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