Chapter 16

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The Molecular Basis of Inheritance
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1

spell DNA

deoxyribonucleic acid

2

F. Griffith

In 1928, F. Griffith was working with two strains of Streptococcus pneumoniae. When he mixed the remains of heat-killed pathogenic bacteria with harmless bacteria, some bacteria were changed into disease-causing bacteria. These bacteria incorporated external genetic material in a process called transformation, which results in a change in genotype and phenotype. Scientists later determined that DNA was the molecule that transformed bacteria.

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proposed a double-helical model for the structure of deoxyribonucleic acid (who)(when)

James Watson, Francis Crick; 1953

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heritable factors (who)

Gregor Mendel

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genes on chromosomes (who)

Thomas Hunt Morgan

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DNA replication

The process by which a DNA molecule is copied

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2 chemical components of chromosomes

DNA and protein

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pathogenicity (when)(who)(how)

1928; Frederick Griffith; while trying to develop a vaccine against pneumonia. He was studying the bacterium streptococcus pneumoniae. Concluded that living nonpathogenic R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells that enabled the R cells to make capsules.

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streptococcus pneumoniae

bacterium that causes pneumonia in mammals

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pathogenic

disease-causing bacterium

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nonpathogenic

harmless bacterium

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transformation

a change in genotype and phenotype due to the assimilation of external DNA by a cell

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identified the transforming substance to be DNA (who)

Oswald Avery, Maclyn McCarty, and Colin Macleod

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virus

consist of DNA (or sometimes RNA) contained in a protein coat. The reproduce by infecting a cell and take over the cell's metabolic machinery

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bacteriophage

viruses that infect bacteria

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A. Hershey and M. Chase

In 1952, A. Hershey and M. Chase showed that DNA was the genetic material of a phage known as T2 that infects the bacterium Escherichia coli.

Hershey and Chase devised an experiment using radioactive isotopes to determine whether it was a phage's DNA or protein that entered the bacteria and served as the genetic material of T2 phage.

They grew T2 with radioactive sulfur to tag phage proteins and radioactive phosphorus to tag phage DNA.

After infecting separate samples of E. coli with differently labeled T2 cells, they blended and centrifuged the samples to isolate the bacterial cells from the lighter viral particles.

In the protein sample, radioactivity was found in the liquid and did not enter the bacterial cells. In the DNA sample, most of the radioactivity was found in the bacterial cell pellet.

They concluded that viral DNA is injected into the bacterial cells and serves as the hereditary material for viruses.

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E. Chargaff

In 1950, E. Chargaff noted that the percentages of the four nitrogenous bases in DNA were species specific. He also determined that the number of A and T was approximately equal as well as the G and C.

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Chargaff's rules

  1. the base composition of DNA varies between species
  2. for each species, the percentages of A and T bases are roughly equal and the percentages of G and C bases are roughly equal.
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DNA nucleotide

Phosphate, Sugar (deoxyribose), Nitrogenous base (GCAT)

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DNA structure

pg. 117 study guide

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Guanine

...

22

Cytosine

...

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Adenine

...

24

Thymine

...

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double helix

the presence of two strands

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antiparallel

sugar-phosphate backbone subunits run in opposite directions

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phosphodiester bond

the bond between the phosphate group and the sugar in a polynucleotide moleucle

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hydrogen bond

the bond between the nitrogenous bases that hold the strands together

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semiconservative model

the two strands of the parental molecule separate, and each functions as a template for synthesis of a new, complementary strand

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conservative model

the two parental strands reassociate after acting as templates for new strands, thus restoring the parental double helix

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dispersive model

all four strands of DNA following replication have a mixture of old and new DNA

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origins of replication

site where replication of a chromosome begins

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the E. coli chromosome, like many other bacterial chromosomes, is circular and has a single origin

...

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a eukaryotic chromosome may have hundreds or even a few thousand replication origins

...

35

replication fork

a Y-shpaed region at the end of a replication bubble where the parental strands of DNA are being unwound

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helicases

enzyme that unwinds the helix and separates the parental strands at each replication fork.

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single-strand binding proteins

keep the separated strands apart while they serve as templates.

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topoisomerase

breaks, swivels, and rejoins the parental DNA ahead of the replication fork, relieving the strain caused by unwinding

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Primase

enzyme that joins about 5-10 RNA nucleotides base-paired to the parental strand to form the Primer needed to start the new DNA strand.

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DNA polymerases

connect nucleotides to the growing end of a new DNA strand

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A nucleotide lines up with its complementary base on the template strand; it loses two phosphate groups, and thy hydrolysis of this pyrophosphate to two inorganic phosphates provides the energy for polymerization.

...

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DNA polymerase III

...

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DNA polymerase I

replaces the RNA primer with DNA nucleotides

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DNA ligase

enzyme that joins the sugar-phosphate backbones of the fragments

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Initial pairing errors in nucleotide placement may occur as often as 1 per 100,000 base pairs

...

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mismatch repair

other enzymes remove and replace incorrectly paired nucleotides that have resulted from replication errors (colon cancer)

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nucleotide excision repair

the damaged strand is cut out by a nucleases and the gap is correctly filled through the action of a DNA polymerase and ligase.

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nuclease

DNA-cutting enzyme

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nucleotide excision in skin cells

in skin cells, nucleotide excision repair frequently corrects thymine dimers caused by ultraviolet rays in sunlight.

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xeroderma pigmentosum

inherited defect in a nucleotide excision repair enzyme. Individuals with this disorder are hypersensitive to sunlight, if mutations in skin cells are left untreated, skin cancer results.

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telomeres

multiple repetitions of a short nucleotide sequence (TTAGGG in humans) at the ends of chromosomes that protect an organism's genes from being eroded during successive DNA replications.

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Telomeres two protective functions

  1. specific proteins associated with telomeric DNA prevent the staggered ends of the daughter molecule from activating the cell's systems for monitoring DNA damage.
  2. telomeric DNA acts as a kind of buffer zone that provides some protection against the organism's genes shortening.
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telomerase

enzyme that lengthens telomeres in germ cells but not most somatic cells.

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chromatin

in eukaryotes, each chromosome consists of a single extremely long DNA double helix associated with a large amount of protein.

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histones

small, positively charged proteins that bind tightly to the negatively charged DNA.

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nucleosome

  • Unfolded chromatin appear as a string of beads, each bead a ....the basic unit of DNA packing
  • consists of DNA wound twice around a protein core of eight histones, two each of the main histone types (H2A, H2B, H3, and H4)
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linker DNA

the string between beads


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