Chapter 16: The Molecular Basis of Inheritance
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Comprehensive review flashcards covering Chapter 16: The Molecular Basis of Inheritance, including key historical experiments, DNA structure, replication enzymes, repair mechanisms, and chromatin organization.
100 Terms
What fundamental biological requirement does genetic information fulfill across generations?
Genetic information provides for the continuity of life through the transmission of heritable information.
How does DNA replication allow genetic information to be inherited at both cellular and generational levels?
DNA replication allows genetic information to be inherited from a parent cell to daughter cells by mitosis (or cell division) and from generation to generation by meiosis (or sexual reproduction).
What is the structural difference between an unduplicated chromosome and a duplicated chromosome?
An unduplicated chromosome consists of a single double-stranded DNA molecule, whereas a duplicated chromosome consists of two identical sister chromatids joined together, each containing an identical double-stranded DNA molecule.
What are the two primary chemical components that make up chromosomes?
DNA and protein.
Why did early researchers believe that protein, rather than DNA, was the genetic material?
Proteins were known to possess enormous heterogeneity and specificity of function, constructed from 20 different amino acids, whereas nucleic acids appeared too uniform and simple with only four bases.
What organism did Frederick Griffith study in his landmark 1928 experiments on heredity?
The bacterium Streptococcus pneumoniae.
How did the virulent and nonvirulent strains of Streptococcus pneumoniae in Griffith's experiments differ?
The virulent S strain had a smooth polysaccharide capsule that made it pathogenic (disease-causing), whereas the nonvirulent R strain lacked a capsule, made rough colonies, and was nonpathogenic (harmless).
What happened when Griffith injected living pathogenic S cells into mice as a control?
The mice contracted pneumonia and died.
What happened when Griffith injected living nonpathogenic R cells into mice as a control?
The mice remained healthy and survived.
What was the result when Griffith injected heat-killed S cells into mice?
The heat-killed S cells were nonpathogenic on their own, and the mice remained healthy and lived.
What surprising result did Griffith observe when he injected mice with a mixture of heat-killed S cells and living R cells?
The mice died, and living, pathogenic S cells capable of reproduction were recovered from their blood.
What experiment and biological discovery are illustrated in this diagram?
Frederick Griffith's 1928 experiment showing that hereditary information from heat-killed pathogenic S cells transformed harmless living R cells into living pathogenic S cells.
How is transformation defined in molecular genetics?
A change in genotype and phenotype due to the assimilation of external DNA by a cell.
What substance did Oswald Avery, Maclyn McCarty, and Colin MacLeod identify as the transforming agent in 1944?
DNA.
How did Avery, MacLeod, and McCarty prove that DNA, and neither protein nor RNA, was the transforming agent?
They treated heat-killed bacteria with enzymes that specifically degraded RNA, protein, or DNA; transformation was blocked only when DNA was destroyed.
What is a bacteriophage?
A virus that infects and replicates within bacteria.
What biological structure is represented in this diagram, and what are its four labeled parts?
A T2 bacteriophage; its labeled structures are the phage head, DNA, tail sheath, and tail fiber.
How do the tail fibers of a bacteriophage initiate infection of a bacterial host cell?
The tail fibers bind specifically to receptor surface proteins located on the bacterial host cell wall.
What action does the phage tail sheath perform during bacterial infection?
The sheath contracts to inject the phage DNA into the bacterial cell.
Once inside the host bacterium, how does phage DNA direct viral replication?
Phage DNA directs production of phage proteins and copies of the phage genome using host and viral enzymes, taking advantage of components within the cell.
Which three separate sets of proteins self-assemble to build new phage particles inside an infected bacterium?
Proteins self-assemble into phage heads, tails, and tail fibers.
How does a bacteriophage destroy a host bacterial cell at the end of the lytic cycle?
The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter until the cell swells and bursts, releasing new phage particles.
Which two scientists performed the definitive 1952 blender experiments showing that phage genetic material is DNA?
Alfred Hershey and Martha Chase.
Why did Hershey and Chase use a radioactive isotope of sulfur (35S) to label viral protein?
Because protein contains sulfur (in amino acids such as methionine and cysteine), whereas DNA does not.
Why did Hershey and Chase use a radioactive isotope of phosphorus (32P) to label viral DNA?
Because nearly all of the phage's phosphorus is in its DNA backbone, while proteins lack phosphorus.
In the Hershey-Chase experiment, what mechanical method separated the empty viral coats from the bacteria?
Agitation in a blender to shear the viral parts from the cell surfaces, followed by centrifugation to separate bacterial cells into a pellet.
Where was radioactivity detected when Hershey and Chase tested bacteriophages labeled with radioactive sulfur (35S)?
Radioactivity remained in the supernatant fluid outside the bacterial cells.
Where was radioactivity detected when Hershey and Chase tested bacteriophages labeled with radioactive phosphorus (32P)?
Radioactivity was found inside the bacterial cells in the pellet.
What major conclusion did Hershey and Chase draw from their phage experiments?
DNA, not protein, enters bacterial cells and functions as the genetic material of phage T2.
What is Erwin Chargaff's first rule regarding DNA base composition?
DNA base composition varies among different species.
What is Erwin Chargaff's second rule regarding DNA base ratios within an organism?
Within any single species, the percentage of adenine equals thymine, and the percentage of guanine equals cytosine.
According to Chargaff's data, what is the approximate percentage of adenine and thymine in human DNA?
Adenine is approximately 30.3% (or 30.4%) and thymine is approximately 30.1%.
According to Chargaff's data, what is the percentage of adenine in wheat DNA?
28.1%.
According to Chargaff's data, what is the percentage of adenine in E. coli DNA?
24.7%.
What are the three chemical components of a single DNA nucleotide?
A pentose sugar (deoxyribose), a nitrogenous base, and a phosphate group.
To which carbon of the pentose sugar is the nitrogenous base attached in a nucleotide?
The 1′ carbon.
To which carbon of the pentose sugar is the phosphate group attached in a nucleotide?
The 5′ carbon.
What chemical group is attached to the 3′ carbon of the pentose sugar in a nucleotide?
A hydroxyl group (−OH).
Which two scientists built the first molecular model of DNA and were awarded the 1962 Nobel Prize?
James Watson and Francis Crick.
Who also shared the 1962 Nobel Prize with Watson and Crick for contributions to DNA structure discovery?
Maurice Wilkins.
What technique did Rosalind Franklin use to produce data critical to discovering the DNA double helix?
X-ray crystallography (producing X-ray diffraction images such as Photo 51).
What structural properties of DNA were deduced directly from Rosalind Franklin's X-ray photo?
The photo implied that DNA is helical, made up of two strands (a double helix), with nitrogenous bases spaced 0.34nm apart and a consistent width of 2nm.
What is the structural characteristic that defines purine nitrogenous bases?
Purines are nitrogenous bases containing two organic rings (a six-membered ring fused to a five-membered ring).
What is the structural characteristic that defines pyrimidine nitrogenous bases?
Pyrimidines are nitrogenous bases containing a single six-membered organic ring.
Which nitrogenous bases belong to the purine family?
Adenine (A) and Guanine (G).
Which nitrogenous bases belong to the pyrimidine family?
Cytosine (C), Thymine (T), and Uracil (U).
Which nitrogenous base is found only in DNA and never in RNA?
Thymine (T).
Which nitrogenous base is found only in RNA and never in DNA?
Uracil (U).
Which nitrogenous bases are found in both DNA and RNA?
Adenine (A), Guanine (G), and Cytosine (C).
Why must a purine always pair with a pyrimidine inside the double helix?
Purine-purine pairs would be too wide and pyrimidine-pyrimidine pairs too narrow; pairing a purine with a pyrimidine produces a uniform 2nm diameter.
How many hydrogen bonds form between adenine and thymine?
Two hydrogen bonds.
How many hydrogen bonds form between guanine and cytosine?
Three hydrogen bonds.
How did Watson and Crick's base-pairing rule explain Chargaff's rules biochemically?
Because an A on one strand must pair with a T on the partner strand, and a G with a C, the total amount of adenine must equal thymine, and guanine must equal cytosine.
What structural feature of DNA is highlighted in this untwisted chemical diagram?
The antiparallel arrangement of the sugar-phosphate backbones (5′→3′ opposite 3′→5′) with complementary purine and pyrimidine base pairing stabilized by hydrogen bonds.
What type of chemical bond links consecutive nucleotides along a single strand of DNA?
Covalent phosphodiester bonds linking the sugar of one nucleotide to the phosphate group of the next.
What is meant when the two strands of a DNA molecule are described as 'antiparallel'?
The two sugar-phosphate backbones run in opposite directions relative to each other, with one oriented 5′→3′ and the other 3′→5′.
What characterizes the 5′ end of a single DNA strand?
A free phosphate group attached to the 5′ carbon of the terminal deoxyribose sugar.
What characterizes the 3′ end of a single DNA strand?
A free hydroxyl group (−OH) attached to the 3′ carbon of the terminal deoxyribose sugar.
What is the semiconservative model of DNA replication?
A replication mechanism in which each daughter molecule consists of one parental (old) strand and one newly synthesized strand.
What is the conservative model of DNA replication?
A hypothesis proposing that the two parental strands reassociate after acting as templates, completely restoring the original parental double helix.
What is the dispersive model of DNA replication?
A hypothesis proposing that each strand of both daughter DNA molecules contains a mixture of interspersed old and newly synthesized DNA fragments.
What three hypothesized models of DNA replication are tested in this centrifuge tube experiment?
The conservative, semiconservative, and dispersive models of DNA replication.
How did Matthew Meselson and Franklin Stahl produce 'heavy' DNA for their experiment?
They cultured E. coli for several generations in a growth medium containing the heavy isotope of nitrogen (15N).
What did Meselson and Stahl do after growing E. coli in heavy 15N medium?
They transferred the bacteria to a growth medium containing only 14N, a lighter isotope of nitrogen.
What density banding pattern did Meselson and Stahl observe after one round of DNA replication in 14N?
A single hybrid band of intermediate density between 15N and 14N.
Which replication model was ruled out by the single hybrid band observed after the first bacterial replication in 14N?
The conservative model, which predicted two separate bands (one heavy parental band and one light new band).
What density banding pattern did Meselson and Stahl observe after the second round of replication in 14N?
Two distinct bands: one light density band and one intermediate hybrid band.
Which replication model was ruled out by the observation of two distinct bands after the second round of replication?
The dispersive model, which predicted only a single progressively lighter hybrid band.
Which model of DNA replication matched the data obtained by Meselson and Stahl?
The semiconservative model.
What is an origin of replication?
A specific site on a DNA molecule with a specific nucleotide sequence where replication begins.
How does the number of origins of replication differ between E. coli and eukaryotic cells?
E. coli has a single origin on its circular chromosome, whereas eukaryotic chromosomes have hundreds to thousands of replication origins.
How is the replication of very long eukaryotic DNA molecules speeded up?
Multiple replication bubbles form simultaneously along the DNA molecule and eventually fuse together.
What is a replication fork?
A Y-shaped region at each end of a replication bubble where parental strands of DNA are being unwound.
In which direction can a newly made DNA strand elongate?
Only in the 5′→3′ direction.
Why can DNA polymerase only synthesize a new strand in the 5′→3′ direction?
DNA polymerases can add nucleotides only to the free 3′ end of a growing strand, never to the 5′ end.
What is the leading strand during DNA replication?
The new complementary DNA strand synthesized continuously toward the replication fork in the mandatory 5′→3′ direction.
What is the lagging strand during DNA replication?
The discontinuously synthesized DNA strand that elongates in a 5′→3′ direction away from the replication fork.
What are Okazaki fragments?
Short segments of DNA synthesized on the lagging strand away from the replication fork during DNA replication.
How are Okazaki fragments joined together into a continuous DNA strand?
DNA ligase joins the sugar-phosphate backbones of all the Okazaki fragments together.
What is the function of helicase during DNA replication?
Untwists and separates the parental double helix at the replication fork.
What is the function of single-strand binding protein (SSBP)?
Holds the unwound parental DNA strands apart and stabilizes them during replication.
Why is single-strand binding protein (SSBP) categorized differently from helicase, primase, and ligase?
SSBP is a structural binding protein, not an enzyme.
What is the function of topoisomerase during DNA replication?
Relieves strain and overwinding ahead of the replication fork by breaking, swiveling, and rejoining DNA strands.
What is the function of primase during DNA replication?
Synthesizes an RNA primer to initiate DNA synthesis by providing a free 3′−OH group.
What is the role of DNA polymerase III (DNA pol III) in E. coli?
Adds DNA nucleotides to new strands, elongating both the leading strand and Okazaki fragments.
What is the role of DNA polymerase I (DNA pol I) in E. coli?
Removes the RNA primer from the 5′ end of Okazaki fragments and replaces it with DNA nucleotides.
What process and cellular structure are illustrated in this diagram?
A DNA replication fork illustrating the continuous synthesis of the leading strand and discontinuous synthesis of the lagging strand by replication enzymes.
What is the function of a nuclease enzyme in DNA repair?
Cuts DNA or RNA, either removing one or a few damaged bases or hydrolyzing DNA completely into component nucleotides.
What is a thymine dimer, and what causes it?
Covalent linkage between adjacent thymine bases on a DNA strand caused by ultraviolet (UV) radiation, which causes the double helix to buckle.
How is a thymine dimer repaired by nucleotide excision repair?
A nuclease cuts out the damaged DNA segment, DNA polymerase fills in the missing nucleotides using the intact strand as template, and DNA ligase seals the backbone.
What are telomeres and what two protective functions do they perform in eukaryotic cells?
Repetitive, noncoding nucleotide sequences at the ends of eukaryotic chromosomes; specific proteins associate with them to prevent activating cellular DNA-damage monitoring systems, and they act as buffer zones protecting protein-coding genes from shortening.
Why do telomeres become shorter during each round of DNA replication?
Because the lagging strand cannot replace the terminal RNA primer with DNA at the 5′ end of a linear DNA molecule, resulting in progressive erosion.
How is telomere shortening connected to human physiology?
Telomere shortening is connected to the aging process of tissues and the organism as a whole.
What types of cells produce the enzyme telomerase, and why is this enzyme important?
Eukaryotic germ cells; telomerase catalyzes the lengthening of telomeres, restoring their original length and preventing vital genes from being lost in gametes.
Why are cancer cells able to divide indefinitely while most normal body cells have a limited life span?
Telomerase activity is abnormally high in cancer cells, maintaining telomere length and preventing cell senescence.
What structural unit of chromatin is illustrated in this drawing, and what are its parts?
A nucleosome, the basic unit of DNA packing, consisting of DNA wrapped around an octamer of histone proteins, separated by linker DNA.
What is the structural difference between heterochromatin and euchromatin?
Heterochromatin is highly condensed and compacted, whereas euchromatin is less compacted and more dispersed.
How does the organization of heterochromatin affect gene expression compared to euchromatin?
Heterochromatin is largely inaccessible to transcription machinery so its genes are generally silenced, while euchromatin is accessible and its genes can be transcribed.
How has our understanding of interphase chromosome organization in the nucleus changed from the 'bowl of spaghetti' model?
Interphase chromosomes occupy specific discrete chromosome territories within the nucleus rather than being randomly entangled.
In the Chapter 16 Test Your Understanding section, what is the answer to question 1?
A.