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http://www.dnai.org/timeline/index.html
• Rosalind Franklin - was the X-ray
crystallographer who took the
photo that enabled Watson and
Crick to deduce the DNA shape.
• James Watson and Francis Crick 1953 proposed and built the first
model of DNA.
What is the importance of DNA?
• DNA controls
– Heredity information
– Protein synthesis
– All cell activities
Let’s Review Chromosome Structure
• Contain DNA and
protein packed tightly
together called
chromatin.
• Protein is called
histones.
• Histones form a beadlike structure called
nucleosomes
What do nucleosomes do?
• Help to form enormous lengths of DNA.
They make sure everything can get packed
in a small amount of space.
Let’s Review Chromosome Structure
What is the structure of DNA?
• Made of subunits
called nucleotides
DNA Structure
Each Nucleotide contains:
– Phosphate Group
– Five-carbon sugar -deoxyribose
– Nitrogen Base
What are the four Nitrogenous
Bases?
•
•
•
•
Adenine
Guanine
Thymine
Cytosine
What are the groups of Nucleotides?
• Purines - double
ring of A and G
• Pyrimidines single ring of C
and T
Chargaff’s Rule or Base-Pairing
Rule
• Adenine pairs
with Thymine
• Guanine pairs
with Cytosine
Describe the arrangement of the
DNA molecule.
• Double Helix “twisted ladder”
• DNA is only found in the nucleus
(too big)
•Sides of ladder contain alternating
sugar and phosphates
One side 3`-5`
Other side 5`- 3`
• Rungs contain a
purine and pyrimidine
held together by a
weak hydrogen bond
• (A-T has 2 bonds
• C-G has 3 bonds)
• The sequence of bases
on one strand is
complementary to the
other strand
Why does DNA need to replicate?
Before a cell divides, it duplicates its
DNA (S phase of Interphase). This
ensures that the new cells will have a
complete set of DNA molecules.
http://www.johnkyrk.com/
DNAreplication.html
Describe DNA replication.
• DNA unwinds
• Enzymes (helicases) open up the helix by
breaking the H+ bonds b/w the bases
•The point where the DNA separates is
called the replication fork
•Free DNA nucleotides in the
nucleus pair up w/ the exposed
nitrogen bases according to the
base-pair rules
• The enzyme DNA polymerase moves along the
strands
• DNA polymerase also proofreads the new strands
Identify: Replication Forks, Helicase’s actions, DNA
Polymerase, Semi-Conservative Strands, Antiparallel
direction
DNA Replication
• In Eukaryotes there are usually many replication
forks that begin in the middle and move in both
directions
– If it did not happen this way is would take 16 days to
copy one DNA molecule in a fruit fly
– There are about 6,000 simultaneous replication forks so
it takes only 3 minutes
– Human DNA is copied in segments with a replication
fork approx. every 100,000 nucleotides
Biotech DNA Replication Animation
What is Protein Synthesis?
• Proteins (long chains
of amino acids) are
made in all cells.
• Proteins are similar
– Slight differences
make large differences
in shape and function
of cells.
Protein Synthesis
• Involves a new molecule called
RNA or Ribonucleic Acid
What is RNA?
• Differs from DNA in 3 ways
– RNA is single stranded
– RNA contains the 5-carbon sugar
Ribose
– Has the nitrogen base Uracil instead
of Thymine
3 Forms of RNA
• Messenger RNA
(mRNA)– RNA copy of a
gene that acts as
a blueprint for a
protein
3 Forms of RNA
Ribosomal RNA (rRNA)– found in the ribosomes
3 Forms of RNA
Transfer RNA (tRNA)– carries amino acids in the
cytoplasm to the ribosomes
How is the DNA supposed to get the
information it encodes out to the
ribosomes which carry out protein
synthesis?
• There must be a
messenger
This messenger is mRNA!
So, how is mRNA made?
• By the process of transcription.
– Transferring information from DNA
to RNA
– Occurs in the nucleus of the cell.
How doesTranscription start?
• Enzyme RNA polymerase binds to
the beginning of a gene on a region
of DNA called the promoter
– A promotor is a specific sequence of
DNA that is a start signal
• DNA unwinds and separates
• RNA polymerase moves along the
bases of one side of DNA and pairs
the exposed nitrogen bases with the
complementary RNA nucleotide
– A-U
– C-G
• RNA detaches from DNA and
DNA zips back up
• Transcription happens at a rate of
about 60 nucleotides per second
until RNA polymerase reaches a
stop signal (terminator)
What songs do the "notes of DNA"
dictate?
• So, now we've made mRNA in the nucleus. So
where does this newly synthesized molecule go
from here?
• The next step after transcription is translation, the
process of making proteins.
• Now that the mRNA has the DNA's instructions,
the molecule must travel OUT of the nucleus to
the CYTOPLASM where protein synthesis takes
place.
Let's examine the "players" in this
process.
•
•
•
•
•
•
•
the Ribosome
tRNA (transfer RNA)
the A site
the P site
Codons
Anticodons
Amino Acids
So how can we bring the amino
acids down to the mRNA?
• The tRNA molecule acts as a "taxi" whose
job is to read the code from the mRNA and
bring the corresponding amino acid into
place.
• Every tRNA molecule has its own set of
three bases which is called an anticodon.
• This anticodon is complementary to mRNA
codons.
Even though there are only 20 amino acids
that exist, there are actually 64 possible
tRNA molecules:
• 4 X 4 X 4 = 64 possible
combinations
HOW IN THE WORLD CAN ONLY 20
AMINO ACIDS CREATE THE
PRACTICALLY INFINITE NUMBER OF
PROTEINS PRESENT IN THE
BODY?!!??
• The key to all the variety is that the 20
amino acids can be linked in different
combinations and in different numbers. For
example,
• alanine-valine-tryptophan........serine
is a different protein than
• valine-serine-tryptophan........alanine
So, let's review this…
• A series of three nucleotide bases on a DNA
molecule is called a triplet;
• A set of three nucleotide bases on an
mRNA molecule is called a codon; and
• A set of three nucleotide bases on a tRNA
molecule is called an anticodon
Protein synthesis occurs in three
stages: Initiation, Elongation and
Termination.
• Protein synthesis is initiated when an mRNA,
a ribosome, and the first tRNA molecule
come together.
• The first A-U-G codon on the 5' end of the
mRNA acts as a "start" signal.
• Initiation is complete when
the methionine tRNA
occupies one of the two
binding sites on the
ribosome.
• Since this first site is the site where the
growing peptide will reside, it's known
as the P site.
• This is where the growing Protein will
be.
There is another site just to the 3'
direction of the P site; it is known as
the A site.
This is where the incoming tRNA will
Attach itself
ELONGATION
• The only tRNA which will successfully
attach is the one whose anticodon IS
COMPLEMENTARY to the codon of the A
site on the mRNA.
TERMINATION
• A "stop" codon (U-A-A, U-G-A, or U-A-G)
signals the end of the process.
PRACTICE PROBLEM
• DNA sequence A T T C G T A A G
• mRNA sequence U A A G C A U U C
• tRNA sequence A U U C G U A A G
http://wwwclass.unl.edu/biochem/gp2/
m_biology/animation/gene/g
ene_a2.html
http://ww
w.pbs.org/
wgbh/aso/t
ryit/dna/in
dex.html#