chapter 12
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CHAPTER 12DNA and RNA
12-1: DNA How was DNA discovered?
Fredrick Griffith Oswald Avery Hershey & Chase Watson & Crick
• Frederick Griffith
Griffith (cont.) Experimented with bacteria and mice Cultured both harmless and pneumonia
causing bacteria. Exposed mice to a mixture of both
harmless and heat killed pneumonia causing bacteria Mice came down with disease
Conclusion: Griffith called it transformation
Cells can be transformed when coming into contact with other types of cells
Harmless bacteria transformed into disease causing strains.
What was this disease causing agent and how did it transform the other cell?
Oswald Avery Set out to answer the previous question. Repeated Griffith's work, took extract from heat
killed bacteria. Treated extract with enzymes that break down:
Proteins CHO’s Lipids Even RNA
Same results Then repeated with enzymes that break down DNA Result:
No transformation
Hershey and Chase (cont.) Used radioactive markers to determine if
proteins or DNA was injected by viruses The tracers could be followed from the virus
to the bacteria. Injected with:
Phosphorus-32 (in DNA, not in proteins) Sulfur-35 (in proteins, not in DNA)
Results: Only Phosphorus-32 was transferred
At this point, scientists knew DNA was the “culprit” for where genes are contained.
Still needed to know:1. How did DNA carry genes generation to
generation?2. How did DNA code for traits?3. How was copied?
The Components and Structure of DNA
Nucleotides 5-carbon sugar
Deoxyribose Ribose
Phosphate Nitrogen base
Chargaff’s rule Determined complementary nature of DNA.
X-rays were used to “see” the general structure of DNA
Appeared like this:
Watson & Crick Determined the
shape had to be a double helix
Two strands with complementary base pairs in between that are bonded together.
12-2: Chromosomes and DNA Replication
Chromosome structure Histones
Proteins that chromatin is wrapped around Chromatin
Condensed DNA
Nucleosome = histones + chromatin
Duplicating DNA “Replication” Each new cell after
mitosis gets exact copy
Steps of replication
1. DNA is “unzipped” by protein called DNA helicase (breaks hydrogen bonds between compl. bases)
2. DNA polymerase reads the sequence and adds base pairs that complement.
3. DNA polymerase also proofreads along the existing strand.
4. DNA ligases “put together” chunks of copied base segements.
12-3 RNA and Protein Synthesis
RNA Structure
Single-stranded Ribose-phosphate backbone Contains nitrogen base uracil instead of thymine
It bonds with adenine on DNA
Types of RNA mRNA – messenger RNA
transcription rRNA – ribosomal RNA
Make up components of the ribosomes tRNA – transfer RNA
translation
Transcription DNA gives code to RNA for making proteins Similar to replication except now code is
copied to RNA (has uracil) RNA polymerase unzips the DNA strand and
begins to add bases that complement one of the strands.
YouTube - Transcription
How does it know where to start? Promoters
Specific sequences of base pairs that RNA polymerase can only bind to in order to initiate transcription.
RNA editing Introns and Exons
Introns Sequences that code for nothing
Exons Sequences that directly code for sections of
proteins Sequences that are “expressed” as protein
Eventually enzymes go back and cut the introns out and splice together the exons to have a fully functioning mRNA.
The Genetic Code 20 different amino acids Each is coded for by a segment of 3 base
pairs Codon Most amino acids have multiple codons Also codons for starting and stopping
transcription
Translation Copying mRNA into a sequence of amino
acids. mRNA attaches to ribosome Ribosome “reads” looks for a “start codon” Two tRNA with “anticodon” that
complement the strand are attached to mRNA by the ribosome.
Temporary hydrogen bonds allow the tRNA to be bound to mRNA long enough to form a “peptide bond” between the two amino acids.
YouTube - Translation
Genes and Proteins These proteins that are produced
ultimately go to make: Strucutral proteins (eye color, physical
features) Enzymes (control all cellular activities, ex:
digesting lactose) Hormones (producing
testosterone/estrogen) Combined all of these factors ultimately
make us what we are.
12-4 Mutations Mutations
Changes in the genetic sequence Types of mutations
Point mutations Occur in one (or few) base(s) of the DNA
sequence Include:
Substitutions Sometimes little to no effect on amino acid
sequence, however sometimes can be cataclysmic
Sickle-cell anemia
Frameshift mutations Caused by insertions or deletions of bases,
shifting the way the mRNA is read. Shifts the “reading frame” Usually has dramatic effects on the
formation of the protein – often rendering it useless
Chromosomal mutations Deletions
Duplications
Inversions
Translocations
Significance of mutations Most often mutations ultimately show little
no effect on the protein that is supposed to be made.
However, when it does have an effect the new protein formed can be:
DetrimentalIncreases organisms chance of
dying an not passing mutated gene on.
BeneficialIncreases the organisms chance of
survival and reproductibility – therefore passing it down
CREATES GENETIC VARIABILITY!This is often how asexually
reproducing organisms evolveSlow process
12-5: Gene Regulation How does an organism “know” when to
turn the gene on or off? Example: How does your body turn on the
lactase gene?
Regulation of the lac operon in E.Coli Operon
Groups of genes that code for specific protein lac operon in E.Coli codes for protein that
breaks down lactose
Again, transcription begins at the sequence called the promoter
Just “below” the promoter are “operator” (O) sites These sites are areas where “repressors” can
bind In most cases repressors are bound to O site,
preventing transcription Turning off the gene Just like a room, when you are not in it –
TURN OFF THE LIGHT!
When lactose is present, it binds with the repressor changing its shape, forcing it off the O site
Allows RNA polymerase to begin transcription
YouTube - Lac Operon
How is this done in eukaryotic cells? Much more complex than lac operon. Invovles sequence of base pairs near
promoter called “TATA box” Sequence of TATATATA… or TATAAA….
Positions RNA polymerase All cells contain the entire Genetic Code
BUT any one cell will use a small fraction of those genes. Heart cells use different genes than brain cells.
Development and Differentiation
Cells that change into specific types of specialized cells
Hox genes Genes that control this differentiation early in
development Mutations involving hox genes can have HUGE
effect on outcome of organism Pax 6
Gene found in Drosophila and mice that controls eye development
Inserted mouse Pax 6 gene into the “knee” of Drosophila embryo Grew an eye on its leg
YouTube - Evolution Genetic Tool Kit
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