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Chapter 12 : DNA and RNA
What does DNA look like?
What are the elements that makeup DNA?
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What is DNA?
What is DNA?Discovered by Watson & Crick in 1953A long molecule made up of units, called nucleotidesGenes are made of DNADescribed as a twisted ladder, or a double helix
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Made up of nucleotides Each nucleotide has 3 parts
* a sugar called deoxyribose* a phosphate group* a nitrogen base
There are 4 nitrogen bases:purines - adenine, guaninepyrimidines - cytosine, thymine
* Every purine pairs with a pyrimidine in order to make a DNA chain
DNA Structure
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Purines Pyrimidines
Adenine Guanine Cytosine Thymine
Phosphate group Deoxyribose
Figure 12–5 DNA Nucleotides
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•Twisted double helix made of two strands
•Each strand of the helix is a chain of nucleotides
•Held together by hydrogen bonds
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Always Together….Great Couple
A & T G & C
•Every nucleotide is paired with a another from the opposite strand.
•Each pair is specific (Chargaff’s Rule).
•Adenine and Thymine pair together
•Guanine and Cytosine pair together
Base Pairing of Nucleotides
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Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Figure 12–7 Structure of DNA
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How is DNA organized in a chromosome?
• A nucleus of ONE human cell has more than 1 meter of DNA!!!
•Chromatin: a substance consisting of DNA tightly coiled around proteins called histones.
•The DNA and histones together are called nucleosomes (bead-like structure)
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Figure 12-10 Chromosome Structure of Eukaryotes
Chromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix
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Chromosome
E. coli bacterium
Bases on the chromosome
Prokaryotic Chromosome Structure
Section 12-2
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How can DNA use its double-stranded structure to its advantage for replication???
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DNA Replication (Refer to Figure 12-11)When does this occur in the cell cycle?
1) Enzymes un-twist and unzip the molecule (break H bonds between base pairs).
2) Each strand serves as a template
3) Free nitrogen bases form bonds and make complementary strands which follow the base pairing rules.
Template
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Figure 12–11 DNA Replication
Growth
Growth
Replication fork
DNA polymerase
New strand
Original strand DNA
polymerase
Nitrogenous bases
Replication fork
Original strand
New strand
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DNA vs. RNA
RNA – also a long chain of nucleotides (5-carbon sugar, phosphate group, nitrogenous base)
Differences:
1. RNA sugar = ribose, instead of deoxyribose
2. RNA – usually single-stranded
3. RNA does not have thymine. 1. Uracil instead.
2. Adenine and Uracil pair
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RNA is in charge of assembling Amino Acids into Proteins
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Transcription: a sequence of DNA is copied into an RNA strand Transcribe the DNA molecule below:
TAATAGCGCATTACGATTATCG
AUUAUCGCGUAAUGCUAAUAGC
•RNA will only start and stop at specific regions of the DNA called promoters.
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•We need codons for Protein Synthesis.
•They are directions to make proteins
•Every set of directions tells you where to START and where to STOP (start and stop codons)
•AUG: START UAA, UAG, UGA: STOP
•Proteins are made by joining amino acids into long chains (POLYPEPTIDES)
•Each polypeptide consists of a combo of any or all 20 amino acids.
•Amino acids contain 3 nucleotides. The 3 nucleotides are read as a code called a codon.
•A CODON specifies a single AA that is added to the polypeptide.
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Translation Explained
tRNA UAC mRNA AUGCGCAUAACGCAU
Start Codon
Anticodon
methionine
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Figure 12–17 The Genetic Code
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Translation Practice
Make a polypeptide (chain of amino acids) chain from the mRNA molecule
AUGAUCGCGUAUUGCUACUAG - mRNA
methionine-isoleucine-alanine-tyrosine-cysteine-tyrosine STOP
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Mutations - changes in the DNA sequence Gene mutation- changes in a single gene Chromosomal mutation changes in the
entire chromosome (containing many genes)1) Point Mutations - substitution of one nucleotide for
another
2) Frame Shift Mutations - shifting of the genetic code due to insertion or deletion of nucleotide
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Mutation AnalogyTHE FAT CAT ATE THE RAT
substitution THE FAT CAT ATE THE CAT *The letter “C” was substituted for the “R”
insertion THE FAT CAT ATE THE RAT THC EFA TCA TAT ETH ERA T
*Because the “C” was added, all other letters shifted down, thereby changing the amino acids that are made.
C
Deletion THE F T CAT ATE THE RAT
THE FTC ATA TET HER AT*Again, the amino acids will change b/c the “F” was removed
A
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Mutation Practice
What will the new amino acid be if the 5th nucleotide is substituted with an adenine?
AUGA CGCGUAUUGCUACUAG - mRNAU
What will the new amino acid sequence be if a guanine is inserted between the 9th and 10th nucleotide ?
ASPARAGINE
G
GUA = VALINE
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Putting it all together
What is the amino acid sequence that forms from the following DNA molecule? (DNA synthesis)
TACTACACCGTATAACAGGGCCTAGCAACT
Template
ATGATGTGGCATATTGTCCCGGATCGTTGA
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(Transcription)DNA - TACTACACCGTATAACAGGGCCTAGCAACT
mRNA - AUGAUGUGGCAUAUUGUCCCGGAUCGUUGA
(Translation)
amino acid sequence
methionine-methionine-tryptophan-histidine-isoleucine-valine-proline-aspartic acid-arginine-stop