 3.a.1 – DNA, and in some cases RNA, is the primary source of heritable information (16.1 & 16.2).  3.c.1 – Changes in genotype can result in changes in phenotype (16.2).

u Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? u Two possibilities u Two possibilities: u Protein u DNA

 Protein:  Until 1940s, evidence for protein was STRONG!  Very complex structure  High specificity of function  DNA:  Simple structure  Not much known about it (early 1900’s)

u Pneumonia in mice u Two strains: u S – pathogenic (caused pneumonia) u R - harmless

u Something turned the R cells into S cells (in 4 th experiment) u Transformation u Transformation - the assimilation of external genetic material by a cell u And…the pathogenic trait was inherited by all new offspring!

 Griffith used heat  Heat denatures proteins  DNA – heat stable  Then, could proteins still be the genetic material?  Griffith’s results were contrary to accepted views

 Repeated Griffith’s experiments, but added specific fractions of S cells  Result - only DNA transformed R cells into S cells

 Experiment not believed  Why?  Scientists thought bacteria make-up was considerably different from humans/other living organisms

u Genetic information of a virus or phage u Phage u Virus that attacks bacteria and reprograms host to produce more viruses (by injecting its own DNA)

 DNA and/or RNA core  Enclosed by envelope  Made of protein  To reproduce, a virus must attach to a cell and inject its genetic info (either RNA/DNA) INTO the cell

u Hershey/Chase knew viruses reproduced, but didn’t know what was injected… u Two main chemicals: u Protein u DNA

u Radioactive isotope tracers u Protein - CHONS, can trace with 35 S u DNA - CHONP, can trace with 32 P

 Used phages labeled with one tracer or the other and looked to see which tracer entered the infected bacteria cells  Hershey - Chase movie Hershey - Chase movie

 DNA enters the host cell, but the protein did not  Therefore, DNA is the genetic material that is passed down

 Used X-ray crystallography data  Used model building  Result - Double Helix Model of DNA structure  One page paper, 1953

 Also used x-ray crystallography  Determined DNA had two strands  Died in 1958  Her colleague got Nobel Prize (because Franklin published under his name!)

nucleotides u Made of nucleotides: (3 parts) 1. Deoxyribose Sugar (5-C ring) 2. Phosphate (PO 4 -) 3. Nitrogen Bases: A,T,C,G Purines: A,G Pyrimidines: C,T

 Polymer of sugar - phosphate  2 backbones present  Phosphate of one nucleotide is attached to sugar of the next  Alternates sugar-phosphate

 Bridge the backbones together  Purine + Pyrimidine = 3 rings  Keeps a constant distance between the 2 backbones  Nucleotide held together by H-bonds

 Studied chemical composition of DNA  Found:  the nucleotides were found in certain ratios  % composition differed between species

 A = T  G = C  Example: in humans A = 30.9% T = 29.4% G = 19.9% C = 19.8%

 Explained by double helix model  %A = %T, 3 ring distance  %G = %C, 3 ring distance

Purines Pyrimidines

 Published a second paper (1954) that speculated on the way DNA replicates  Proof of replication given by others

u The process of making more DNA (from existing DNA) u Completed during S-phase of Interphase u Problem: When cells replicate, the genome must be copied exactly u How is this done?

1. Conservative 1. Conservative –  one old strand, one new strand 2. Semiconservative 2. Semiconservative –  each strand is 1/2 old, 1/2 new 3. Dispersive 3. Dispersive –  strands are mixtures of old and new

u Grew bacteria on two isotopes of N u Started on 15 N, switched to 14 N u Looked at weight of DNA after one, then 2 rounds of replication u Results: u Confirmed the Semiconservative Model of DNA replication u Parent strand serves as a template

 DNA splits by breaking the H-bonds between the backbones.  Then DNA builds the missing backbone using the old backbone as a template.  DNA is replicated in only a few hours.

 Specific sites on the DNA molecule that start replication.  Recognized by a specific DNA base sequence.  Proteins/enzymes initiate replication

u Ex u Ex: bacteria (E. coli) u Circular DNA u 1 origin site u Replication runs in both directions from the origin site

 Many origin sites.  100s/1000s  Replication bubbles fuse to form new DNA strands.  Faster replication (usually)  Replication also runs in both directions from origin site

 Done so by DNA Polymerases  Adds DNA triphosphate monomers to the growing replication strand  These triphosphate contain the complementary nucleotides  Matches A to T and G to C

 Exergonic rxn  Comes from the triphosphate monomers.  Loses two phos as each monomer/nucleotide is added.  Similar to ATP cycle  ATP contains ribose sugar  DNA = deoxyribose

 The two DNA strands run antiparallel to each other  Two “ends” of strand  3` - sugar/OH end  5` = phosphate end  New DNA strand can only elongate in the 5`  3` direction  Old DNA strand 3’  5’

toward the replication fork u Continuous replication toward the replication fork in the 5`  3` direction u Leading strand is a NEW strand that’s being added

away from the replication fork u Discontinuous synthesis away from the replication fork short segments u Replicated in short segments as more template becomes opened up u Lagging strand is also NEW!

Replication fork animation

u DNA Polymerase cannot initiate DNA synthesis (by itself) primer u Nucleotides can be added (only to an existing chain). This nucleotide chain is called a primer

u Made of RNA u 10 nucleotides long primase u Added to DNA by an enzyme called primase u DNA is then added to the RNA primer (to finish replication) u A primer is needed for each DNA elongation site u This is called “Priming”

u DNA Ligase u DNA Ligase - joins all DNA fragments together u Helicase u Helicase - unwinds the DNA double helix u DNA polymerase u DNA polymerase – elongation, replacement of RNA primer with DNA

u Single-Strand Binding Proteins u Single-Strand Binding Proteins - help hold the DNA strands apart u Primase u Primase – priming (adds RNA section to existing chain)

 1 in 10 billion base pairs  About 3 mistakes in our DNA each time it’s replicated

u DNA Polymerase u DNA Polymerase self-checks and corrects mismatches u DNA Repair Enzymes u DNA Repair Enzymes - a family of enzymes that checks and corrects DNA u Replication overview Replication overview

 50+ different DNA repair enzymes known  Failure to repair may lead to cancer or other health problems  Ex: u Xeroderma Pigmentosum u Xeroderma Pigmentosum -Genetic condition where a DNA repair enzyme doesn’t work u UV light causes damage, which can lead to cancer

Cancer Protected from UV

 T-T binding from side to side causing a bubble in DNA backbone  Often caused by UV light

 Cuts out the damaged DNA  DNA Polymerase fills in the excised area with new bases  DNA Ligase seals the backbone

 DNA Polymerase can only add nucleotides in the 5`  3` direction  Therefore, it can’t complete the ends of the DNA strand  Result:  DNA gets shorter and shorter with each round of replication

 Repeating units of TTAGGG ( X) at the end of the DNA strand  Protects DNA from unwinding and sticking together  Telomeres shorten with each DNA replication

 Serve as a “clock” to count how many times DNA has replicated  When the telomeres are too short, the cell dies by apoptosis

 Telomeres are involved with the aging process  Limits how many times a cell line can divide

 Enzyme that uses RNA to rebuild telomeres  Can make cells “immortal”  Found in cancer cells  Found in germ/sex cells  Limited activity in active cells (such as skin cells)  Control of telomerase may stop cancer, or extend the life span

 Recognize scientists and the experiments that lead to the understanding of the molecular basis of inheritance.  Identify the double helix composition and structure of DNA.  Identify the process and steps of DNA replication.  Recognize the problems in replicating the ends of the DNA molecules.  Give an example of DNA proofreading and repair.  Gain familiarity with the packing of DNA into a Eukaryotic chromosome.

 You do NOT need to memorize the names of the steps and particular enzymes involved in DNA replication EXCEPT for the following:  DNA polymerase  Ligase  RNA polymerase  Helicase  Topoisomerase