Lecture 2: Discovery of DNA

What is molecular biology?

• study of molecular basis of biological activity between bio-molecules in a cell (DNA/RNA/proteins) their biosynthesis and the regulation of these interactions

How do your heart and brain cells express different genes?

• not all genes in every single cell type are the same → cells are specialized

How is PCR used in forensics?

• PCR is used to amplify and analyze DNA

How can bacteria be engineered to synthesize a human protein?

• insulin is produced from bacteria

• bacterial DNA + plasmid DNA + human insulin gene (DNA) → recombinant DNA → PCR used to amplify → human insulin is extracted and purified

How do viruses invade and take over cells?

• HIV invades T cells and copies itself inside the host and destroys it

• Coronavirus uses host cell to replicate and create new viral contents

How can you design therapeutics to inhibit the expression of any gene of interest?

• CRISPR - cells are removed from a patient → CRISPR/Cas9 edit gene sequences in cells → modified cells expanded in culture and returned to patient

How can your entire genome be sequenced and disease genes be identified?

• gene sequencing is used to identify disease genes

What is the central dogma?

the flow of genetic information goes from:

DNA (replicated) → transcribed to RNA → translated to proteins

How many amino acids compose a protein?

20 amino acids

How many nucelotides compose DNA?

• 4 nucleotides

What did early scientists believe was the genetic material and why?

• they though protein was the genetic material because it was more complex than DNA

  • DNA = 4 nucleotides

  • protein = 20 amino acids

What did scientists use to study DNA?

bacterial cells

What are 3 unique properties of bacteria that helped in studying DNA?

• simple

• prokaryotic (no nucleus or other membrane bound organelles)

• have ribosomes

Which experiment helped us understand that DNA carries information?

• Avery, McCarty, and Macleod

Describe the Griffith Experiment

• used strains of human pathogenic streptococcus pneumoniae

  • 4 controls:

    • living smooth pathogenic cells → mouse dies

    • living rough nonpathogenic cells → mouse ok

    • heat killed smooth pathogenic cells → mouse ok

    • mix of heat killed smooth pathogenic and living nonpathogenic rough cells → mouse dies

  • living smooth cells were found in the mouse’s blood sample

Describe the Avery, McCarty, and MacLeod Experiment

• living smooth pathogenic cells were killed with heat, then the ‘active’ material was extracted

• the extracted material was then incubated with 3 different enzymes:

  • RNAse

  • DNAse

  • Protease

• only the DNAse + extracted material control yielded no transformation

RNAse vs DNAse vs Protease

RNAse - degrades RNA

DNAse - degrades DNA

Protease - degrades proteins

What did the Avery, McCarty, and MacLeod Experiment prove?

proved that DNA is important for transformation, not RNA or proteins

Define Transformation

• genetic alteration of a cell that happens from the direct uptake of genetic material from its surroundings

What is a bacteriophage?

• virus that infects and replicates in bacteria by attaching to the surface and injecting its DNA

From which experiments do we know that DNA is the genetic material?

• Avery, Macleod, McCarthy

• Hershey Chase

Which experiment was responsible for understanding that DNA is the injected material and not protein?

• Hershey and Chase Experiment

How was the first part of the Hershey and Chase experiment conducted?

Protein Coat:

1. phage incubated with radioactive sulfur → sulfur integrates into protein coat and becomes radioactive

2. phage infects and injects DNA into bacteria

3. phage + bacteria get blended up and centrifuged → supernatent = radioactive phage protein ; pellet = genetic material

How was the second part of the Hershey and Chase experiment conducted?

DNA

1. phage incubated with radioactive phosphorus → phosphorus integrates into DNA and becomes radioactive

2. phage injects DNA into bacteria

3. phage + bacteria get blended up and centrifuged → supernatent = protein coat ; pellet = radioactive DNA

What did the Hershey Chase Experiment prove?

• that DNA is injected, not protein

• DNA is the cells genetic material

What are 3 properties of genetic material?

1. it carries information

2. it is capable of being copied

3. is capable of undergoing change

Scrapie is an infectious brain disease in sheep (related to Kuru
and CJD in humans and mad cow disease in cows).

Researchers isolated brain extracts from sick sheep and injected
them into brains of hamsters, and observed the following:

-Untreated brain extract => infectious
- RNase treated => still infectious
- DNase treated => no longer infectious
- Proteinase treated => still infectious

Which molecule is the “genetic material”?

DNA

What did researchers used to believe about base concentrations?

• they used to think that the concentrations of all bases were equal

• A = T = C = G

What did Chargaff discover?

• the amounts of the 4 bases were as follows:

  • A = 30.3%

  • T = 30.3%

  • G = 19.5%%

  • C = 19.9%

What is Chargaff’s rule?

• % A (adenine) = % T (thymine)

• % G (guanine) = % C (cytosine)

• the bases form weak hydrogen bonds

What is x-ray diffraction?

• technique that uses x-rays to create pictures of molecules bc of short wavelengths allowing x-rays to bounce off individual atoms

• x-rays move through helical shape and refract at angles perpendicular to the helix

What did Rosalind Franklin Discover?

DNA is a helix and consists of 2 strands

What is the general structure of DNA?

• made of 2 coiled strands called a double helix

• sides are made of pentose sugar deoxyribose bonded to phosphate groups by phosphodiester bonds

• center is made of nitrogen bases bonded together by weak hydrogen bonds