CHAPTER 10: Molecular Biology of the Gene Name ______________

Chapter 12: Biotechnology

Introduction

A. Viruses infect organisms by

1. binding to receptors on a host’s target cell & injecting viral genetic material into the cell

2. hijacking the host cell’s own molecules and organelles to produce new copies of the virus.

B. The host cell is destroyed, and newly replicated viruses are released to continue the infection.

C. Viruses are not generally considered alive because they are not cellular and cannot reproduce on their

own.

D. Because viruses have much less complex structures than cells, they are relatively easy to study at the

molecular level.

The Structure of the Genetic Material

10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material

1. Until the 1940s, the case for proteins serving as the genetic material was stronger than the case for

DNA.

a. Proteins are made from 20 different amino acids

b. DNA was known to be made from just four kinds of nucleotides.

2. Studies of bacteria and viruses ushered in the field of molecular biology, the study of heredity at the

molecular level, and revealed the role of DNA in heredity.

3. In 1928, Frederick Griffith discovered that a “transforming factor” could be transferred into a

bacterial cell. He found that

a. when he exposed heat-killed pathogenic bacteria to harmless bacteria, some harmless bacteria

were converted to disease-causing bacteria and

b. the disease-causing characteristic was inherited by descendants of the transformed cells.

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4. In 1952, Alfred Hershey and Martha Chase used bacteriophages to show that DNA is the genetic ma-

terial of T2, a virus that infects the bacterium Escherichia coli (E. coli).

a. Bacteriophages (or phages for short) are viruses that infect bacterial cells.

b. Phages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus to detect

DNA.

c. Bacteria were infected with either type of labeled phage to determine which substance was injected

into cells and which remained outside the infected cell.

d. The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphorus-

labeled DNA was detected inside cells.

e. Cells with phosphorus-labeled DNA produced new bacteriophages with radioactivity in DNA but

not in protein.

10.2 DNA and RNA are polymers of nucleotides

1. DNA and RNA are nucleic acids.

2. One of the two strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain).

3. A nucleotide is composed of a nitrogenous base, five-carbon sugar, and phosphate group.

a. The nitrogen bases are grouped according to their shape as either pyrimidines or purines.

Nucleotides

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4. The nucleotides are joined to one another by a sugar-phosphate backbone.

Deoxyribose (Found in DNA) Ribose (Found in RNA)

Phosphate Ester Bond

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5. Each type of DNA nucleotide has a different nitrogen-containing base:

a. adenine (A), cytosine (C), thymine (T), and guanine (G).

6. RNA (ribonucleic acid) is unlike DNA in that it uses the sugar ribose (instead of deoxyribose in

DNA) and RNA has the nitrogenous base uracil (U) instead of thymine.

7. Hydrogen Bonds- weak bond between covalently bonded hydrogen and an atom with an unshared

pair of electrons

a. Hydrogen acts (+) while oxygen or nitrogen acts (-)

b. Adenine and Thymine have 2 hydrogen bonds; Cytosine and guanine have 3 hydrogen bonds

Label the following: cytosine, guanine, adenine, thymine, phosphate, phosphodiester bond, hy-

drogen bond, 3’ end, 5’ end, deoxyribose

10.3 DNA is a double-stranded helix

1. In 1952, after the Hershey-Chase experiment demonstrated that the genetic material was most likely

DNA, a race was on to describe the structure of DNA and explain how the structure and properties of

DNA can account for its role in heredity.

2. In 1953, James D. Watson and Francis Crick deduced the secondary structure of DNA, using

a. X-ray crystallography data of DNA from the work of Rosalind Franklin and Maurice Wilkins

b. Erwin Chargaff’s observation that in DNA the amount of adenine was equal to the amount of thy-

mine and the amount of guanine was equal to that of cytosine. %A=%T and %G=%C

3. Watson and Crick reported that DNA consisted of two polynucleotide strands wrapped into a double

helix.

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a. The sugar-phosphate backbone is on the outside.

b. The nitrogenous bases are perpendicular to the backbone in the interior.

c. Specific pairs of bases give the helix a uniform shape.

4. In 1962, the Nobel Prize was awarded to

a. James D. Watson, Francis Crick, and Maurice Wilkins.

b. Rosalind Franklin probably would have received the prize as well but for her death from cancer in

1958. Nobel Prizes are never awarded posthumously.

5. The Watson-Crick model gave new meaning to the words genes and chromosomes. The genetic in-

formation in a chromosome is encoded in the nucleotide sequence of DNA.

DNA Replication

10.4 DNA replication depends on specific base pairing

1. In their description of the structure of DNA, Watson and Crick noted that the structure of DNA sug-

gests a possible copying mechanism.

2. DNA replication follows a semiconservative model.

a. The two DNA strands separate.

b. Each strand is used as a pattern to produce a complementary strand, using specific base pairing.

c. Each new DNA helix has one old strand with one new strand.

10.5 DNA replication proceeds in two directions at many sites simultaneously

1. DNA replication begins at the origins of replication where

a. DNA unwinds at the origin to produce a “bubble,”

b. replication proceeds in both directions from the origin, and replication ends when products from

the bubbles merge with each other.

2. DNA replication occurs in the 5 ́to 3 ́direction.

a. Replication is continuous on the 3´ to 5 ́template.

b. Replication is discontinuous on the 5 ́to 3 ́template, forming short segments (okazaki fragments)

3. Two key proteins are involved in DNA replication.

a. DNA ligase joins small fragments into a continuous chain.

b. DNA polymerase adds nucleotides to a growing chain and proofreads and corrects improper base

pairings.

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4. DNA polymerases& DNA ligase also repair DNA damaged by harmful radiation & toxic chemicals.

5. DNA replication ensures that all the somatic cells in a multicellular organism carry the same genetic

information.

The Flow of Genetic Information from DNA to RNA to Protein

10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits

1. DNA specifies traits by dictating protein synthesis.

2. The molecular chain of command is from DNA in the nucleus to RNA and RNA in the cytoplasm to

protein.

3. Transcription is the synthesis of RNA under the direction of DNA.

4. Translation is the synthesis of proteins under the direction of RNA.

5. The connections between genes and proteins

a. The initial one gene–one enzyme hypothesis based on studies of inherited metabolic diseases.

b. The one gene–one enzyme hypothesis was expanded to include all proteins.

c. Most recently, the one gene–one polypeptide hypothesis recognizes that some proteins are com-

posed of multiple polypeptides.

10.7 Genetic information written in codons is translated into amino acid sequences - the sequence of nucleotides

in DNA provides a code for constructing a protein.

1. Protein construction requires a conversion of a nucleotide sequence to an amino acid sequence.

2. Transcription rewrites the DNA code into RNA, using the same nucleotide “language.”

3. 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

nonoverlapping three-base “words” called codons.

4. Translation involves switching from the nucleotide “language” to the amino acid “language.”

5. Each amino acid is specified by one of the 64 codons.

6. Some amino acids have more than one possible codon.

10.8 The genetic code dictates how codons (found on mRNA) are translated into amino acids

1. Characteristics of the genetic code

a. Three nucleotides specify one amino acid.

b. 61 codons correspond to amino acids.

c. AUG codes for methionine and signals the start of transcription.

d. 3 “stop” codons signal the end of translation (UAA, UAG, and UGA)

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2. Table of Codons

10.9 Transcription produces genetic messages in the form of RNA

1. An RNA molecule is transcribed from a DNA template by a process that resembles the partial

synthesis of a DNA strand during DNA replication.

2. RNA nucleotides are linked by the transcription enzyme RNA polymerase.

3. Specific sequences of nucleotides along the DNA mark where transcription begins and ends.

4. The “start transcribe” signal is a nucleotide sequence called a promoter.

5. Transcription begins with initiation, as the RNA polymerase attaches to the promoter.

6. During the second phase, elongation, the RNA grows longer.

7. As the RNA peels away, the DNA strands rejoin.

8. Finally, in the third phase, termination, the RNA polymerase reaches a sequence of bases in the DNA

template called a terminator, which signals the end of the gene.

9. The polymerase molecule now detaches from the RNA molecule and the gene.

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10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA

1. Messenger RNA (mRNA)

a. encodes amino acid sequences and conveys genetic messages from DNA to the translation ma-

chinery of the cell, which in

i. prokaryotes occurs in the same place that mRNA is made

ii. eukaryotes mRNA must exit the nucleus via nuclear pores to enter the cytoplasm.

c. Eukaryotic mRNA has introns, interrupting sequences that separate while exons, the coding re-

gions.

2. Eukaryotic mRNA undergoes processing before leaving the nucleus.

a. RNA splicing removes introns and joins exons to produce a continuous coding sequence.

b. A cap and tail of extra nucleotides are added to the ends of the mRNA to facilitate the export of the

mRNA from the nucleus & protect the mRNA from attack by cellular enzymes, and help ribo-

somes bind to the mRNA.

10.11 Transfer RNA molecules serve as interpreters during translation

1. Transfer RNA (tRNA) molecules function as a language interpreter, converting the genetic message

of mRNA & into the language of proteins.

2. Transfer RNA molecules perform this interpreter task by

a. picking up the appropriate amino acid

b. using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the

mRNA.

10.12 Ribosomes build polypeptides by the process translation occurring on the surface of the ribosome.

1. Ribosomes coordinate the functioning of mRNA and tRNA and, ultimately, the synthesis of

polypeptides.

2. Ribosomes have two subunits: small and large.

3. Each subunit is composed of ribosomal RNAs and proteins.

4. Ribosomal subunits come together during translation.

5. Ribosomes have binding sites for mRNA and tRNAs.

10.13 An initiation codon marks the start of an mRNA message

1. Translation can be divided into the same three phases as transcription:

a. initiation, elongation, and termination.

2. Initiation brings together mRNA, tRNA bearing the first amino acid, and the two subunits of a ribo-

some.

3. Initiation establishes where translation will begin.

4. Initiation occurs in two steps.

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a. An mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the

start codon, AUG and codes for methionine.

b. A large ribosomal subunit joins the small subunit, allowing the ribosome to function.

i. The first tRNA occupies the P site, which will hold the growing peptide chain.

ii. The A site is available to receive the next tRNA.

TRANSLATION

10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation

1. Once initiation is complete, amino acids are added one by one to the first amino acid.

2. Elongation is the addition of amino acids to the polypeptide chain.

3. Each cycle of elongation has three steps.

a. 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.

b. Peptide bond formation: The new amino acid is joined to the chain.

c. Translocation: tRNA is released from the P site and the ribosome moves tRNA from the A site in-

to the P site.

4. Elongation continues until the termination stage of translation, when

a. the ribosome reaches a stop codon, the completed polypeptide is freed from the last tRNA, and

b. the ribosome splits back into its separate subunits.

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10.15 Review: The flow of genetic information in the cell is DNA → RNA → protein [Central Dogma]

1. Transcription is the synthesis of RNA from a DNA template. In eukaryotic cells,

a. transcription occurs in the nucleus

b. the mRNA must travel from the nucleus to the cytoplasm.

2. Translation can be divided into four steps, all of which occur in the cytoplasm: amino acid

attachment, initiation of polypeptide synthesis, elongation, and termination.

10.16 Mutations can change the meaning of genes

1. A mutation is any change in the nucleotide sequence of DNA.

2. Mutations can involve large chromosomal regions or just a single nucleotide pair.

3. Mutations within a gene can be divided into two general categories.

a. Base substitutions involve the replacement of one nucleotide with another.

i. have no effect at all, producing a silent mutation,

ii. change the amino acid coding, producing a missense mutation, which produces a different

amino acid,

iii. lead to a base substitution that produces an improved protein that enhances the success of the

mutant organism and its descendant

iv. change an amino acid into a stop codon, producing a nonsense mutation.

b. Mutations can result in deletions or insertions that may alter the reading frame (triplet grouping)

of the mRNA, so that nucleotides are grouped into different codons, lead to significant changes in

amino acid sequence downstream of the mutation, and produce a nonfunctional polypeptide.

4. Mutagenesis is the production of mutations.

5. Mutations can be caused by

a. spontaneous errors that occur during DNA replication or recombination or

b. mutagens, which include high-energy radiation such as X-rays and ultraviolet light and chemicals.

Chapter 10: Molecular Biology of the Gene

Word Parts

anti- = opposite

capsa- = a box

exo- = out, outside,

-genesis = origin, birth

helic- = a spiral

intro- = within

liga- = bound or tied

lyso- = loosen

lyto- = loosen

muta- = change;

-gen = producing

-phage = to eat

poly- = many

pro- = before;

-phage = to eat

retro- = backward

semi- = half

trans- = across;

-script = write

virul- = poisonous

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Vocabulary

1. molecular biology- The study of the molecular basis of genes and gene expression; molecular

genetics.

2. bacteriophage- A virus that infects bacteria; also called a phage.

3. nucleotide- An organic monomer consisting of a five-carbon sugar covalently bonded to a

nitrogenous base and a phosphate group. Nucleotides are the building blocks of nucleic acids.

4. sugar-phosphate backbone- The alternating chain of sugar and phosphate to which the DNA and

RNA nitrogenous bases are attached; like the sides of a ladder.

5. deoxyribonucleic acid (DNA)- A double-stranded helical nucleic acid molecule consisting of

nucleotide monomers with deoxyribose sugar and the nitrogenous bases adenine (A), cytosine

(C), guanine (G), and thymine (T). Capable of replicating, is an organism's genetic material.

6. thymine (T)- A single-ring nitrogenous base found in DNA; pyrimidine

7. cytosine (C)- A single-ring nitrogenous base found in DNA and RNA; pyrimidine

8. adenine (A)- A double-ring nitrogenous base found in DNA and RNA; purine

9. guanine (G)- A double-ring nitrogenous base found in DNA and RNA; purine

10. uracil (U)- A single-ring nitrogenous base found in RNA; pyrimidine

11. double helix- The form of native DNA, referring to its two adjacent polynucleotide strands

wound into a spiral shape.

12. semiconservative model- Type of DNA replication in which the replicated double helix consists

of one old strand, derived from the old molecule, and one newly made strand.

13. DNA polymerase- An enzyme that assembles DNA nucleotides into polynucleotides using a

preexisting strand of DNA as a template. (replication enzyme)

14. DNA ligase- An enzyme, essential for DNA replication, that catalyzes the covalent bonding of

adjacent DNA strands; used in genetic engineering to paste a specific piece of DNA containing a

gene of interest into a bacterial plasmid or other vector. (like glue)

15. transcription- The synthesis of messenger RNA (mRNA) from a DNA template.

16. translation- The synthesis of a polypeptide using the genetic information encoded in an mRNA

molecule. There is a change of "language" from nucleotides to amino acids.

17. triplet code- A set of three-nucleotide-long DNA that specify the amino acids for polypeptide

chains; a general term for a set of three nucleotides in a sequence

18. codon- A three-nucleotide sequence in mRNA that specifies a particular amino acid or

polypeptide termination signal; the basic unit of the genetic code.

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19. genetic code- The set of rules that dictates the correspondence between RNA codons in an

mRNA molecule and amino acids in protein.

20. RNA polymerase- An enzyme that links together the growing chain of RNA nucleotides during

transcription, using a DNA strand as a template.

21. terminator- A special sequence of nucleotides in DNA that marks the end of a gene. It signals

RNA polymerase to release the newly made RNA molecule and then to depart from the gene.

22. messenger RNA (mRNA)- The type of ribonucleic acid that encodes genetic information from

DNA and conveys it to ribosomes, where the information is translated into amino acid sequences.

23. introns-internal noncoding regions of a gene, that remains in the nucleus

24. exons- parts of the gene that are expressed (coding sequence) and leave the nucleus

25. RNA splicing- The removal of introns and joining of exons in eukaryotic RNA, forming an

mRNA molecule with a continuous coding sequence; occurs before mRNA leaves the nucleus.

26. transfer RNA (tRNA)- A type of ribonucleic acid that functions as an interpreter in translation

Each tRNA molecule has a specific anticodon, picks up a specific amino acid, and conveys the

amino acid to the appropriate codon on mRNA.

27. anticodon- On a tRNA molecule, a specific sequence of three nucleotides that is

complementary to a codon triplet on mRNA.

28. ribosome- A cell structure consisting of RNA and protein organized into two subunits and

functioning as the site of protein synthesis in the cytoplasm. The ribosomal subunits are

constructed in the nucleolus.

29. ribosomal RNA (rRNA)- The type of ribonucleic acid that, together with proteins, makes up

ribosomes; the most abundant type of RNA in most cells.

30. start codon- On mRNA, the specific three-nucleotide sequence (AUG) to which an initiator

tRNA molecule binds, starting translation of genetic information.

31. P site- One of two of a ribosome's binding sites for tRNA during translation. The P site holds the

tRNA carrying the growing polypeptide chain. (P stands for peptidyl tRNA.)

32. A site- One of two of a ribosome's binding sites for tRNA during translation. The A site holds the

tRNA that carries the next amino acid in the polypeptide chain. (A stands for aminoacyl tRNA.)

33. stop codon- In mRNA, one of three triplets (UAG, UAA, UGA) that signal gene translation

to stop.

34. mutation- A change in the nucleotide sequence of an organism's DNA; mutation also can occur

in the DNA or RNA of a virus; the ultimate source of genetic diversity. Types of mutations

include silent mutation, missense mutations, and nonsense mutations.

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35. reading frame- The way in which a cell's mRNA-translating machinery groups the mRNA

nucleotides into codons.

36. mutagenesis- The creation of a mutation.

37. mutagen- A chemical or physical agent that interacts with DNA and causes a mutation.

38. virus- A microscopic particle capable of infecting cells of living organisms and inserting its

genetic material. Viruses are generally not considered to be alive because they do not display

all of the characteristics associated with life.

39. prophage- Phage DNA that has inserted by genetic recombination into the DNA of a prokaryotic

chromosome.

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EXTRA- not notes for DNA chapter

38. virus- A microscopic particle capable of infecting cells of living organisms and inserting its

genetic material. Viruses are generally not considered to be alive because they do not display

all of the characteristics associated with life.

39. capsid- The protein shell that encloses a viral genome. It may be rod-shaped, polyhedral,

or more complex in shape.

40. lytic cycle- A type of viral replication cycle resulting in the release of new viruses by lysis

(breaking open) of the host cell.

41. lysogenic cycle- A type of bacteriophage replication cycle in which the viral genome is

incorporated into the bacterial host chromosome as a prophage. New phages are not produced,

and the host cell is not killed or lysed unless the viral genome leaves the host chromosome.

42. prophage- Phage DNA that has inserted by genetic recombination into the DNA of a prokaryotic

chromosome.

43. emerging virus- A virus that has appeared suddenly or has recently come to the attention of

medical scientists.

44. AIDS (acquired immunodeficiency syndrome)- Acquired immunodeficiency syndrome; the late

stages of HIV infection, characterized by a reduced number of T cells and the appearance of

characteristic opportunistic infections.

45. HIV (human immunodeficiency virus)- Human immunodeficiency virus, the retrovirus that

attacks the human immune system and causes AIDS.

46. reverse transcriptase- An enzyme used by retroviruses that catalyzes the synthesis of DNA

on an RNA template.

47. retrovirus- An RNA virus that reproduces by means of a DNA molecule. It reverse-transcribes

its RNA into DNA, inserts the DNA into a cellular chromosome, and then transcribes more

copies of the RNA from the viral DNA. HIV and a number of cancer-causing viruses are

retroviruses.

48. viroid- A plant pathogen composed of molecules of naked, circular RNA several hundred

nucleotides long.

49. prion- An infectious form of protein that may multiply by converting related proteins into more

prions. Prions cause several related diseases in different animals, including scrapie in sheep and

mad cow disease.

50. transformation- The incorporation of new genes into a cell from DNA that the cell takes up from

the surrounding environment.

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51. transduction- The transfer of bacterial genes from one bacterial cell to another by a phage.

52. conjugation- The union (mating) of two bacterial cells or protist cells and the transfer of DNA

between the two cells.

53. F factor- A piece of DNA that can exist as a bacterial plasmid. The F factor carries genes for

making sex pili and other structures needed for conjugation, as well as a site where DNA

replication can start. F stands for fertility.

54. plasmid- A small ring of independently replicating DNA separate from the main chromosome(s).

Plasmids are found in prokaryotes and yeast.

55. R plasmid- A bacterial plasmid that carries genes for enzymes that destroy particular antibiotics,

thus making the bacterium resistant to the antibiotics.

The Genetics of Viruses and Bacteria

10.17 Viral DNA may become part of the host chromosome

1. A virus is essentially “genes in a box,” an infectious particle consisting of

a. a bit of nucleic acid,

b. wrapped in a protein coat called a capsid, and in some cases, a membrane envelope.

2. Viruses have two types of reproductive cycles.

a. In the lytic cycle,

i. viral particles are produced using host cell components,

ii. the host cell lyses, and viruses are released.

b. Lysogenic cycle

i. Viral DNA is inserted into the host chromosome by recombination.

ii. Viral DNA is duplicated along with the host chromosome during each cell

division.

iii. The inserted phage DNA is called a prophage. Most prophage genes are inactive.

iv. Environmental signals can cause a switch to the lytic cycle, causing the viral

DNA to be excised from the bacterial chromosome and leading to the death of the host cell.

10.18 CONNECTION: Many viruses cause disease in animals and plants

1. Viruses can cause disease in animals and plants.

2. DNA viruses and RNA viruses cause disease in animals.

3. A typical animal virus has a membranous outer envelope and projecting spikes of

glycoprotein.

4. The envelope helps the virus enter and leave the host cell.

5. Many animal viruses have RNA rather than DNA as their genetic material. These

include viruses that cause the common cold, measles, mumps, polio, and AIDS.

6. The reproductive cycle of the mumps virus, a typical enveloped RNA virus, has seven major steps:

a. entry of the protein-coated RNA into the cell,

b. uncoating—the removal of the protein coat, RNA synthesis—mRNA synthesis using a viral en-

zyme,

c. protein synthesis—mRNA is used to make viral proteins,

d. new viral genome production—mRNA is used as a template to synthesize new

viral genomes,

e. assembly—the new coat proteins assemble around the new viral RNA, and

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f. exit—the viruses leave the cell by cloaking themselves in the host cell’s plasma

membrane.

7. Some animal viruses, such as herpes viruses, reproduce in the cell nucleus.

8. Most plant viruses are RNA viruses.

a. To infect a plant, they must get past the outer protective layer of the plant.

b. Viruses spread from cell to cell through plasmodesmata.

c. Infection can spread to other plants by insects, herbivores, humans, or farming tools.

9. There are no cures for most viral diseases of plants or animals.

10.19 EVOLUTION CONNECTION: Emerging viruses threaten human health

1. Viruses that appear suddenly or are new to medical scientists are called emerging

viruses. These include the

a. AIDS virus, Ebola virus, West Nile virus, and SARS virus.

2. Three processes contribute to the emergence of viral diseases:

a. mutation—RNA viruses mutate rapidly.

b. contact between species—viruses from other animals spread to humans.

c. spread from isolated human populations to larger human populations, often over great distances.

10.20 The AIDS virus makes DNA on an RNA template

1. AIDS (acquired immunodeficiency syndrome) is caused by HIV (human immunodeficiency virus).

2. HIV

a. is an RNA virus, has two copies of its RNA genome, carries molecules of reverse transcriptase,

which causes reverse transcription, producing DNA from an RNA template.

3. After HIV RNA is uncoated in the cytoplasm of the host cell,

a. reverse transcriptase makes one DNA strand from RNA,

b. reverse transcriptase adds a complementary DNA strand,

c. double-stranded viral DNA enters the nucleus and integrates into the chromosome, becoming a

provirus, the provirus DNA is used to produce mRNA,

d. the viral mRNA is translated to produce viral proteins, and

e. new viral particles are assembled, leave the host cell, and can then infect other cells.

10.21 Viroids and prions are formidable pathogens in plants and animals

1. Some infectious agents are made only of RNA or protein.

a. Viroids are small, circular RNA molecules that infect plants. Viroids

i. replicate within host cells without producing proteins and

ii. interfere with plant growth.

b. Prions are infectious proteins that cause degenerative brain diseases in animals.

Prions

i. appear to be misfolded forms of normal brain proteins,

ii. which convert normal protein to misfolded form.

10.22 Bacteria can transfer DNA in three ways

1. Viral reproduction allows researchers to learn more about the mechanisms that regulate DNA replica-

tion and gene expression in living cells.

2. Bacteria are also valuable but for different reasons.

a. Bacterial DNA is found in a single, closed loop chromosome.

b. Bacterial cells divide by replication of the bacterial chromosome and then by binary fission.

c. Because binary fission is an asexual process, bacteria in a colony are genetically identical to the

parent cell.

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3. Bacteria use three mechanisms to move genes from cell to cell.

a. Transformation is the uptake of DNA from the surrounding environment.

b. Transduction is gene transfer by phages.

c. Conjugation is the transfer of DNA from a donor to a recipient bacterial cell through a cytoplas-

mic (mating) bridge.

4. Once new DNA gets into a bacterial cell, part of it may then integrate into the recipient’s chromo-

some.

10.23 Bacterial plasmids can serve as carriers for gene transfer

1. The ability of a donor E. coli cell to carry out conjugation is usually due to a specific piece of DNA

called the F factor.

2. During conjugation, the F factor is integrated into the bacterium’s chromosome.

3. The donor chromosome starts replicating at the F factor’s origin of replication.

4. The growing copy of the DNA peels off and heads into the recipient cell.

5. The F factor serves as the leading end of the transferred DNA.

6. An F factor can also exist as a plasmid, a small circular DNA molecule separate from the bacterial

chromosome.

a. Some plasmids, including the F factor, can bring about conjugation and move to

another cell in linear form. The transferred plasmid re-forms a circle in the recipient cell.

7. R plasmids

a. pose serious problems for human medicine by carrying genes for enzymes that destroy antibiotics

Bacteria Viruses

Cell wall & cell membrane Protein coat

Reproduce on own- various ways Need a host to reproduce

No organelles but metabolize No organelles

Beneficial/harmful Harmful mostly (Helpful?)

Antibiotics kill Antibiotics ineffective

Immunizations Immunizations

DNA DNA or RNA

Immune system fights Immune system fights