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DNAThe Code of Life
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Topics Covered
• The DNA Molecule
• DNA Replication
• How DNA works
• Transcription
• Translation
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DNADeoxyribonucleic acid
Discovered by Watson and Crick
Made up of nucleotides
Double helix• 2 coiled strands (twisted
ladder)• Strands are
complimentary
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Nucleotide
THREE PARTSSugar
• Deoxyribose (in DNA)
• Ribose (in RNA)Phosphate groupNitrogen Base
S
P
NB
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The Four Nucleotides
C•Cytosine (C)
T•Thymine (T)
A•Adenine (A)
G
•Guanine (G)PURINES
PYRIMIDINES
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A
Two Complimentary Strands
BASE PAIRING(Chargaff’s Rule)
• A – T or T – A
• G – C or C - G
AT
C
G
C
G
T
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The Double Helix
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DNA Replication• Why does DNA need to replicate (copy)
itself?– MITOSIS
• Replication preserves the sequence of bases in an organism’s DNA
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DNA Replication: Step 1
• The DNA molecule unzips
TACGATTAA
ATGCTAATT
TACGATTAA
ATGCTAATT
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Replication: Steps 2 & 3• DNA
polymerase adds the correct complimentary nucleotide to each exposed strand
• A complimentary strand is created for each original strand
TACGATTAA
ATGCTAATT
TAATT
ATTAA
A TT AG CC G
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DNA, Genes & Chromosomes
• Chromosomes – long strands of DNA– How many
chromosomes do humans have?
• Genes – part of a chromosome– Each gene codes for a
different protein
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ProteinsGene Expression
• The use of genetic material to make a protein
• Protein ultimately determines a trait
Amino Acids• Building blocks of
proteins
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How does DNA code for a protein?
• Nucleotide bases “spell out” the message
• ORDER is important– OIGLOBY– BIOLOGY
• DNA alphabet has 4 letter– G A T C
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Protein Synthesis
• Transcription– An RNA copy of DNA is made
• Translation– RNA is translated in to a protein
DNA RNAtranscription
Proteintranslation
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RNA• Ribonucleic acid• Messenger for
DNA
• Single stranded
• No thymine (T) has uracil (U) instead
A
C
G
U
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TranscriptionAn RNA copy of DNA is made
•Called mRNA• Only one side of DNA is copied
Occurs in the Nucleus
Once mRNA is made it leaves the nucleus (through a pore)
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TRC: Step 1
• The DNA molecule unzips
TACGATTAA
ATGCTAATT
TACGATTAA
ATGCTAATT
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TRC: Step 2• RNA
polymerase binds to one strand of DNA at the promoter site
TACGATTAA
ATGCTAATT
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TRC: Step 3• RNA pol. Pairs
each nucleotide base with its complimentary base
• One strand Only!!
TACGATTAA
ATGCTAATT
AU
CUAAUU
G
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TRC: Step 4
• New messenger RNA (mRNA) leaves the nucleus through a nuclear pore
A U G C U A A U U
nucleuscytoplasm
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Translation
• RNA translated into a protein• Two types of RNA work together
– mRNA – messenger– tRNA – transfer
• Occurs in the cytoplasm• Genetic Code is the translator• Each “word” is 3 letters
– GCA = alanine (see genetic code)
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Genetic Code
• CODON – three letter “word” in mRNA– Specify a particular amino acid
• ANTICODON– Three leter “word” in tRNA– Complimentary to the codon
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The Genetic Code
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TRL: Step 1
• mRNA “start” codon binds to a ribosome– AUG = start– AUG =
methionineA U G C U A A U U
ribosome
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TRL: Steps 2 & 3
• tRNA picks up an amino acid in the cytoplasm and carries to ribosome
• tRNA anticodon binds to complimentary mRNA codon
A U G C U A A U U
U A C
met
U A C
met
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TRL: Steps 4 & 5
• Amino acid detaches from the tRNA and attaches to a growing protein chain
• tRNA leaves to find another amino acid A U G C U A A U U
U A C
metU A C
met
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A U G C U A A U U
met
A U G C U A A U U
met
TRL: Step 6
• Continues as mRNA passes through ribosome and one tRNA after another is selected to match the mRNA codons
G A U
leu
G A Uleu
met
G A U
leuG A U
leu
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TRL: Step 7
• Ends when a “stop” codon is reached and the newly assembled protein is released into the cell
A U G C U A A U U
leu
met
stop
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AT
C
G C
A
T
G
C
G
A
T