Transcript DNA Structure and Function

DNA Structure
and Function
Griffith
• Griffith showed some heredity material
could move into live harmless bacteria
and make a lethal strain
Griffith’s Experiment
Mice injected with
live cells of
harmless strain
R.
Mice injected with
live cells of killer
strain S.
Mice injected
with heat-killed
S cells.
Mice injected with
live R cells plus
heat-killed S
cells.
Mice live. No live
R cells in their
blood.
Mice die. Live S
cells in their
blood.
Mice live. No live
S cells in their
blood.
Mice die. Live S
cells in their
blood.
Heat killed S strain, but releases the killer genes
that the R strain incorporated.
Virus
•
•
•
•
Basically only two parts
DNA inside
Protein Coat outside
Carries genetic material – in which part?
genetic
material
viral coat
bacterial
cell wall
plasma
membrane
sheath
base plate
tail fiber
cytoplasm
Hershey-Chase
• Experiment with viruses showed that
the genetic information was in DNA, not
protein.
virus particle
labeled with 35S
virus particle
labeled with 32P
bacterial cell
label
outside cell
Hershey and
Chase showed
DNA carries
genetic
information
label inside cell
The Hershey-Chase experiment: phages
Fig. 9.5a
Fig. 9.5bc
Fig. 9.6a
Fig. 9.6b
Watson
and
Crick
Rosalind Franklin’s X-ray
Crystallography
DNA
• Deoxyribonucleic Acid = DNA
• Made up of nucleotides
• Nucleotides have three parts
– Sugar
– Phosphate group
– Nitrogenous base
• Sugar-phosphates make the DNA back
bone that is covalently bonded
phosphate
group
adenine (A)
base with a
double-ring
structure
guanine (G)
base with a
double-ring
structure
thymine (T)
base with a
single-ring
structure
cytosine (C)
base with a
single-ring
structure
sugar (ribose)
Nitrogenous bases
•
•
•
•
Four different nitrogenous bases
Have one or two rings
Form 2 or 3 hydrogen bonds
Bases can only pair one way:
– A-T
– C-G
• The sequence of nitrogenous bases
carries the genetic information
or
or
one base pair
DNA Structure
• Forms a double helix
• Two complementary strands held
together by hydrogen bonds
Fig. 9.5a
Fig. 9.5bc
Meselson- Stahl
• Heavier isotope falls to bottom of flask
• Timed to capture each new generation
of bacteria
• Shows radiation diluted by half each
generation, didn’t stay together.
• Showed semi-conservative replication
Fig. 9.6a
Fig. 9.6b
DNA replication
• Semiconservative – one old and one
new strand in each daughter molecule
• Each original strand acts as a template
to form a new complementary strand
DNA Replication
Three enzymes:
• Helicase – unwinds DNA
• DNA Polymerase adds new nucleotides
off the template
– Works in one direction only
– One side makes separate fragments
• Ligase seals up the fragments
– Proofreads DNA, fixes mistakes
Three Enzymes
DNA Replication
Helicase
Unwinds helix
Polymerase
adds
nucleotides
Ligase
Seals fragments
continuous
assembly on
one strand
newly
forming
DNA
strand
discontinuous
assembly on
other strand
one
parent
DNA
strand
DNA Replication
• Starts in several spots
• Pretty rapid process.
• Very accurate, few errors
Chromosomes
• DNA Replication forms the sister
chromatids just before Mitosis or
meiosis
Fig. 9.10
Mutations
• When cells are dividing, the DNA strands are
apart.
• A change in the DNA has no complementary
strand to fix it.
• These changes get incorporated into new
strand
• They are passed on in all the new divisions.
• Dividing cells collect mutations, can become
cancerous
– Skin, lungs, liver
Transcription
DNA
Translation
protein
RNA
nucleus
• DNA to RNA
• Copies only select
genes, not all at once
• Each gene is on only
one strand of DNA, not
the complimentary
strand
cytoplasm
•
•
•
•
RNA to Protein
In cytoplasm
Uses ribosome
Can make multiple
copies
• Relatively short lived
RNA
• Always a single strand
• Use Ribose as a sugar
• Uses Uracil
– and Adenine, Cytosine, Guanine
• mRNA carries genetic info. From
nucleus to cytoplasm
• tRNA carries amino acids to ribosome,
links the genetic code
• rRNA makes up most of ribosome
URACIL (U)
base with a
single-ring
structure
phosphate
group
sugar
(ribose)
DNA  RNA  protein
Chromosome during transcription
Transcription
• At Initiation RNA polymerase binds start
of gene and uncoils DNA.
• At Elongation RNA polymerase moves
along the gene briefly binding
nucleotides to DNA (only about 10
nucleotides at a time), as the RNA
nucleotides join together in a making a
single complimentary strand
• At Termination the mRNA moves out of
nucleus, detaches and DNA recoils
RNA polymerase
DNA
transcribed DNA winds up again
newly forming RNA
transcript
DNA to be transcribed unwinds
DNA template at the
assembly site
Fig. 9.11
growing RNA transcript
3’
5’
3’
5’
direction of transcription
5’
3’
m RNA modification
• new pre-mRNA includes extra
nucleotides called introns must be cut
out.
• The exons remain to go on to the
cytoplasm carrying the information for
the protein synthesis.
Fig. 9.17
Translation
• mRNA code directs sequence of amino acids
in protein.
• Uses ribosomes to assemble proteins
• At Initiation a tRNA attaches to the mRNA and
the ribosome subunits combine.
– Start codon is AUG
• At Elongation the ribosome moves down the
mRNA assembling the amino acids
– Only 6 nucleotides at at time
– Each triplet codes for one amino acid
• At Termination a stop codon causes the
protein chain and the ribosome and mRNA to
separate from each other.
base sequence of gene region
mRNA
amino acids
arginine
glycine
tyrosine
tryptophan tyrosine
Genetic Code uses
triplets of
Nucleotides to place
amino acids in
sequence
Fig. 9.13
Fig. 9.14
Fig. 9.15
Fig. 9.16
Mutations
• a Point Mutation is a single base pair
nucleotide substitution
– May cause a single amino acid change, or none
• Insertions and Deletions (adding or removing
nucleotides) reset the reading frame and
change subsequent amino acids.
– Missense makes a new amino acid chains
– Nonsense adds stop codons and synthesis cuts
off.
Fig. 9.23
original base triplet
in a DNA strand
a base substitution
within the triplet (red)
During replication, proofreading
enzymes make a substitution:
possible outcomes:
or
original, unmutated
sequence
a gene mutation
Mutations
mRNA
parental DNA
arginine
glycine
tyrosine
tryptophan asparagine amino acids
altered mRNA
arginine
glycine
leucine
leucine
glutamate
DNA with
base insertion
altered aminoacid sequence
Polyribosomes – make multiple copies of the protein
at the same time on the same mRNA
Fig. 9.18
Transcription
mRNA
rRNA
protein
subunits
mRNA
ribosomal
transcripts
subunits
tRNA
tRNA
Translation
amino
acids,
tRNAs,
ribosomal
subunits
Protein
From DNA to protein