dna, genes, and biotechnology
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DNA, Genes, and Biotechnology. Chapter 22. Structure of the Hereditary Material. Experiments in the 1950s showed that DNA is the hereditary material Scientists raced to determine the structure of DNA 1953 - Watson and Crick proposed that DNA is a double helix. - PowerPoint PPT PresentationTRANSCRIPT
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DNA, Genes, and Biotechnology
Chapter 22
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Structure of the Hereditary Material
• Experiments in the 1950s showed that DNA is the hereditary material
• Scientists raced to determine the structure of DNA
• 1953 - Watson and Crick proposed that DNA is a double helix
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Structure of Nucleotides in DNA
• Each nucleotide consists of– Deoxyribose (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases– Adenine, Guanine, Thymine, Cytosine
(A) (G) (T) (C)
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Nucleotide Bases
phosphate group
deoxyribose
ADENINE (A)
THYMINE (T)
CYTOSINE (C)
GUANINE (G)
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Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen bonds between bases
• A binds with T, C with G
• Molecule is a double helix
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Structure of DNA
A gene is a sequence of nucleotides in a DNA molecule
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DNA Structure Helps Explain How It Duplicates
• DNA is two nucleotide strands held together by hydrogen bonds
• Hydrogen bonds between two strands are easily broken
• Each single strand then serves as template for new strand
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DNA Replication
newnew old old
• Each parent strand remains intact
• Every DNA molecule is half “old” and half “new”
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Base-Pairing during
Replication
Each old strand serves as the template for complementary new strand
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Errors in DNA Replication
• Mistakes can occur during replication
• Enzymes may detect and correct the problem, restoring the proper DNA sequence
• When the error is not corrected, the result is a mutation
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Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
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Effect of Base-Pair Substitution
original base triplet in a DNA strand
As DNA is replicated, proofreadingenzymes detect the mistake andmake a substitution for it:
a base substitution within the triplet (red)
One DNA molecule carries the original, unmutated sequence
The other DNAmolecule carries a gene mutation
POSSIBLE OUTCOMES:
OR
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Frameshift Mutations
• Insertion– Extra base added into gene region
• Deletion– Base removed from gene region
• Both shift the reading frame
• Result in many wrong amino acids
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Frameshift Mutation
ARGININE GLYCINE TYROSINE TRYPTOPHAN ASPARAGINE
ARGININE GLYCINE LEUCINE GLUTAMATELEUCINE
mRNA
PARENTAL DNA
amino acid sequence
altered mRNA
BASE INSERTION
altered amino-acid sequence
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Transposable Elements
• DNA segments that move spontaneously about the genome
• When they insert into a gene region, they usually inactivate that gene
• Neurofibromatosis
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DNA to RNA to Proteins
transcriptionDNA RNA
translation protein
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Three Classes of RNAs
• Messenger RNA– Carries protein-building instruction
• Ribosomal RNA– Major component of ribosomes
• Transfer RNA– Delivers amino acids to ribosomes
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Base-Pairing during Transcription
• A new RNA strand can be put together on a DNA region
DNA
DNA DNA
RNAG C A T
C G T A
G C A U
C G T A
base-pairing in DNA replication base-pairing in transcription
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Promoter
• A base sequence in the DNA that signals the start of a gene
• For transcription to occur, RNA polymerase must first bind to a promoter
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Gene Transcription
transcribed DNA winds up again
DNA to be transcribed unwinds
mRNAtranscript
RNA polymerase
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Transcript Modificationunit of transcription in a DNA strand
exon intron
mature mRNA transcript
poly-A tail
5’
5’ 3’
3’
snipped out
snipped out
exon exonintron
cap
transcription into pre-mRNA
3’ 5’
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Gene Regulation
• Most cells of your body carry the same genes
• Each uses only a tiny subset at any time, and some are never turned on
• Regulatory proteins can speed up or halt transcription
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Genetic Code
• Set of 64 base triplets• Codons
– Nucleotide bases read in blocks of three
• 61 specify amino acids• 3 stop translation
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Code Is Redundant• Twenty kinds of amino acids are specified by
61 codons
• Most amino acids can be specified by more
than one codon
• Six codons specify leucine
– UUA, UUG, CUU, CUC, CUA, CUG
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tRNA Structurecodon in mRNA
anticodon in tRNA
amino acid OH
tRNA molecule’s attachment site for amino acid
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Ribosomestunnel
small ribosomal subunit large ribosomal subunit intact ribosome
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Three Stages of Translation
Initiation
Elongation
Termination
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Initiation
• Initiator tRNA binds to small ribosomal subunit
• Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon
• Large ribosomal subunit joins complex
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Binding Sites on Large Subunit
binding site for mRNA
P (first binding site for tRNA)
A (second binding site for tRNA)
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Elongation
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Termination
• A stop codon in the mRNA moves onto the ribosomal binding site
• No tRNA has a corresponding anticodon
• Proteins called release factors bind to the ribosome
• mRNA and polypeptide are released
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Making Recombinant DNA
5’
3’
G
C T T A A
A A T T C
G
G A A T T C
C T T A A G3’
5’
one DNA fragment another DNA fragment
3’
5’
Restriction enzymes cut DNA into fragments
Fragments base-pair at sticky ends
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Using Plasmids
• Plasmid is small circle of bacterial DNA
• Foreign DNA can be inserted into plasmid – Forms recombinant plasmids
– Plasmid is a cloning vector
– Can be used to deliver DNA into another cell
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Using Plasmids
DNA fragments+enzymes
recombinantplasmids
host cells containing recombinant plasmids
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Polymerase Chain Reaction
• Sequence to be copied is heated• Primers are added and bind to ends of
single strands• DNA polymerase uses free nucleotides
to create complementary strands• Doubles number of copies of DNA
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PCR (1)double-stranded DNA to copy
DNA heated to 90°– 94°C
primers added to base-pair with ends
mixture cooled; base-pairing of primers and ends of DNA strands
DNA polymerasesassemble new DNA strands
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PCR (2)mixture heated again; makes all DNA fragments unwind
mixture cooled; base-pairing between primers and ends of single DNA strands
DNA polymerase action again doubles number of identical DNA fragments
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DNA Sequencing:Reaction Mixture
• Copies of DNA to be sequenced• Primer• DNA polymerase• Standard nucleotides• Modified nucleotides
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Reactions Proceed
• Nucleotides are assembled to create complementary strands
• When a modified nucleotide is included, synthesis stops
• Result is millions of tagged copies of varying length
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Recording the Sequence
T C C A T G G A C CT C C A T G G A C
T C C A T G G AT C C A T G G
T C C A T G
T C C A T
T C C A
T C C
T C
T
electrophoresisgel
one of the many fragments of DNA migratingthrough the gel
one of the DNA fragmentspassing through a laser beam after moving through the gel
T C C A T G G A C C A
•DNA is placed on gel•Fragments move off gel in size order; pass through laser beam•Color each fragment fluoresces is recorded on printout
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The Human Genome Project
Goal - Map the entire human genome
• Initially thought by many to be a waste of resources
• Process accelerated when Craig Ventner used bits of cDNAs as hooks to find genes
• Sequencing was completed ahead of schedule in early 2001
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Surprising Discoveries
• Coding regions (exons) make up only 1.5% of our DNA
• About half of remaining DNA is repeated segments
• Around 1.4 million SNPs in the human genome (single nucleotide polymorphisms)
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Using Human Genes• Even with gene in hand it is difficult to
manipulate it to advantage• Viruses usually used to insert genes
into cultured human cells but procedure has problems
• Very difficult to get modified genes to work where they should
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DNA Fingerprints
• Unique array of DNA fragments
• Inherited from parents in Mendelian fashion
• Even full siblings can be distinguished from one another by this technique
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Tandem Repeats
• Short regions of DNA that differ substantially among people
• Many sites in genome where tandem repeats occur
• Each person carries a unique combination of repeat numbers
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RFLPs
• Restriction fragment-length polymorphisms• DNA from areas with tandem repeats is cut
with restriction enzymes• Because of the variation in the amount of
repeated DNA, the restriction fragments vary in size
• Variation is detected by gel electrophoresis
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Biotechnology Concerns
• Mutation of transgenic bacteria or viruses could yield new pathogens
• Bioengineered plants and animals could alter ecological balance
• Genetic screening could lead to discrimination
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Engineered Bacteria
• Transgenic bacteria can be used to grow medically valuable proteins– Insulin, interferon, blood-clotting factors
– Vaccines
• Human gene is inserted into bacteria, which are then grown in huge vats
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Engineered Plants and Animals
• Transgenic goats produce human tissue plasminogen activator (tPA)
• Chinese hamster ovary cells produce blood-clotting factor VIII
• Aspen plants produce less lignin and more cellulose
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Jefferson’s Genes
• It was suspected that Jefferson had at least one child with his slave, Sally Hemings
• DNA evidence showed that a modern male descendant of Sally Hemings has a Y chromosome that had been passed down to him through the generations from Thomas Jefferson