Download DNA: deoxyribonucleic acid

DNA: deoxyribonucleic acid
DNA history, structure, replication,
and gene expression
(transcription, translation, protein
synthesis)
DNA: the basics
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Double helix (twisted ladder, double spiral)
Chromosomes are made of it (46)
Genes are made of it (25,000 or so)
Each gene is the code for a protein
DNA is the genetic code, a chemical blueprint to
create an organism
• Made of nucleotides: sugar, phosphate, base
What are bases?
• Bases form the code for information in DNA
• Four bases: cytosine, guanine, adenine, thymine
(CGAT)
– Four bases means there are 4 nucleotides
– Four bases means there are 4 symbols in the code.
• Base pairing rule
– adenine always bonds with thymine (AT)
– Guanine always bonds with cytosine (CG)
History of DNA
• People thought proteins must be the genetic code
because there are lots of them
• 1928: Griffith “transformation” experiment showed
that genes were not alive (protein or DNA?)
• 1940: Chromosomes made of DNA
– Genes made of protein or DNA?
• 1944: Avery shows DNA was genetic material in
Griffith’s experiment
• 1952: Hershey and Chase “bacteriophage”
experiment showed DNA was genetic material.
• 1947: Chargaff discovered base pairing rule
The heroes of DNA
• 1952: Rosilind Franklin made x-ray
photo of DNA crystal. Showed it was a
helix shape.
• 1953: Watson and Crick
– Proved DNA was a double helix
– Bases are the “letters” in the code
– Explained replication
Rosalind Franklin (in Paris)
1953 article in Nature
Watson and Crick
Watson
Crick
DNA replication: copying DNA
• An enzyme unzips the helix--”helicase”
• Nucleotides move in--bases must “compliment”
• Each half forms a “complimentary” strand
replacing the missing half.
• An enzyme controls this--”polymerase”
Protein Synthesis
• Genes are for making proteins.
• Step one
– Transcription:
• make a copy of the gene with mRNA. The enzyme RNA
polymerase controls this. Also, there’s a promoter or start
“gene” and there are stop “genes”
• send the the messenger RNA out of the nucleus to the
ribosomes
• Step two
– Translation
• The ribosome uses the order of the nucleotide bases to line up
amino acids in the right order to make the protein
• tRNA (transfer RNA) brings the amino acids to the ribosome
• Remember: proteins are long chains of amino acids.
mRNA gene
copy has the code for the order of the amino acids. Ribosomes make
proteins.
RNA is different
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RNA is a single helix
RNA has ribose instead of deoxyribose
RNA has uricil instead of thymine!!!
3 flavors of RNA: mRNA, tRNA, and rRNA
Copying the gene follows base pairing rule just like
DNA replication.
• Making DNA
– DNA:
CGATTAGC
– DNA copy: GCTAATCG
• Making RNA
– DNA: CGATTAGC
– mRNA: GCUAAUCG (notice U goes where T should be!!!)
The genetic code
• 3 mRNA bases = codon (complimentary to
DNA codon)
• 1 codon = 1 amino acid
• tRNA has an anticodon that is the compliment
to the codon
• Like this
– DNA CGA TTA GCA
– mRNA GCU AAU CGU
– tRNA CGA UUA GCA
codons
anticodons
• How do you know which amino acids are
used?
– Use the “Punnett square” for genes
– Notice the stop codons!! (UUA, UAG, UGA)
The “Central Dogma”
• Flow of genetic information in a cell
transcription
DNA
replication
RNA
translation
protein
Final note on transcription of mRNA
• Exon: the part of the DNA that is part of
the gene
• Intron: part of the DNA that is not part fo
the gene! “Introduced” by viruses and
stuff
• The introns have to be removed to
make the mRNA.
MUTATIONS: most are recessive
• Types
– Point: change one base
– Point shift: change the order
– Deletion
– Inversion
– Insertion
• Dominant mutations include dwarfism
and Huntington’s disease
Mutations and cancer
• Cancer: mitosis out of control
• Carcinogen--chemicals and stuff like
radiation and UV light that cause cancer-damage or change DNA!
• Mutagens--things that cause mutations in
genes. Somatic or body cell mutations
cause cancer. Germ cell (gametes)
mutations cause birth defects.
Cancer: mutations in somatic cells
• Benign tumors don’t spread
• Malignant tumors spread to other parts of
the body to cause new tumors
• Metastasize: when cancer spreads
• Chemotherapy--chemicals that treat
cancer usually interfere with mitosis
Sickle Cell Anemia
Genetic Disease
 Heterozygous individuals – carriers
 Homozygous individuals – diseased
Hemoglobin
 Found in red blood cells
 Carries oxygen to tissues
SCA Results from Defective Hemoglobin
 Hemoglobins stick together
 Red blood cells damaged
Complications from low oxygen supply to tissues
 Pain, organ damage, strokes, increased infections, etc.
Incidence highest among Africans and Indians
 Heterozygotes protected from Malaria
GENE TECHNOLOGY
• Human Genome Project:
– map all our genes
– Other genomes have been mapped, too.
• Stem cell therapy
• Genetic engineering/recombinant DNA:
– Bacteria given a human gene to make insulin
– GMO: genetically modified organisms (corn with a bacteria gene for a
pesticide). “frankinfood”
– Glowing genes from jellyfish or lightning bugs to track genes
• Gene therapy: use viruses to replace bad genes
• Forensics (CSI)
– Genetic/DNA fingerprinting uses electrophoresis
• Genetic tools
– Restriction enzymes: cut DNA up
– PCR: duplicates DNA