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Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
CP Bio Chapter 12-13 DNA Function
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
History: Learning about DNA
Frederick Griffith 1928
a. Tried to make a vaccine for pneumoniab. Used mice and two strains of bacteria
- one harmless (“R type”)- one caused pneumonia (“S type”)
c. Live R alone and dead S alone did not cause immune response
d. Mixed live R with dead S mice got sick and DIED
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How did harmless bacteria turn into deadly?
Live R (‘rough’) – no diseaseLive S (‘smooth’) – pneumoniaMix Live R and dead S - pneumonia
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Griffith’s conclusionSomething from dead S cells transformed
living R cells into living S cells
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Avery, McLeod, and McCartey 1944What cellular substance changed Griffith’s bacterial cells from harmless to deadly?
Used a series of enzymes on bacterial cultures
- destroyed specific molecules in the cells
- carbs, proteins, lipids, DNA
Found that when DNA was destroyed, bacterial cells did NOT change
CONCLUSION: DNA is the substance that can change bacterial cells
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The Hershey-Chase experiment 1952Background: Viruses enter cells and change them - make cells produce copies of virus.
Problem: Which part of virus enters cells? Is it the protein coat? Or the DNA?
Experiment: Grow bacteria, add phage virus tagged with radioactive isotope
- use sulfur radioactivity in proteins (capsid)
- use phosphorus radioactivity in DNA
Which enters bacterial cells?
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Hershey-Chase Experiment
Grow bacteria in culture with tagged
phage. Virus infects bacteria
Is radioactivity in the liquid (virus), or
in the cells (bacteria)?
Centrifuge separates cells
from culture liquid
Blender shakes phage loose from
bacterial cells.
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Which part enters cells?
When phage was tagged with phosphorus bacterial cells became radioactive
Conclusion: DNA entered cells, but protein did not
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Finding the structure of DNA
Franklin & Wilkins 1952- X-ray pictures of DNA
crystals- showed double helix shape
Watson & Crick 1953 discovered structure of DNA
Erwin Chargaff 1950 - base-pairing rules
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• Two strands, held together by hydrogen bonds – Between complementary bases
• four nitrogen bases: adenine, thymine, cytosine, guanine• Deoxyribose sugar
DNA is a double helix
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DNA replication:
• For cell division• Starts in several places
at once• Each original strand is
template for a new strand• Proceeds until entire
strands are duplicated• Copies stay together at
centromere
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Ch. 13 DNA Function – “Gene Expression”
Flow of genetic information: DNA to RNA to protein
• DNA (genotype) codes for proteins
– Proteins make the phenotype
• A gene is one section on a DNA molecule• Has instructions to make one polypeptide
• CODE is the sequence of DNA bases
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RNA
• ONE strand• Ribose sugar• Uracil base (no thymine)
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Three kinds of RNAMessenger RNA (mRNA)– carries code from DNA in nucleus to ribosome
Ribosomal RNA (rRNA) – makes up ribosome, along with proteins
Transfer RNA (tRNA) – carries one amino acid to ribosome and matches to mRNA code
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1. The DNA of the gene is transcribed into RNA
2. The gene is translated into a polypeptide
Figure 10.6A
DNA
Transcription
RNA
Protein
Translation
Two stages in protein synthesis
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mRNA synthesis - writes the gene onto a messenger molecule
RNApolymerase
RNA nucleotides
Direction of transcription Template
Strand of DNA
Newly made RNA
TC
AT C C A A T
TG
G
CC
AATTGGAT
G
U
C A U C C AA
U
Stage 1: Transcription – in nucleus
DNA unzips
mRNA leaves nucleus
Copy ONE side of DNA
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Transcription – writes DNA code onto mRNA
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1. In the nucleus, the DNA unzips
2. RNA nucleotides line up along one strand of the DNA, follow base pairing rules
3. Messenger RNA (mRNA) is single strand, detaches from DNA
4. leaves the nucleus
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RNA is processed before leaving the nucleus• Noncoding segments called introns are edited out
• Coding segments called exons are spliced together
• A cap and tail are added to the ends
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13.2 How is the genetic code read?• The “words” of the DNA “language” are
written in sets of three bases – codons
• Codons spell the amino acid sequence - primary structure of a protein
Stage 2: Translation- in ribosome
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Translating the genetic code
T A C T T C A A A A T C
A T G A A G T T T T A G
A U G A A G U U U U A G
Transcription
Translation
RNA
DNA
Met Lys PhePolypeptide
Startcondon
Stopcondon
Strand to be transcribed
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UUG
All organisms use the same code
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1. Ribosome attaches to mRNA
2. Transfer RNA (tRNA) brings amino acids to ribosome
How RNA helps
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3. Begins at “start” codon
4. Amino acids are set in sequence, according to the code on mRNA
5. Ends at “stop” codon
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Genetic information: DNA to RNA to protein
Sequence of codons primary structure of protein
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change in the DNA base sequence• Errors in replication or by mutagens
Mutations can change the meaning of genes
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Ch. 13- What turns genes ON and OFF?In bacteria, control genes are next to code genes
Lac operon – gene is OFF when lactose absent
- ON when lactose presentRepressor protein on DNA blocks RNA polymerase - Lactose removes repressor transcription
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Gene Control in EukaryotesControl genes are NOT near code genes
Many proteins interact to help mRNA form
• Transcription Factors
• Control genes may be on different chromosomes from coding genes