Download DNA Biology - De Anza College

DNA BIOLOGY
• Double – stranded, twisted ladder
• Rungs are paired nucleotides
• Complementary pairing: hydrogen bonds
– A pairs with T
– G pairs with C
• Given any sequence, can write
complementary strand
• DNA Replication
– Copy in each cell
– Doubles before mitosis or meiosis
– Double helix unzips
– New complementary strand for each side 
two helices
DNA polymerase synthesizes new strands
• Transcription: DNA  mRNA
• Translation: mRNA  protein
• RNA Structure
– Ribose instead of deoxyribose
– Uracil instead of thymine
– Single stranded
TRANSCRIPTION
• DNA unwinds – girase enzyme
• Complementary mRNA synthesized by
RNA polymerase
• Specific sequences indicate start and stop
points
TRANSLATION
• Nucleotide code  amino acids
• Codon = triplet of nucleotides (64 different
combinations)
• Why three? 20 amino acids are possible
• Transfer RNA (tRNA) is translator
– One end has anticodon(complementary to
codon)
– Other end has correct amino acid
Ribosomes
• Two subunits lock together around mRNA
• When “AUG” is found, Met-tRNA binds =
initiation
• Ribosome moves along mRNA
• Amino acids added to chain = elongation
• STOP codon ends protein chain =
termination
MUTATIONS
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Changes in the sequence of DNA
Point mutation: one nucleotide
Some change protein, others don’t (silent)
UCU = serine UCC = serine UGU =
cysteine
• Addition or deletion of nucleotide =
frameshift mutation
• Disruptive because all downstream
codons changed
RECOMBINANT DNA
• Genetic code same in all creatures
• Genes can be moved to other creatures
• Example: Eukaryotic genes that code for
the molecule insulin – moved to bacteria
– Cut: restriction enzyme
– Paste: DNA ligase
– Plasmid: small circular molecule of DNA
found in bacteria
Examples of Genetic Engineering
• Plant crops: pest resistance, herbiside
resistance
• Farm animals: disease resistance, growth
hormones, increased milk production
• Environmental cleanup – bacteria that eat
oil
• Medicines and vaccines
• Cloning from adults – would human clones
be the same person?
Forensic DNA Analysis
• Polymerase chain reaction (PCR)
– cell – free replication of DNA
• Crime scene: blood spot, hair, skin cells, etc.
• DNA fingerprint
– Repeated sequences between genes vary for
individuals
– Restriction enzymes and electrophoresis
CANCER: Growth of a Malignant
Tumor
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One cell breaks constraints of cell cycle
Multiplies out of control
Forms a mass of cells = tumor
If confined, benign; if spreading, malignant
At certain size, nutrients do not fuse to
center
• Tumor can make chemicals that stimulate
formation of blood vessels
• Blood vessels bring nutrients; cells escape
– metastasize.
• Spread to lymph nodes, lungs
• Death results from cncerous cells
replacing normal cells, blocking blood
vessels or air passages
• Rules that Cancer Cell Break
– Divide only when called for
– Divide 60 times maximum (telomeres)
– Divide only when attached to solid surface
– Suicide when DNA is damaged
• Proteins (Genes) that Control Cell Division
– Stimulators (growth factors) = protooncogenes
– Inhibitors (p 53) = tumor suppressors
– DNA damage to cells can change these
genes
– Proto-oncogene  oncogene
– Example: Growth factor overproduced
– Inhibitor genes damaged  loss of tumor
suppressor
– Several mutations needed to cause cancer
– Mutations accumulate throughout life
• Genetic inheritance
• Viruses
• Chemicals that damage DNA or stimulate cell
division = carcinogens
• Radiation (cosmic rays, UV, radon, medical Xrays)
TREATMENTS
• Surgery – can cure if still early in disease,
all cells removed
• Radiation – can be directed to particular
part of body; kills all rapidly dividing cells
by DNA damage; side effects – nausea,
sterility, hair loss, anemia
• Chemotherapy – reaches all parts of the
body; side effects similar to radiation
• Experimental drugs and gene therapy
PREVENTION
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Quit smoking!!!
Less meat, more vegetables
Avoid sun exposure
Detect early: women breast exams, Pap
smears; men – prostate checks