Chapter 14: DNA

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Chapter 14 DNA
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The Library of Life

• Genetic information is stored in
  deoxyribo-nucleic acids (DNA).
• The human genome is composed of about 3.3
  trillion bases.
• The error rate for replication of DNA is about
  one base in a billion.
• This chapter describes how scientists discovered
  that genes are composed of DNA.

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Key Concepts

• Genes are composed of DNA.
• Four nucleotides are the building blocks of DNA.
  Each of these nucleotides contains one of four
  nucleotide bases adenine, cytosine, guanine, or
  thymine.
• DNA consists of two strands of nucleotides
  twisted together into a spiral. The two strands
  of DNA are held together by hydrogen bonds that
  form between adenine and thymine and between
  cytosine and guanine.
• Each strand of DNA has an enormous number of
  bases arranged one after another. The sequence of
  bases in DNA differs among species and among
  individuals within a species. These differences
  are the basis of inherited variation.
• Because adenine pairs only with thymine and
  cytosine pairs only with guanine, each strand of
  DNA can serve as a template from which to
  duplicate the other strand.
• DNA in cells is damaged constantly by factors
  such as heat energy and ultraviolet light. If
  this damage were not repaired, the organism would
  die.

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The Search for the Genetic Material

• By the early 1900s, geneticists knew that genes
  controlled the inheritance of characters and
  genes were located on chromosomes.
• Chromosomes are composed of DNA and protein.
• The first step in the quest to understand the
  physical structure of genes was to determine
  whether DNA or protein was the genetic material.
• Initially, most geneticists thought that protein
  was the genetic material.
• Key experiments conducted between 1928 and 1952
  convinced geneticists that DNA was the genetic
  material.

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The Search for the Genetic Material

• Harmless bacteria can be transformed into deadly
  bacteria
• Fredrick Griffiths experiments showed that
  nonpathogenic bacteria could be transformed into
  pathogenic bacteria with heat-killed pathogenic
  bacteria (see Figure 14.1).
• DNA can transform bacteria
• Oswald Avery and his colleagues published a paper
  in 1944 that showed that DNA was the only
  compound able to transform bacteria from
  nonpathogenic to pathogenic.
• The genetic instructions of viruses are contained
  in DNA
• Hershey and Chase determined that only the DNA
  portion of a bacterial virus was responsible for
  taking over the metabolism of its host (see
  Figure 14.2).

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The Search for the Genetic Material

• Harmless bacteria can be transformed into deadly
  bacteria
• Fredrick Griffiths experiments showed that
  nonpathogenic bacteria could be transformed into
  pathogenic bacteria with heat-killed pathogenic
  bacteria (see Figure 14.1).
• DNA can transform bacteria
• Oswald Avery and his colleagues published a paper
  in 1944 that showed that DNA was the only
  compound able to transform bacteria from
  nonpathogenic to pathogenic.
• The genetic instructions of viruses are contained
  in DNA
• Hershey and Chase determined that only the DNA
  portion of a bacterial virus was responsible for
  taking over the metabolism of its host (see
  Figure 14.2).

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The Three-Dimensional Structure of DNA

• In 1951, Linus Pauling became the first
  researcher to determine the three-dimensional
  structure of a protein.
• The three-dimensional structure of DNA was
  determined two years later.
• DNA is a double helix
• In 1953, Watson and Crick published a paper that
  proposed that DNA was a double helix (see Figure
  14.3).
• The paper contained two key points there were
  two strands of nucleotides in DNA and only
  certain bases could pair with each other.
• Adenine pairs with thymine, and cytosine pairs
  with guanine (the strands are complementary).
• Differences in DNA sequences are the basis of
  inherited variation.

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How DNA Is Replicated

• In a paper published later in 1953, Watson and
  Crick proposed the mechanism for DNA replication
  (see Figure 14.5).
• Each new molecule of DNA contains one parental
  strand and one newly synthesized strand.
• DNA polymerase is the enzyme that replicates DNA.

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Repairing Damaged DNA

• There are many opportunities for error in
  replicating DNA.
• DNA is constantly damaged by radiation,
  chemicals, and collisions with molecules within
  the cell.
• Few mistakes are made in DNA replication
• DNA replication has an error rate of about one in
  10,000 bases.
• Proofreading enzymes reduce the error rate to one
  in 10 million bases.
• When an incorrect base avoids proofreading, a
  mismatch error occurs.
• Mismatch repair enzymes repair 99 percent of
  mismatches, which reduces the overall chance of
  error in DNA replication to one in 1 billion
  bases.
• When an error in DNA replication is not repaired,
  the change in the sequence of DNA is called a
  mutation.
• Normal gene function depends on DNA repair
• There are three steps in DNA repair recognition,
  removal, and replacement (see Figure 14.7).
• When DNA repair mechanisms fail to work, diseases
  such as xeroderma pigmentosum can occur (see
  Figure 14.8).

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Repairing Damaged DNA

• There are many opportunities for error in
  replicating DNA.
• DNA is constantly damaged by radiation,
  chemicals, and collisions with molecules within
  the cell.
• Few mistakes are made in DNA replication
• DNA replication has an error rate of about one in
  10,000 bases.
• Proofreading enzymes reduce the error rate to one
  in 10 million bases.
• When an incorrect base avoids proofreading, a
  mismatch error occurs.
• Mismatch repair enzymes repair 99 percent of
  mismatches, which reduces the overall chance of
  error in DNA replication to one in 1 billion
  bases.
• When an error in DNA replication is not repaired,
  the change in the sequence of DNA is called a
  mutation.
• Normal gene function depends on DNA repair
• There are three steps in DNA repair recognition,
  removal, and replacement (see Figure 14.7).
• When DNA repair mechanisms fail to work, diseases
  such as xeroderma pigmentosum can occur (see
  Figure 14.8).

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Highlight Sunburns and Parsnips

• Psoralens are compounds contained in plants such
  as celery, limes, and parsnips.
• Psoralens can damage DNA in the presence of UV
  light.
• Mutations of DNA that result from sunburn can be
  the first step on the path to skin cancer.

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The Scientific Process Rosalind Franklin
Crucial Contributor to the Discovery of DNAs
Structure

• Rosalind Franklin provided X-ray photographs of
  crystallized DNA to Watson and Crick, which aided
  them in determining the structure of DNA.