Chapter 12.1 Notes - Identifying the Substance of Genes
Fredrick Griffith – the 1st scientist to figure out what genes are made of.
 Used mice in his experiments with S-strain bacteria which caused pneumonia and Rstrain which had no effects on the mice.
 He then heated the bacteria to kill the S-strain and then injected it into the mice, and
the mice were fine. But when he injected the heat killed bacteria with the harmless Rstrain bacteria, the mice died.
Oswald Avery – in 1944 found that if the DNA was destroyed, then transformation of the
diseased bacteria did not occur, therefore DNA was the transforming molecule.
Alfred Hershey and Martha Chase – 1952 used viruses to study DNA
 Viruses are Non-living particles that can infect living cells.
 Bacteriophage – a kind of virus that infects bacterial cells
o A bacteriophage sticks to the surface of a cell and injects its genetic material into
it.
o The viral genes make many new bacteriophages, which destroy the bacterium.
o When the cell splits open, hundreds of new viruses burst out.
 Hershey and Chase used a bacteriophage that had a DNA core and a protein coat. They
wanted to find out which part of the virus (DNA core or Protein coat) entered the
bacterial cell.
 They found that the DNA was found in the bacterium and that DNA is the molecule of
heredity.
Roles of DNA
 Storing Information
o The main job of DNA is to store info
o Genes control the pattern of development. The instructions that cause a single
cell to develop into an oak tree, a gold fish, or a dog must be written in the DNA
of each organism
 Copying information
o A cell must make a complete copy of every one of its genes before the cell
divides.
o Within weeks of learning DNAs structure, scientists suggested a way DNA could
be copied.
 Transmitting information
o Genes are passed down from parent to offspring. Therefore, DNA molecules
must be carefully sorted and passed along when the cells divide.
o This is especially important in meiosis for the reproductive cells. The loss of any
DNA during meiosis might mean the loss of valuable genetic information that
offspring might need to survive.
Chapter 12.2 Notes – The Structure of DNA
DNA – Deoxyribonucleic Acid
 Can be copied and passed down from one generation to the next
 Specifies how proteins are made
 DNA is a nucleic acid made up of nucleotides joined into long strands or chains by
covalent bonds.
Nucleic acids are long molecules found in cell nuclei
Nucleotides are the building blocks of nucleic acids.
These nucleotides are made up of 3 basic parts:
 5 carbon sugar (deoxyribose)
 a phosphate group
 a nitrogenous base
Nitrogenous bases are the bases that have nitrogen in them
DNA has 4 types of nitrogenous bases:
 adenine (A)
 guanine (G)
 cytosine (C)
 thymine (T)
The nucleotides are joined together by covalent bonds formed between the sugar of one and
the phosphate group of the next.
The nitrogenous bases stick out sideways from the nucleotide chain.
The nucleotides can be joined together in any order.
The structure of DNA makes it very good at absorbing UV light.
Chargaff’s Rule
 Erwin Chargaff had discovered that the percentages of A and T bases are almost equal in
any sample of DNA. The same was true for C and G bases.
 DNA samples from organisms as different as bacteria and humans obeyed this rule.
 Chargaff’s Rule – AKA Base pairing rules
o A=T
o C=G
Franklin’s X-ray
Her X-ray diffraction picture proved the structure of DNA was a helix and made of 2 strands.
Watson and Crick
 In 1953, Watson got a hold of Franklin’s X-ray picture. Using what they saw they were
able to correctly put together a model for the DNA structure that was accurate.
 DNA was a double helix – like a twisted ladder
 Antiparallel Strands
o The 2 strands of DNA run in opposite directions – “antiparallel”
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o The nitrogenous bases on both strands meet in the center of the molecule.
Hydrogen Bonding
o Hydrogen bonds form between the nitrogenous bases
o Hydrogen bonds are weak bonds that can easily be broken
Base pairing
o Adenine (A) paired with Thymine (T)
o Cytosine (C ) paired with Guanine (G)
o Only these are paired together
Watson and Crick’s model explained Franklin’s X-ray, and Chargaff’s findings.
Chapter 12.3 DNA Replication
Replication – the process of copying DNA prior to cell division; makes sure that each daughter
cell has the same complete set of DNA molecules.
 The DNA molecule first separates into 2 strands
 Then the process makes 2 new strands following the base pairing rules
 The 2 strands of the double helix separate making a replication fork
 As each new strand forms, new bases are added following base pairing rules.
The Role of Enzymes
 DNA replication is carried out by enzymes that pull apart a molecule of DNA by breaking
the hydrogen bonds that hold together the base pairs (A=T; C=G)
 When the hydrogen bonds are broken they unwind into 2 strands with each strand
serving as a template for making new DNA strands.
 DNA Polymerase – the principal enzyme involved in DNA replication
o An enzyme that joins individual nucleotides to make a new strand of DNA
o Produces the sugar – phosphate bonds that join nucleotides together to form the
new strands for DNA
o It also checks each new DNA strand, so that each molecule is a close copy of the
original.
Telomeres – the tips of chromosomes; repetitive DNA at the end of a eukaryotic chromosome
 DNA in these regions is hard to replicate so cells use a special enzyme called telomerase
to fix the problem
 Makes it less likely that genes will be damaged or lost during replication of rapidly
dividing cells
 In normal adult cells, telomerase may be switched off, but in cancer cells they are
switched on which may be one of the reasons cancer cells are able to grow and divide so
rapidly.
Replication in Prokaryote Cells
 Have a single, ring-shaped DNA
molecule in the body of the cell
 DNA replication starts when special
proteins bind to one starting point on
the chromosome
 Replication usually goes in 2 directions
until the whole chromosome is copied
 The 2 strand of DNA are separated
when the cell splits into 2 new cells.
Replication in Eukaryote Cells
 DNA is much more complicated and is
housed inside of the nucleus
 Chromosomes are larger than
prokaryotes
 Replication can begin at several places
on the DNA
 Replication moves in both directions
 Proteins check to make sure the copies
are accurate
 Mistakes can happen and the order of
the bases in the DNA changes. When
this happens the information on a gene
may change. Sometimes this can have
serious consequences
 All of the chromosomes stay together
until Anaphase of mitosis.