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Central Dogma of Biology
 DNAmRNAprotein
 DNA TRANSCRIBES to mRNA
 Process is called transcription
 mRNA TRANSLATES to proteins
 Process is called translation
 mRNA actually makes amino acids,
which come together to make
proteins
Nucleic Acids
 RNA
 Single Strand
 Ribose sugar
 A=U
 G=C
 Uracil is the
nitrogenous base
used instead of
THYMINE
 DNA
 Double strand
 Deoxyribose sugar
 A=T
 G=C
 DNA parent strand makes 2
daughter strands…one fast,
smart daughter strand
(leading) and one, slower, nofast daughter strand (lagging)
 Leading strand (runs 3’ to 5’)
 Lagging strand (runs 5’ to 3’)
In the 1950’s, no one knew how
DNA was replicated…
 3 possibilities
 Conservative replication
 One completely new helix made from
old one
 Semi-conservative replication
 New molecule of DNA would contain
one old strand and one new strand
 Dispersive replication
 Each new molecule of DNA would be
made out of old bits and new bits
1958 Meselsohn and Stahl

Mathew Meselsohn and Franklin Stahl

Used E. coli bacteria (common, harmless bacteria in human gut)

Grew E. coli in food source that contained ammonium chloride for a source of nitrogen

Experiment relied on variation in structure of nitrogen atoms (used isotope of nitrogen…N-15)
 7 protons but neutrons vary=isotope=lil bits a radiation given off so they are traceable (ISOTOPES)
 Most have 7 neutrons so relative atomic mass is 14
 Some have 8 neutrons so relative atomic mass is 15

Bacteria used the N-15 to make their DNA

They were able to divide many times to make many copies of their DNA with the N-15 isotope
 Nearly all their DNA only contained this heavy N-15 isotope
 This DNA is heavier than normal DNA with normal N-14

Some of these new bacteria were transferred to another petri dish containing a food source with lighter N-14

Some were left to replicate just once (50 minutes) and others left long enough for just two, three and four times of
replication

DNA was extracted from this group

Samples were placed in Cesium chloride and spun in a centrifuge

The heavier the DNA, the closer to the bottom of the tube it came to rest
Results
How does DNA replicate itself?
 Template mechanism
 DNA Replication
 Process of copying the DNA molecule
 What phase of the CELL CYCLE?

S-phase….
 2 strands of double helix separate
(Unzips)
 Each strand acts as a negative for
making the new complementary strand
 Nucleotides line up one by one
following base pairing rules
 Enzymes (DNA Polymerase and DNA
Ligase) link nucleotides together to
form 2 new DNA strands called the
daughter strands
DNA Replication Simplified
 Unzip parent DNA
 Hydrogen bonds b/t bases break
 Nucleus contains ACTIVATED nucleotides (a little bit different than
normal nucleotides)
 Contain 2 EXTRA phosphate groups (total of 3 now)
 Add activated nucleotides to the 2 template strands of DNA
 Enzyme connects the innermost phosphate of the activated
nucleotide to the nucleotide already on the growing strand
 Two extra phosphate are released back into the nucleus
 DNA parent strand makes 2 daughter strands…one fast, smart daughter strand (leading)
and one, slower, not-so-fast daughter strand (lagging)
 Leading strand (runs 3’ to 5’)
 Lagging strand (runs 5’ to 3’)
 Attach fragments on lagging strand
Origins of Replication
 Specific site on DNA where replication




begins
 DNA Helicase: enzyme that binds to
origin site and unwinds DNA in both
directions
Copying goes outward in both directions
making replication “bubbles”
Parent strands open up as daughter strands
grow on both sides
Eukaryotic DNA has many origins of
replication on a single DNA strand
 Makes copying faster
Eventually bubbles merge making two new
strands
 Each new strand has a part from the
original and a part from the new
 Semi-conservative replication
 Means that parent of the original DNA double
helix is always kept as part of the new DNA
copies
DNA Polymerases
 Enzymes
 Make covalent bonds between nucleotides of the new strands
 Fast, accurate process
 Error only one in a billion nucleotides
 Brings over ACTIVATED nucleotides to
unzipped DNA strand and drops them off
More Players in DNA Replication
 DNA polymerase can only read a
strand that is running 3-prime to 5prime…
 New strand is built running from 5’ to 3’
 DNA polymerase works non-stop
adding nucleotides onto the strand that
runs in the 3’ to 5’ direction
 Therefore, Only one strand is made by a
smooth, and continuous process…
 The other strand is put together in bits and
pieces…
 Each little section of nucleotides is
called an “Okazaki Fragment”
 These are then “glued” together to make
one, continuous strand in the end by
another enzyme… DNA Ligase
Important Enzymes
 DNA Helicase
unzips
 DNA Polymerase
Adds nucleotides
 DNA Ligase
Attaches/glues
Central Dogma of Biology
 DNAmRNAprotein
 DNA TRANSCRIBES to mRNA
 Process is called transcription
 mRNA TRANSLATES to proteins
 Process is called translation
 mRNA actually makes amino acids,
which come together to make
proteins
DNAmRNAamino acids/polypeptide chain (Proteins)
 DNA codes for an RNA strand
 The every 3 bases on the RNA strand code
for a specific amino acid
 CODON: three sequential bases that
code for a specific a.a. (20 a.a. total)
 Amino acid are strung together to make
a protein (primary structure)
 Change DNA will change RNA which will
change amino acids, which change protein
Legend:
Transcription of DNA to RNA to protein: This
dogma forms the backbone of molecular
biology and is represented by four major
stages.
1. The DNA replicates its information in a
process that involves many enzymes:
replication.
2. The DNA codes for the production of
messenger RNA (mRNA) during
transcription.
3. In eukaryotic cells, the mRNA is
processed (essentially by splicing) and
migrates from the nucleus to the cytoplasm.
4. Messenger RNA carries coded information
to ribosomes. The ribosomes "read" this
information and use it for protein synthesis.
This process is called translation.
Ala: Alanine
Phe: Phenylalanine
Lys: Lysine
Pro: Proline
Thr: Threonine
Cys: Cysteine
Gly: Glycine
Leu: Leucine
Gln: Glutamine
Val: Valine
Asp: Aspartic acid
His: Histidine
Met: Methionine
Arg: Arginine
Trp: Tryptophane
Glu: Glutamic acid
Ile: Isoleucine
Asn: Asparagine
Ser: Serine
Tyr: Tyrosisne
DNAmRNAProtein
 Transcription
 Different form of the same message
 DNA makes single stranded RNA (U
replaces T)
 RNA leaves nucleus
 Translation
 Translate from nucleic acid language
to amino acid language
 Uses codons, 3-base “word” that
codes for specific a.a.
 “code” for an amino acid
 Several codons make a “sentence” that
translates to a polypeptide (protein)
The Genetic Code
 Am. Biochemist Marshall Nirenberg began
to crack the genetic code in the 1960s
 Built RNA model with uracil, called poly U,
conducted experiments with it and figured out
UUU coded for amino acid phenylalanine
 Scientists used his procedures to figure out the
other amino acids represented by codons
 Stop codons: UAA, UGA, UAG
 SIGNAL END OF GENETIC MESSAGE
 Start codon: AUG
 SIGNAL TO START TRANSLATING an RNA
transcript
Start
Codons
Stop
Codons
 AUG
 UAA
 UGA
 UAG
Three Types of RNA
 mRNA
 tRNA
 rRNA
Three Types of RNA… #1
 mRNA (messanger RNA)
 RNA transcribed from DNA template
 RNA polymerase (enzyme) links RNA nucleotides together
 Modified in nucleus before if exits
 RNA splicing: process in which Introns are removed and exons re joined
together to make a continuous coding mRNA molecule
 Introns
 Internal non-coding regions of DNA and mRNA
 Space fillers
 They are cut out of mRNA before it is allowed to leave the nucleus
 Process is called RNA splicing (processing)
 Exons
 Coding region of DNA and mRNA that will be translated (Expressed)
 VERY important part of mRNA…it is carrying the message from DNA
(def can’t cut this out)
Three Types of RNA…#2
 tRNA (transfer RNA)
 The interpreter
 Translate 3-letter base codes into
amino acids
 Carries anti-codon on one end (three
letters opposite of what is on mRNA)
 Carries amino acid on other end
 Anti-codon recognizes codon and
attaches
Three Types of RNA…#3
 rRNA (ribosomal RNA)
 Found in ribosome
 Ribosome composed of 2 subunits:
 Small subunit for mRNA to attach
 Large Subunit for two tRNAs to
attach
 “P” site: holds the tRNA carrying
the growing polypeptide chain
 “A” site: holds the tRNA that is
carrying the next a.a. to be added
to the chain
 When stop codon (UAA, UAG, UGA)
is reached, translation ends and
polypeptide is released from tRNA by
hydrolysis
DNA and Sickle Cell Anemia
 Review hemoglobin
 2 alpha chains
 2 beta chain
 Sickle Cell Anemia
 Inherited blood disorder
 SUBSTITUTION Mutation to DNA sequence that codes for beta chains
 One DNA nucleotide is replaced with a different nucleotide
 Normal amino acid sequence for beta chain:
 VAL-HIS-LEU-THR-PRO-GLU-GLU-LYS HbA allele (normal hemoglobin allele)
 DNA triplet code for Glu is CTT
 Mutated amino acid sequence for beta chain:
 VAL-HIS-LEU-THR-PRO-VAL-GLU-LYS HbS allele (sickle cell allele)
 DNA triplet code for CTT is changed to CAT, which no longer codes for Glu, but instead
Valine
Effects of Sickle Cell Mutation
 Glutamic acid
 Found on outside of hemoglobin
 Hydrophilic aa
 Interacts with water molecules, makes hemoglobin soluble (good!)
 Valine
 Hydrophobic aa
 Does not interact with water, makes hemoglobin less soluble (BAD!)
 When abnormal hemoglobin is in area of LOW oxygen concentration,
they stick together because the outside is now hydrophobic
 Abnormal hemoglobins form long chains of insoluble fibers
 These pull the red blood cells that contain the abnormal hemoglobin inwards and out
of shape (become sickle shaped instead of round)
 Sickled RBC cannot move easily through blood
 Get stuck in capillaries
 Possibly fatal