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 35 S virus particle labeled with 32 P bacterial cell label outside cell label inside cell Hershey and Chase showed DNA 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

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

newly forming DNA strand one parent DNA strand 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 DNARNA Translation protein nucleuscytoplasm 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 againDNA 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.

arginineglycinetyrosine tryptophan base sequence of gene region mRNA amino acids

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 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 with base insertion altered amino- acid sequence arginineglycinetyrosinetryptophanasparagine arginineglycineleucineglutamateleucine

Polyribosomes – make multiple copies of the protein at the same time on the same mRNA

Fig. 9.18

mRNArRNAtRNA Translation amino acids, tRNAs, ribosomal subunits mRNA transcripts protein subunits ribosomal subunits tRNA Transcription Protein

From DNA to protein