I. Central Dogma
• "Central Dogma": Term coined by Francis
Crick to explain how information flows in cells.
II. DNA Replication
A. Steps
1. DNA helicase (enzyme) attaches to DNA
molecule.
2. Single Strand Binding Proteins ("SSBP's"):
Keep the strand open
3. Helicase moves along DNA breaking
hydrogen bonds- “unzips” DNA into two
strands.
4. Each strand now has unpaired nitrogen
bases.
4. Primase: Puts down a small RNA primer which
is necessary for DNA polymerase to bind to at
the origin.
5. Free floating nucleotides in the nucleus form
hydrogen bonds with unpaired nitrogen bases.
6. DNA Polymerase bonds together nucleotides
7. Topoisomerase: Rotates the DNA to decrease
torque (which would shred the helix)
8. Ligase (enzyme) repairs DNA
a) Final result = 2 exact copies of DNA
* Each copy = 1 “old” strand and 1 “new”
strand
B. Replication occurs at many sites along
DNA
If unzipped old segment = C-C-A-T-G-A-G-T
What will the new segment be?
C. Leading and Lagging Strand
1. Nucleotides are added to the 3' end of the
DNA strand
a. DNA replication can only occur in the 5' to 3'
direction
2. DNA is "anti-parallel": Both strands have
opposite 5' to 3' orientations (one is "upsidedown" compared to the other)
3. Leading strand made continuously
4. The other strand ("lagging strand") made in
smaller, discontinuous fragments
a) "Okazaki fragments"
D. Telomeres
1. Each round of replication shortens the 5' end
of the lagging strand (by about 100-200 bp)
2. If this continued indefinitely, chromosomes
would get shorter and shorter after each
replication.
a. Information would start to be lost
3. Telomeres at end of eukaryotic chromosomes
a. short, repeating DNA sequence
4. TELOMERASE - enzyme responsible for
replicating the ends of eukaryotic chromosomes
a. add more telomere sequence during replication
III. RNA (Ribonucleic Acid) and
Transcription
A. Structure of RNA
# of strands
DNA
RNA
2 Strands
1 Strand
Type of Sugar Deoxyribose Ribose Sugar
sugar
Nucleotide
Base pairs
A-T
C-G
A-Uracil
C-G
Nucleic Acids
16
B.3 types of RNA
*All made in the nucleus and
travel to the ribosomes
1. Messenger RNA (mRNA)
a. Single straight strand
b. Transmits DNA information
c. Serves as template (pattern)
for making proteins
2. Transfer RNA (tRNA)
a. Single folded strand
b. Complimentary bases pair up
c. Also involved in protein synthesis
3. Ribosomal RNA (rRNA)
a. Globular form
b. Part of ribosome structure
C. Transcription – process of making RNA
from DNA
1. Protein enzyme called RNA
polymerase binds to DNA.
2. RNA polymerase separates portion of
DNA into two separate strands.
3. Free floating nucleotides in nucleus
match their nitrogen bases with
bases of “unzipped” DNA.
DNA base code
= C-G-A-T-A
Complimentary RNA = G-C-U-A-U
4. RNA polymerase forms bonds
(hydrogen) between nitrogen
bases.
5. Polymerase connects
nucleotides by bonding sugars
to phosphates
6. Enzyme releases new RNA
strand when it reaches “stop
sign” on DNA.
GENE
DNA
mRNA
mRNA
Cytoplasm of
cell
Nucleus
Transcription
happens in the
nucleus. An RNA
copy of a gene is
made.
Then the mRNA that
has been made moves
out of the nucleus
into the cytoplasm
Once in the
cytoplasm, the mRNA
is used to make a
protein
D. Processing mRNA
1. A modified nucleotide is added to the 5' end
of the transcript.
2. A tail of several hundred adenine residues is
put on the 3' end of the transcript.
3. These modifications function in nuclear
export and maintenance of the mRNA
4. Exons and Introns
a. Eukaryotic genes contain large stretches of
non-coding DNA ("introns") interspersed
between coding DNA ("exons")
b. To produce a functional protein, the introns
must be removed
c. the exons must be spliced together by a
spliceosome
5. Why Introns?
a. Not really answered.
b. Evolutionary baggage? Selfish genes?
c. We do know that having multiple exons in a
gene allows eukaryotes to make multiple
functional proteins from one gene
("alternative splicing")
IV. TRANSLATION - Protein
Synthesis
A.Proteins
1. Many amino acids linked by peptide
bonds
2. 100’s to 1000’s of AA’s per protein
3. 20 different AA’s
4. Sequence of AA’s determine
structure and function of each
protein
B. Codon – a group of 3 sequential bases
of an mRNA
1. 64 different codons
2. mRNA
ucuuagcuagcg
-How many codons?
3. Each codon codes for:
a. 1 of the 20 amino acids
b. Start or stop codons
C. Anticodon
1. 3 base sequence at the bottom of tRNA
a) Matches the codon on mRNA strand
• The code was cracked largely by Marshall
Nirenberg
• Put synthetic RNA into "cell free" E. coli
extract and analyzed the polypeptides that
were made.
• Nobel Prize: 1968
D. Translation – putting amino acids (AA’s)
together to build protein from info in
mRNA
1. mRNA and tRNA transcribed from DNA in
nucleus.
2. This RNA exits the nucleus through pores.
3. mRNA travels to ribosomes.
4. Free floating AA’s are brought to
ribosomes by tRNA.
5. Protein always starts with methionine
(aug) AA
6. A second AA on tRNA enters
ribosome. Codon and anticodon pair
up and peptide bonds form between
AA’s.
7. When a stop codon (UAG, UAA, or UGA) is
encountered, a release factor binds to the A-site.
8. The polypeptide chain is released.
9. The ribosome disassembles.
The Process of Translation
Figure 8.9
E. Translation in prokaryotes
1. Since prokaryotes do not have a nucleus,
transcription and translation can be coupled.
2. Polyribosomes: simultaneous translation of a
transcript (even while transcript is still being
made.
V. Mutations
A. Types
1. Point mutations: One DNA base is replaced by
another DNA base.
2. Frame-shift mutations: DNA bases are inserted or
deleted
3. Each type of mutation can have different effects,
depending on the situation.
B. Point Mutations
1. Silent - substitution changes a codon to
another codon for the same amino acid.
2. Missense - substitution changes a codon to a
codon for a different amino acid
3. Nonsense- substitution changes a codon to a
stop codon
C. Frameshift
1. Shift the codons
2. Insertions – additions of a nucleotide
3. Deletion – loss of a nucleotide
4. Duplication – repeating sequences of codons
Review questions
What does DNA polymerase do?
What does Helicase do?
What does ligase do?
Match the bases below.
5’ – A T C G T A – 3’
List 3 differences between RNA/DNA.
What are the 3 types of RNA?
Where does RNA go after it is made?
What does RNA attach to when it leaves the
nucleus?
Amino Acids are the building block of ________.
What type of RNA brings an amino acid?
When does translation stop?
Where is the codon located? Anticodon?
What is a codon?