Molecular Biology
DNA Structure
DNA Replication
RNA Structure
Transcription
Translation
The DNA Puzzle
 Scientists knew chromosomes were composed of
DNA and proteins
 They did not know which one actually stored
genetic information
 Most assumed proteins because their structure was
so much more varied and complex
 In 1928 Griffith performed an elegant experiment
which proved bacterial transformation could occur
 1944 Avery Mcleod and McCarty proved the
agent that transformed the bacteria was DNA
 Chargaff in 1947 determined that nitrogen base
percentages were species-specific and that A=T
and C=G was true for DNA
 In 1952, Hershey and Chase used bacteriophages
to show that DNA was genetic material using
radioactive tags for sulfur and phosphorus
 Wilkins and Franklin determined by x-ray
crystallography photographs that width of the
molecule was 2nm with 2 strands
 Watson and Crick built models to conform to
experimental results and came up with a double
helix in which pyrimidines and purines were
paired.
DNA Structure
 You should already know or recall that DNA is
composed of nucleotides, each having one of four
nitrogen bases: adenine, thymine, cytosine, and
guanine.
 The shapes of the nitrogen bases fit together in
such a way that they are complementary. Adenine
always bonds to Thymine and Cytosine always
bonds to Guanine.
 Sugar-phosphate bonds form the backbone of the
helix and hydrogen bonds form the bonds between
the nitrogen bases.
DNA
 When DNA must replicate, each side is a template
for a new strand. This is the semiconservative
model of DNA replication
 It means that one side of each of the two original
strands builds a complementary strand so that two
identical DNA molecules result.
 This is possible because of the base-pairing rules
(A always bonds to T and C always bonds to G)
DNA Replication
 There are a number of things associated with the
replication of DNA that you must remember-think
of them as rules:
1. A new DNA strand can only elongate in the
5`-----3` direction.
2. Each original strand serves as a template
for the formation of a new strand.
(Semiconservative Model)
3. The leading strand elongates continuously
while the lagging strand forms in pieces (called
Okazaki fragments) which are joined by DNA
ligase.(another enzyme)
DNA Replication (cont.)
4. The replication bubble forms when proteins
attach to a specific sequence of nucleotides to
begin the process.
5. Along with the replication bubble, a
replication fork is present and is where the DNA
strands are elongating.
6. DNA Polymerases (enzymes) catalyze the
elongation of the DNA strands.
7. Nucleoside triphosphates supply the energy
for the process.
8. DNA synthesis is primed by RNA and an
enzyme called primase.
Repair and Keeping Ends Intact
 DNA repair is accomplished by using nucleases
(enzymes) to cut out a damaged part of DNA and
then filling it in with the correct sequence.
 The ends of the DNA molecule have telomeres,
strands of non-coding DNA that keep the ends
from becoming shorter as the DNA replicates
again and again. An RNA primer sequence and the
enzyme telomerase accomplish this task.
RNA Structure
 As you should recall, there are a number of
differences between DNA and RNA.
 The first is that the sugar in RNA is ribose, not
deoxyribose.
 The second is that instead of thymine, RNA has
the base uracil, which bonds to adenine.
 Third, it has 3 different shapes related to each
form of RNA. (DNA has only one shape)
 Messenger RNA is linear, Transfer RNA is shaped
like a cloverleaf, and Ribosomal RNA is globular.
RNA’s Functions
 mRNA carries the genetic information from
DNA into the cytoplasm of the cell.
Remember that DNA does not leave the
nucleus.
 tRNA delivers amino acids to the ribosome
to be added to the forming polypeptide
chain.
 rRNA makes up ribosomes where the
polypeptide chain is assembled.
Transcription-Making mRNA
from DNA
 Transcription has three steps:
Initiation
Elongation
Termination
 In initiation, RNA polymerase, an enzyme,
attaches to special regions on the DNA known as
Promoter Regions which are also known as the
TATA Box because those are the nucleotides
found there. At this site, the DNA is unzipped into
two strands.
Transcription (cont.)
 Elongation occurs when the RNA polymerase
assembles RNA nucleotides using one side of the
DNA as a template.Only one side of the DNA
molecule is used and the lengthening strand moves
in the 5` to 3` direction as DNA replication does.
 Termination occurs when the RNA polymerase
reaches a sequence of DNA (often AAAAAAA)
called the Terminator region.
Modifications to mRNA
 Before it leaves the nucleus, the mRNA is altered.
 GTP is added to the 5` end forming a “cap”.
 150-200 adenine nucleotides are added to the 3`
end to form a “poly A tail”. (The two above acts
are believed to be related to gene expression)
 Introns(sequences between coding sequences
called exons) are removed.
 Small ribonucleoproteins (snRNP’s) are the
“cutters” that remove the noncoding sequences.
Translation-from mRNA to
Polypeptide
 All the RNA players move to the
cytoplasm. (mRNA, tRNA, ribosome
subunits)
 An enzyme specific to each amino acid and
ATP attach the correct amino acid to the 3`
end of tRNA.
 The remaining sequence of events follows
on the next several slides.
Initiation of Translation
 The small ribosomal subunit attaches to a special
region near the 3` end of the messenger RNA
molecule.
 A tRNA with the nucleotide sequence UAC
on its anticodon carries the amino acid
methionine which attaches to the “start” codon
AUG on the mRNA.
 The large subunit of the ribosome attaches to the
small, and the mRNA forming a complete
ribosome with the tRNA at the “P” site.
Elongation of Translation
 The next tRNA comes with an amino acid which binds to the A site on




the ribosome.
The release of the methionine and its addition to the next amino acid
occurs, beginning the chain of amino acids that will eventually form
the polypeptide.
As each tRNA is released, the former one moves to the P site,
exposing the A site which allows a new codon to be brought to it. So a
new tRNA with a new amino acid enters the A site.
This continues until the ribosome encounters a “stop” codon and the
polypeptide is released.( Called Termination)
The protein assumes its final shape and structure after interactions
between amino acids and modifications by the ER or Golgi take place.
Elongation
Termination
DNA to RNA to
Proteins