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Section 12-1:
DNA
Chapter 12 – DNA and RNA
Mendel helped to figure out what genes were, but how
exactly where these genes passed on?
In 1928, Frederick Griffith
(British) was doing experiments
with bacteria/mice to figure out
how bacteria make us sick
He found 2 different strains of pneumonia bacteria;
Smooth (S) and Rough (R)
The smooth bacteria caused
death but the rough bacteria
seemed harmless
When Griffith heat killed the
smooth bacteria, the bacteria no
longer killed
Then Griffith mixed heat killed
smooth bacteria with living rough
bacteria (neither should have caused
disease) and it caused death
Griffith called this a transformation
 the rough strain was permanently
changed by the smooth strain
Griffith hypothesized that something
was transferred from strain to strain
 a gene?
In 1944, Oswald Avery (Canadian)
repeated Griffiths experiment and
found similar results
In 1952, Alfred Hershey and
Martha Chase were trying to
verify Avery’s discovery
They were studying viruses,
specifically bacteriophages =
viruses that infect bacteria
Bacteriophages are made of DNA
or RNA inside a protein coat
Hershey and Chase decided if they could
tell if the DNA or the protein was being
injected into the bacteria, they could
determine what genes were made of
They used 2 radioactive markers,
phosphorus-32 and sulfur-35
Proteins contain almost no phosphorus
DNA contains no sulfur
They found all of the radioactivity in the
bacteria was from phosphorus, making
the genetic material DNA, since protein
has no phosphorus
Scientists wanted to know more about the structure of DNA
DNA is a long molecule made up of nucleotides
(which are made of 3 components)
1. 5-carbon sugar called deoxyribose,
2. phosphate group,
3. nitrogen-containing base
There are 4 kinds of nitrogen-containing bases in DNA;
adenine, guanine, cytosine, and thymine
Two belong to a group known as
purines (two rings in their
structure)  A & G
Two belong to a group known as
pyrimidines (one ring in their
structure)  C & T
The sugar and phosphate form the
backbone of the DNA and the nitrogencontaining bases stick out sideways – the
bases can be in any order
This set up of DNA seemed
almost too simple, so of course
scientists did more experiments
to find out more about it
Erwin Chargraff observed that in different organisms that the
percentage of G and C were almost equal and so were the amounts
of A and T
Chargraff’s rules says [A] = [T] and [C] = [G]
In the early 1950’s, Rosalind Franklin (British)
studied DNA using a X-ray diffraction
The photos from the X-rays showed an
X-shaped pattern  showing that
there were 2 strands of DNA twisted
around each other like a helix
Around the same time, James
Watson and Francis Crick were
working to build a model of the
structure of DNA
Watson and Crick’s model of DNA
was a double helix where the 2
strands wound around each other
A double helix looks like a twisted
staircase
Watson
Crick
The 2 strands were held together by
hydrogen bonds between certain
base pairs (nucleotides)– this basepairing and it helped explain
Chargraff ’s rules
A always pairs with T and G always
pairs with C
Section 12-2:
Chromosomes and
DNA Replication
So where is the DNA located within a cell?
Prokaryotes = no nucleus DNA is in cytoplasm
Most prokaryotes have a single circular DNA
molecule – this is called the cell’s chromosome
Eukaryotes = nucleus that’s where DNA is
There are multiple chromosomes 
the number depends on the species
DNA molecules can be surprisingly long
and must be folded into a tiny space
The nucleus of a human cell contains
more than 1 meter of DNA, so how
does it all fit?
In eukaryotes, DNA and proteins are
packed tightly to form chromatin
In the chromatin, DNA is wound
around proteins called histones
The double helix structure helped to
explain how DNA can be replicated
One half of DNA has the info needed to make
the other half through base pairing
The strands are complementary to
one another  when they split up, each
half can help create its other half which
= two new identical DNA molecules
In eukaryotes, DNA replication
occurs at hundreds of places
Why?
In prokaryotes, DNA replication
usually starts at one point and
proceeds in two directions until
the whole chromosome of replicated
At each point, replication happens
in 2 directions until the whole
chromosome is copied
Replication forks = the sites where
separation and replication occur
Each strand serves as a template as to
how to add on base pairs
Ex. A sequence of TACGTT will produce a
complimentary strand of
Several enzymes are involved in the process of DNA replication
DNA helicase is used to “unzip”
the 2 strands of DNA
DNA polymerase joins base pairs to the
DNA molecule  it can also “proofread”
Section 12-3:
RNA and Protein Synthesis
Genes are just instructions coded
on DNA – we just need a way to put
those instructions to work  that’s
where RNA comes in
RNA has a similar structure to DNA but there are 3 major differences
1. The sugar is ribose instead of deoxyribose
2. RNA is a single strand instead of a double strand
3. RNA contains uracil in place of thymine
RNA is basically a disposable copy
of DNA, it can help make lots of
copies without hurting the master
Most RNA is involved in protein synthesis – assembling
amino acids into proteins
There are 3 main types of RNA involved in protein synthesis;
mRNA, rRNA, tRNA
RNA Type
The letter?
mRNA
Messenger
rRNA
Ribosomal
tRNA
Transfer
Its Job
Construction
Analogy
Carries instructions
Blueprints for
from DNA to the rest
the building
of the cell
Makes up ribosomes – The actual build
where protein is made
location
Transfers the amino The workers who
acids to the ribosome bring lumber and
using instructions
supplies
Transcription = the process of copying part of
a nucleotide sequence from DNA to RNA
Transcription requires an enzyme known as
RNA polymerase (similar to DNA polymerase)
RNA polymerase separates the DNA
strands  it then uses one strand of DNA as
a template to form a strand of RNA
Promoter = a region where the enzyme can bind
that has a specific order of nucleotides
The promoter basically tells the enzyme
where to start copying
There is a similar signal to stop copying
Once copied, the RNA does get edited before leaving the
nucleus with instructions for proteins
Parts of the DNA called introns aren’t used for
making proteins  so they get cut out of RNA
Parts of the DNA called exons are used for making
proteins  they get spliced together
Now that we have mRNA instructions,
how do those eventually make proteins
The genetic code = the instructions on
the mRNA  4 bases = 4 letters in the
code
These 4 letters in the code can
create 20 different amino acids 
How?  The code is read 3
letters at a time
Each “word” (called codons) of the
code is 3 bases long
Ex. The code UCGCACGGU
would be broken into groups of 3
bases  UCG-CAC-GGU
Because there are 4 bases, there
are 64 possible 3-base
codons
Some amino acids are coded for by several codons
There is one codon that acts as a start for protein
synthesis and there are three codons that act as a stop
mRNA carries the instructions to the
ribosomes in order to create proteins
(translation)
During translation, the cell uses information
from mRNA to produce proteins
Translation occurs in a series of steps
1. mRNA in the cytoplasm attaches to a ribosome
2. As each codon of the mRNA moves
through the ribosome an amino acid is
brought to the ribosome by tRNA
Each tRNA molecule only carries
one type of amino acid
Each tRNA molecule also has 3
unpaired bases (anticodon) so
that it knows where to match up on
the mRNA  codons and
anticodons are complementary
3. Peptide bonds form between the adjacent
amino acids –the bond between the amino acid
and the tRNA is broken  the tRNA is
available again
4. The amino acid chain grows until a
stop codon – the ribosome then
releases both the mRNA and the
protein
Over all, DNA gets replicated to form new DNA  it then gets
transcribed into RNA  the RNA then gets translated into
proteins
Section 12-4:
Mutations
Mutations are changes in the genetic material
There are 2 main classifications of mutations;
gene and chromosome
Gene mutations are mutations
in just a single gene
Point mutations involve changes in just one
or a few nucleotides
Substitutions happen when one nucleotide
is swapped for another  this usually only
effects one amino acid
Frameshift mutations shift the reading
frame of the nucleotides  this affects
any amino acids after the mutation
Insertions happen when one
nucleotide is inserted in the
wrong place
Deletions happen when
a nucleotide is deleted
Chromosomal mutations involve changes
in larger parts of the chromosome
Deletions – involve the loss of all or part of the chromosome
Duplications – extra parts of the chromosome are made
Inversions – parts of the chromosome are reversed
Translocations – part of the chromosome breaks
off and attaches to another
Mutations can be “neutral”, negative,
or even beneficial
Some mutations causes dramatic
changes that disrupt normal activity
There are many genetic disorders
that are caused by mutations
Harmful mutations are also associated
with cancer