Biology
DNA & the Language of Life
Genes are Made of DNA
• Fredrick Griffith (1928) studied pneumonia
strains (one was harmless while the other was
pathogenic, or disease-causing)
– Made non-harmful strains harmful (transformed)
– Discovered the ‘transforming agent’ was genetic
material
• Oswald Avery (1944) confirmed that the genetic
material was DNA
http://www.google.com/imgres
• Alfred Hershey &
Martha Chase
performed the
‘blender experiments’
using a virus
(specifically a
bacteriophage, or
virus that infects a
bacteria), bacteria,
and radioactively
labeled probes.
– They concluded that
DNA was the genetic
material.
http://www.google.com/imgres
Structure of DNA
Structure determines function.
• DNA is made up of nucleotides.
• A nucleotide is made up of a 3-carbon
sugar called a deoxyribose, a phosphate
group (PO3) and a nitrogenous base.
http://www.google.com/imgres
• There are 4 nitrogenous bases in DNA:
Adenine (A), Guanine (G), Thymine (T), and
Cytosine (C).
• A & G are purines and are larger than T & C
which are pyrimidines.
http://www.google.com/imgres
• Edwin Chargaff (1950’s) discovered that in
different species there is always an equal number
of A’s and T’s and an equal number of C’s and
G’s.
– These findings are Chargaff’s Rules.
• Adenine matches with Thymine and Cytosine
matches with Guanine.
http://www.google.com/imgres
• A purine matches with a pyrimidine. (a large
base matches with a small base, making DNA
have a uniform width throughout).
• Maurice Wilkins and Rosalind Franklin (1952)
used x-rays to photo DNA & discovered that
DNA has a spiral shape.
•
– Unfortunately, this x-ray was taken by another
scientist, James Watson who realized what the x-ray
revealed.
http://kentsimmons.uwinnipeg.ca/cm1504/Image265.gif
• James Watson and Francis Crick
(1953) published their results
(and received the Nobel Prize).
– They received credit for discovering
the shape of DNA.
• DNA is double stranded & a
double helix (or twisted ladder).
• This double helix is formed by 2
strands of nucleotides.
• Each strand is composed of
nucleotides: the sugar and
phosphate bind to each other to
form the “backbone” of the
ladder while the nitrogenous
bases form the “rungs”.
• Each rung is formed with 1
purine bonded with 1 pyrimidine
(A-T or C-G).
http://www.flyfishingdevon.co.uk/salmon/year3/
psy339evolutionarypsychologyroots/watsoncrick-dna.jpg
DNA Replication:
This is DNA copying itself. This occurs during S
phase (synthesis) of Interphase.
What happens?
• DNA unwinds (unzips): enzymes open the base
pairs and hold the double helix apart.
• Each DNA strand acts as a template for DNA
replication for a new complimentary strand
http://www.google.com/imgres
• Nucleotides join the
original strand 1 at a
time
• DNA polymerase is the
enzyme responsible
for the base-pair
matching
• This is called semiconservative
replication because the
new DNA that results
has 1 old strand of
DNA and 1 new strand
of DNA. Watson &
Crick hypothesized
this as well.
http://www.google.com/imgres
http://library.thinkquest.org/04apr/00217/images/content/74-Summary-DNA-Replication.jpg
Gene Protein
• George Beadle & Edward Tatum
hypothesized ‘one gene-one enzyme’
(based on their work w/ bread mold
Neurospora crassa).
• From this hypothesis, it has been
discovered that many genes code for
polypeptides (but they are specific):
one gene-one polypeptide (part of a
protein)
From Genotype to Phenotype
• Protein Synthesis is the production of a protein.
– This is taking the organism’s genotype (genetic makeup)
and translating into the phenotype (the physical traits).
• DNA is made up of nucleotides (bases).
– The bases make up a gene.
– Genes code for the sequence of amino acids (a.a.).
– A.A. code for proteins.
– Therefore, DNA is a template for making proteins.
• An expressed gene is a gene that codes for a protein
that is synthesized. (exon)
There are 2 types of nucleic acids: DNA and RNA.
• RNA= ribonucleic acid.
• RNA differs from DNA
http://images1.clinicaltools.com/images/gene/dna_versus_rna_reversed.jpg
RNA
• Is single stranded
• Contains uracil (U) instead of thyamine
(T)
• Has a ribose instead of a deoxyribose
There are 3 types of RNA:
• mRNA is messenger RNA
• tRNA is transfer RNA
• rRNA is ribosomal RNA
Types of RNA:
tRNA
(Contains rRNA)
http://publications.nigms.nih.gov/thenewgenetics/images/ch1_trans.jpg
Protein Synthesis occurs in 2 stages:
1. Transcription:
• DNA is copied
• RNA is synthesized or
transcribed
• This occurs within the
nucleus
http://biology.unm.edu/ccouncil/Biology_124/Images/transcription.gif
What happens during
transcription?
• RNA polymerase
(enzyme that is
responsible for RNA
synthesis) separates the
DNA double helix &
matches RNA bases
with 1 DNA template
only.
• This is specifically
making mRNA.
• Instead of T, U is
inserted. (U binds with
A, C binds with G)
• Only 1 RNA strand is
made.
http://www.google.com/imgres
• In prokaryotes, mRNA
goes directly to the
ribosome (there is no
nucleus).
• In eukaryotes, mRNA
is spliced.
– Splicing is when
noncoding regions
called introns (junk
DNA) are removed and
coding regions called
exons are sealed
together.
http://faculty.uca.edu/~johnc/RNA%20splicing.gif
• Both introns and
exons are copied
during transcription.
• After splicing, mRNA
leaves the nucleus
and finds a ribosome.
2. Translation:
• This is when nucleic acids are
changed, or translated, into the
language of proteins (amino acids).
• This involves mRNA, rRNA, and tRNA.
• It occurs on a ribosome (either free or
attached to the ER, depending on
where the protein is going).
Translation:
http://users.rcn.com/jkimball.ma.ultra
net/BiologyPages/T/Translation.gif
What happens during translation?
• mRNA attaches to a ribosome.
• A tRNA molecule “transfers” or brings over an a.a.
forming an amino acid chain; With each additional
a.a. the chain grows longer.
• tRNA brings the correct a.a. over based on the
complementary codons & anticodons (base
sequences)
http://www.google.com/imgres
• a codon is a base
sequence on the
mRNA strand. This
codes for a specific
a.a.
• An anticodon is 3
bases found on tRNA
that match the codon.
Example:
• Codon= GCU on
mRNA (codes for
alanine, an a.a.)
• Anticodon= CGA on
tRNA
http://www.google.com/imgres
The Triplet Code
•tRNA wil ultimately
be translated into
amino acids.
•20 amino acids & 64
codons
•3 stop codons (UAA,
UGA, UAG) & 1 start
codon (AUG) which
also codes for
methinionine (met)
http://repository.uwlax.edu/~Bob/assets/Code.jpg
Mutations: Changes in Chromosomes
• Proteins have various functions: they may act within
the cell OR serve a purpose outside of the cell. They
may be activates or repressors (turning genes on or
off).
• A mutation is a random change in the DNA
(sequence of nucleotides).
– This can be chromosomal mutations (which involve entire
chromosomes) or gene mutations (which involve individual
genes).
• A mutagen is an environment factor that causes a
DNA mutation, like radiation and chemicals.
• A carcinogen is a cancer causing agent (this is also
a mutagen).
– These can be tars in cigarette smoke, UV radiation, and
other chemicals.