DNA Structure and Function
Griffith
• Griffith showed some heredity material could move into live harmless bacteria and make a lethal strain
Mice injected with live cells of harmless strain R.
Mice live. No live R cells in their blood.
Mice injected with live cells of killer strain S.
Mice die. Live S cells in their blood.
Mice injected with heat-killed S cells.
Mice live. No live S cells in their blood.
Mice injected with live R cells plus heat-killed S cells.
Mice die. Live S cells in their blood.
Griffith’s Experiment
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
sheath
base plate
tail fiber
cytoplasm
bacterial cell wall
plasma membrane
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
label inside cell
Hershey andChase showedDNA carries genetic information
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
sugar (ribose)
adenine (A) base with a double-ring
structure
thymine (T)base with a single-ring structure
cytosine (C)base with a single-ring structure
guanine (G)base with a double-ring
structure
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
one base pair
or or
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
HelicaseUnwinds helix
Polymerase adds
nucleotides
LigaseSeals fragments
newlyformingDNAstrand
oneparentDNAstrand
continuous assembly on one strand
discontinuous assembly on other 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 RNA
Translation
protein
nucleus cytoplasm
• DNA to RNA• Copies only select
genes, not all at once• Each gene is on only
one strand of DNA, not the complimentary strand
• 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 DNA to be transcribed unwinds
newly forming RNA transcript
DNA template at the assembly site
Fig. 9.11
3’5’
growing RNA transcript
5’
3’ 5’
3’
direction of transcription
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.
arginine glycine tyrosine tyrosinetryptophan
base sequence of gene region
mRNA
amino acids
Genetic Code usestriplets 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
During replication, proofreading enzymes make a substitution:
a base substitution within the triplet (red)
original, unmutated sequence
a gene mutation
possible outcomes:
or
Mutations
mRNA parental DNA amino acids
altered mRNA
DNA withbase insertion
altered amino-acid sequence
arginine glycine tyrosine tryptophan asparagine
arginine glycine leucine glutamateleucine
Polyribosomes – make multiple copies of the proteinat the same time on the same mRNA
Fig. 9.18
mRNA rRNA tRNA
Translation
amino acids, tRNAs, ribosomal subunits
mRNA transcripts
protein subunits
ribosomal subunits
tRNA
Transcription
Protein
From DNA to protein
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