered alive because it
-is not cellular and
-cannot reproduce on its own.
A virus is
-an infectious particle consisting of little more than "genes in a box,"
-a bit of nucleic acid wrapped in a protein coat, and,
-in some cases, an envelope of membrane.
A virus cannot reproduce on its own. It can multiply only by infecting a living cell and directing the cell's molecular machinery to make more viruses.
Viruses that attack bacteria are called bacteriophages ("bacteria-eaters"), or phages for short.
Once they infect a bacterium, most phages enter a reproductive cycle called the lytic cycle.
Some viruses can also reproduce by an alternative route—the lysogenic cycle, in which viral DNA replication occurs without phage production or the death of the cell.
Viruses that infect plant cells can
-stunt plant growth and
-diminish crop yields.
Most known plant viruses have RNA rather than DNA as their genetic material.
There is no cure for most viral plant diseases.
Agricultural scientists focus on
-preventing infection and
-breeding or genetically engineering varieties of crop plants that resist viral infection.
Viroids and Prions
Viruses may be small and simple, but they are giants compared to two other classes of pathogens: viroids and prions.
are small, circular RNA molecules that infect plants.
are thought to be a misfolded form of a protein normally present in brain cells.
Prions cause a number of brain diseases in various animal species, including
-scrapie in sheep and goats,
-chronic wasting disease in deer and elk,
-mad cow disease, and
-in humans, Creutzfeldt-Jakob disease, an extremely rare, incurable, and inevitably fatal deterioration of the brain.
-is the manipulation of organisms or their components to make useful products and
-dates back to the dawn of civilization when people used yeast to make bread and beer and selectively bred livestock.
Biotechnology today means the use of DNA technology, modern laboratory techniques for studying and manipulating genetic material.
Using the methods of DNA technology, scientists can modify specific genes and move them between organisms as different as bacteria, plants, and animals.
-Organisms that have acquired one or more genes by artificial means are called
genetically modified (GM) organisms
. (Technically everything we eat, unless wild, is somehow artificially selected for and bred)
-If the newly acquired gene is from another organism, typically of another species, the recombinant organism is called a
by combining pieces of DNA from two different sources—often from different species—to form a single DNA molecule.
Recombinant DNA technology is widely used in
- the direct manipulation of genes for practical purposes.
-genetically engineered bacteria to mass-produce a variety of useful chemicals, from cancer drugs to pesticides, and
-transferred genes from bacteria to plants and from one animal species to another.
Recombinant DNA Techniques
Bacteria are the workhorses of modern biotechnology.
To manipulate genes in the laboratory, biologists often use
- which are small, circular DNA molecules that duplicate separately from the larger bacterial chromosome.
-are small, circular DNA molecules that duplicate separately from the larger bacterial chromosome,
-can carry virtually any gene and are passed from one generation of bacteria to the next, and
-are key tools for
, the production of multiple identical copies of a gene-carrying piece of DNA. Gene cloning methods are central to the production of useful products from genetically engineered organisms.
Cutting and Pasting DNA with Restriction Enzymes
Recombinant DNA is produced by combining two ingredients:
1. a bacterial plasmid and
2. the gene of interest.
To understand how these DNA molecules are spliced together, you need to learn how enzymes cut and paste DNA.
The cutting tools used for making recombinant DNA are bacterial enzymes called restriction enzymes.
-The DNA sequence recognized by a particular restriction enzyme is called a
-After a restriction enzyme binds to its restriction site, it cuts the two strands of the DNA by breaking chemical bonds at specific points within the sequence, like a pair of highly specific molecular scissors.
To separate and visualize DNA fragments of different lengths, researchers carry out a technique called
, a method for sorting macromolecules, usually proteins or nucleic acids, primarily by their electrical charge and size.
DNA Profiling and Forensic Science
-is the analysis of DNA samples to determine whether they come from the same individual and
-has rapidly transformed the field of
, the scientific analysis of evidence for crime scene investigations and other legal proceedings.
To produce a DNA profile, scientists compare sequences in the genome that vary from person to person.
DNA Profiling Techniques the Polymerase Chain Reaction (PCR)
polymerase chain reaction
-is a technique by which a specific segment of DNA can be amplified: targeted and copied quickly and precisely, and
-permits a scientist to obtain enough DNA from even minute amounts of blood or other tissue to allow a DNA profile to be constructed.
In principle, PCR is simple.
-A DNA sample is mixed with nucleotides, the DNA replication enzyme DNA polymerase, and a few other ingredients.
-The solution is then exposed to cycles of heating (to separate the DNA strands) and cooling (to allow double-stranded DNA to re-form).
-During these cycles, specific regions of each molecule of DNA are replicated, doubling the amount of that DNA.
-The result of this chain reaction is an exponentially growing population of identical DNA molecules.
A DNA molecule within a starting sample is likely to be very long. But, most often, only a very small target region of that large DNA molecule needs to be amplified.
The key to amplifying one particular segment of DNA and no others is the use of
, short (usually 15-20 nucleotides long), chemically synthesized single-stranded DNA molecules.
The primers bind to sequences that flank the target sequence, marking the start and end points for the segment of DNA to be amplified.
In addition to forensic applications, PCR can be used in the treatment and diagnosis of disease. PCR can be used to
-amplify, and thus detect, HIV in blood or tissue samples and
-diagnose hundreds of human genetic disorders by being used with primers that target the genes associated with these disorders.
Short Tandem Repeat (STR) Analysis
How do you prove that two samples of DNA come from the same person?
-makes up much of the DNA that lies between genes in humans and
-consists of nucleotide sequences that are present in multiple copies in the genome.
Short tandem repeats
(STRs) are short sequences of DNA repeated many times, tandemly (one after another), in the genome.
-is a method of DNA profiling and
-compares the lengths of STR sequences at specific sites in the genome.
In forensic cases using STR analysis with the 13 standard markers, the probability of two people having identical DNA profiles is somewhere between one chance in 10 billion and one in several trillion.
By transferring the gene for a desired protein into a bacterium, yeast, or other kind of cell that is easy to grow in culture, scientists can produce large quantities of useful proteins that are present naturally only in small amounts.
DNA technology is used to produce medically valuable molecules, including
-human growth hormone (HGH),
-the hormone erythropoietin (EPO), which stimulates production of red blood cells, and
-vaccines, a harmless variant or derivative of a disease-causing microbe—such as a bacterium or virus—that is used to prevent an infectious disease.
Transgenic plants and animals
Cotton, corn, and potato make their own insecticide
Larger fishes, cows, and pigs from inserted growth hormone gene
- use of transgenic farm animal to produce pharmaceuticals in milk
Transgenic animals may be cloned—nucleus from adult cell introduced into enucleated egg cell produces identical genotype of adult donor
Genetically Modified Organisms in Agriculture
Since ancient times, people have selectively bred agricultural crops to make them more useful.
Today, DNA technology is quickly replacing traditional breeding programs as scientists work to improve the productivity of agriculturally important plants and animals.
Traditional Breeding: Selecting the seeds of plants that have favored traits. Over time those traits become more abundant in a population.
Mutagenesis: Exposing seeds to atomic radiation (Gamma rays).
RNA Interference: technology used to switch genes "on and off", by modifying their expression.
Transgenics: Genes of one organism inserted into another organism.
Human Gene Therapy
Human gene therapy is intended to treat disease by introducing genes into an afflicted person.
-In cases where a single defective gene causes a disorder, the mutant version of a gene may be replaced or supplemented with the normal allele.
-This could potentially correct a genetic disorder, perhaps permanently.
-In other cases, genes are inserted and expressed only long enough to treat a medical problem.
-Ex vivo—outside the body
-In vivo—inside the body
In the past decade, new experimental techniques have generated enormous volumes of data related to DNA sequences.
The need to make sense of an ever-increasing flood of information has spawned the field of
, the application of computational methods to the storage and analysis of biological data.
is the study of complete sets of genes (genomes).
The first targets of genomics research were bacteria, which have relatively little DNA.
As of 2014, the genomes of thousands of species have been published, and tens of thousands more are in progress.
-The majority of organisms sequenced to
date are prokaryotes, including over 4,000
bacterial species and nearly 200 archaea.
Genome sequences have been determined for cells from several cancers, for ancient humans, and for the many bacteria that live in the human intestine.
Interconnections within Systems: Systems Biology
The computational power provided by the tools of bioinformatics allows the study of whole sets of genes and their interactions, as well as the comparison of genomes from different species.
Genomics is a rich source of new insights into fundamental questions about
-regulation of gene expression,
-embryonic development, and
The successes in genomics have encouraged scientists to begin similar systematic studies of the full protein sets that genomes encode (proteomes), an approach called
Because proteins, not genes, actually carry out the activities of the cell, scientists must study when and where proteins are produced and how they interact to understand the functioning of cells and organisms.
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