Download DNA and RNA

DNA and RNA
Chapter 12
How do we know DNA is the
genetic material??
• Scientists were not sure if protein or
DNA was genetic material (both in
chromosomes)
• Griffith (1920s)
– Experimented with mice and two
strains of pneumonia bacteria
• R (rough) – no disease
• S (smooth) – causes disease, virulent
Griffith’s Experiment
• Found that R strain was transformed
by heat-killed S strain
• What substance transformed the R
strain??
• Avery performed further tests in
1940s
– Treated R strain + heat-killed S strain
with:
• Protease – enzyme that destroys protein
• DNase – enzyme that destroys DNA
Avery’s Experiment
Avery’s Experiment
• DNA caused the transformation of R
strain
• Scientists still not convinced!
Hershey and Chase Exp.
• Tested bacteriophages (viruses that
infect bacteria)
• DNA contains phosphorus
• Protein contains sulfur
• Radioactively labeled phosphorus
and sulfur in two batches of phage
• Infected bacteria with phage
• Only radioactive phosphorus found
in bacteria → DNA entered
And the point is . . .
DNA is the
genetic
material, not
protein
DNA Donor
Clone
Who’s
da’
mama?
Surrogate mom
DNA Structure
• Double helix
– 2 strands twisted around each other
• Sugar-phosphate backbone
• 4 nitrogen bases
–
–
–
–
Adenine
Thymine
Guanine
Cytosine
Base Pairs
Nucleotide Structure
• Phosphate group
• Deoxyribose sugar – 5 carbon
• Nitrogenous base
Four Nucleotides
Base Pairing
Source of DNA
A
T
G
C
Streptococcus
29.8
31.6
20.5
18.0
Yeast
31.3
32.9
18.7
17.1
Herring
27.8
27.5
22.2
22.6
Human
30.9
29.4
19.9
19.8
What patterns do you see in this data?
Chargaff’s Rule
• Erwin Chargaff – 1949
– Found that # of T = A and
G=C
– Only explanation is that T
pairs with A and C pairs
with G
– If a DNA sample is 20% A,
what % will C be?
• That’s right . . . 30% (20% A =
20% T, leaves 60% for G and
C, 30% each)
Double Helix
• Wilkins and Franklin (1952)
• Found that DNA was helix shaped
through X-ray pictures
First DNA Model
• James Watson & Francis
Crick (1952)
• Constructed a model of
DNA using others’ data
• Twisted ladder with base
pairs as rungs of ladder
• Hydrogen bonds hold
base pairs together
• Nobel Prize 1958
DNA!
Chromosomes
• Prokaryotes – one circular
chromosome in cytoplasm
• Eukaryotes – many linear
chromosomes in nucleus
– Fruit fly: 8 Human: 46
– Oak tree: 30 Chimpanzee: 48
• DNA is wrapped around
histones (proteins) to fit more in
small space
Chromosome Structure
DNA Replication
• Occurs during S phase of Interphase
• Exact copies made of all DNA
• Three steps:
– Unwind – enzyme is helicase
– Unzip – enzyme is DNA polymerase
– Copy – enzyme is DNA polymerase
• Each strand acts as template for
new strands
DNA Replication
• DNA not copied one base at a time
• Thousands of replication forks on
each chromosome
– Speeds up replication from weeks to
minutes
• Strands are copied in opposite
directions
• Proofreading enzyme checks for
errors
DNA Replication
DNA Replication
• Semi-Conservative
– Half of parent DNA
molecule is conserved
(saved) in daughter DNA
molecule
– Daughter DNA is half old
and half new DNA
DNA Replication
RNA
• Made of nucleotides
– Phosphate
– Ribose sugar
– Nitrogenous base
• Single stranded
• Uracil instead of
Thymine
– A pairs with U
Types of RNA
• Messenger RNA – mRNA
– Carries genetic messages out of
nucleus to ribosome b/c DNA can’t
leave nucleus
• Transfer RNA – tRNA
– Brings amino acids to mRNA at
ribosomes
• Ribosomal RNA – rRNA
– Makes up ribosome
Protein Synthesis
• Each chromosome has hundreds of
genes
• Each gene codes for one protein
• 2 steps:
– Transcription: DNA → mRNA
• In nucleus
– Translation: mRNA → protein
• At ribosome
Transcription
Translation
Transcription
• DNA unwinds, unzips
• RNA polymerase (enzyme) adds
RNA nucleotides to one strand of
DNA
• Promoters in DNA sequence tell
enzymes where gene begins
• Single stranded pre-mRNA leaves,
DNA strands rejoin
Transcription
Translation
• mRNA message is translated into an
amino acid (protein) sequence
• Every 3 RNA bases = one amino
acid
– Called a codon (there are 64!)
• mRNA attaches to ribosome in
cytoplasm
• tRNA matches codon with amino
acid
tRNA
• Clover leaf shaped
strand of RNA
• Has anti-codon at
one end and
corresponding amino
acid at other end
• Anti-codon pairs with
codon on mRNA
Translation
• Starts at AUG – start codon
• tRNA brings in amino acids for each
codon
• Amino acids attached to growing
polypeptide chain (protein)
• Stops at UAA, UAG, or UGA – stop
codons
• Ribosome helps to position all
molecules correctly
Translation
Translating the Code
Practice!
DNA
mRNA
codon
Amino Acid
ATC
UAG
Stop!
TAC
AUG
Start –
Methionine
GAT
CUA
Leucine
CCG
GGC
Glycine
Mutations
• Gene
– Point mut. – substitute a single base
• Changes one amino acid
– Frameshift – insert or delete a single
base
• Changes entire amino acid sequence from
mut. Forward
• Chromosomal
– Deletion, Duplication, Inversion,
Translocation
Gene Mutations
DNA: TAC GCA TCC
mRNA: AUG CGU AGG
AA:
Met-Arg-Thr
Substitution
DNA: TAC GTA TCC
mRNA: AUG CAU AGG
AA:
Met-His-Thr
Gene Mutations
DNA: TAC GCA TGG
mRNA: AUG CGU ACC
AA:
Met-Arg-Thr
Insertion
DNA: TAT CGC ATG G
mRNA: AUA GCG UAC C
AA:
Ile-Ala-Tyr
Gene Mutations
DNA: TAC GCA TGG
mRNA: AUG CGU ACC
AA:
Met-Arg-Thr
Deletion
DNA: TAG CAT GG
mRNA: AUC GUA CC
AA:
Ile-Val-?
Chromosomal Mutations
Causes of Mutations
• Internal
– Mistakes in DNA replication
• External
– Radiation, chemicals, high temps
– Mutagens: chemicals that cause mut.
• Mutations in body cells only affect
that person
• Mutations in sex cells can spread
throughout a population
Gene Regulation
• Genes are turned on or off as
needed by the cell
• Repressors turn genes off by binding
to DNA and preventing RNA
polymerase from binding