CH 10: Molecular Biology of the Gene

– Viruses are invaders that sabotage our cells– Genetic material (nucleic acid) and protein shell– Viral proteins bind to receptors on a host’s target cell– Viral nucleic acid enters the cell

– It may remain dormant by integrating into a host chromosome

– When activated, viral DNA triggers viral duplication, using the host’s molecules and organelles

– The host cell is destroyed, and newly replicated viruses are released to continue the infection

Introduction: Sabotage Inside Our Cells

• Herpes virus and HIV derive their membranous envelopes from the host cell.

• Activation of dormant viruses occurs in a time of stress for the herpes virus.

• For the HIV virus, this occurs during the activation of T cells during the immune response, so the insidious nature of the HIV virus is that it destroys cells at the very time they are recruited to protect the body from invaders.

Introduction: Sabotage Inside Our Cells

How was DNA discovered?

• There were several discoveries that happened that informed scientists that DNA existed BEFORE they actually discovered the structure.

• All discoveries were related and helped scientists come to a full understanding later…

Experiments: Griffith• 1. Frederick Griffith 1928

– Griffith studied 2 types of bacteria

• Type “R” Bacteria :harmless to mice

• Type “S” Bacteria: gave mice a fatal strain of pnemonia.

Experiments: Griffith

Experiments: Griffith– What happened?

• The non-invasive bacteria R was transformed into an invasive/deadly form!

• Descendants of the transformed bacteria carried the invasive characteristic!

Experiments: Griffith– Conclusion:

• Whatever transformation the bacteria made was heritable and could be passed down to the next generation.

Experiments: Hershey and Chase

• 2. Alfred Hershey and Martha Chase 1952

– Hershey and Chase studied viruses.

Experiments: Hershey and Chase

– Structure of a virus:

• Nucleic Acids (like DNA) wrapped in a protein coat.

• Bacteriophage- virus that infects bacteria cells (“bacteria eaters”)

• Phage T2

Experiments: Hershey and Chase

• Viruses can only reproduce by infecting a living cell with its genetic material (nucleic acids) and making the cell copy it.

• By Hershey and Chase studying virsus, there was a better understanding of how DNA is passed down through generations.

• Also, it concluded that viruses are definitely NOT living.

Experiments: Hershey and Chase

Experiments: Hershey and Chase

• How did they label the protein coat vs. the DNA?

• To label the protein only, they used radioactive Sulfur (S) because protein contains sulfur but DNA does not.

• To label the DNA only, they used radioactive because all DNA has phosphorus (P)

The Genetic Code…

• The genetic code is made up of DNA.

• It’s functions are to:– Instruct in the form a of special code.

– Pass information from one generation to the next.

DNA

• Stands for Deoxyribonucleic Acid

• It is located in the nucleus of each cell.

Structure of DNA

• DNA is a polymer, which means it is made of monomers.

– Called nucleotides

Structure of a nucleotide

• Each nucleotide contains:– 5-carbon sugar (Deoxyribose or Ribose)– Phosphate group– Nitrogenous Base

• There are 5 possible Nitrogenous Bases…

Back to DNA…• DNA has a double helix structure.

• It looks like a twisted ladder.

• It is made up of different combinations of the 4 nucleotides.

• Sugars and Phosphates make up the sides of the ladder-like structure.

• Nitrogenous bases make up the steps of the ladder.

• Weak hydrogen bonds (like velcro) hold the nitrogenous bases together from each side.

What does DNA store the information for???

• The information stored in the DNA is used to make proteins.

• Why proteins?

– Proteins can directly make hair, hormones, and enzymes.

– Enzymes are proteins that act as catalysts to facilitate chemical reactions.

– Proteins are important molecules of cells and are responsible for cell movement (cilia and flagella).

RNA• RNA= Ribonucleic Acid

• RNA is a nucleic acid, like DNA, but has different features that make it important.

• Functions of RNA:– Acts as a messenger between NDA (which is stuck in the nucleus) and

ribosomes (which are in the cytoplasm).

– Why does the information from DNA need to get to the ribosome?• To make proteins!

– Therefore, RNA helps carry out protein synthesis.

RNA• Structure of RNA

– The structure of RNA is very similar to DNA except…

• RNA is single stranded

• RNA is composed of Nucleotides that have a Ribose sugar instead of Deoxyribose (still has a phosphate group and a base.)

• The bases are Adenine, Cytosine, Guanine, and Uracil (there is no Thymine!)

RNA

• 3 types of RNA:– mRNA: messenger RNA

• Carries the code from the nucleus to the ribosome.

– tRNA: transfer RNA• Recognizes codons on mRNA and brings specific amino acids

to the ribosome.

– rRNA: ribosomal RNA• Found in the ribosome• Interacts with mRNA and tRNA to ensure proteins are made

correctly.

The discovery of the DNA structure…

• Rosalind Franklin– She x-rayed DNA and saw its double helix

The discovery of the DNA structure…

• Erwin Chargaff– Noticed that the number of

Adenine bases in a sample of DNA was equal to the number of Thymine bases, but not to the number of Guanine or Cytosine.

– Also, the number of Guanine bases was equal to the number of Cytosine, but not to the number of Adenine or Thymine.

– He came up with the base-pairing rule.

The discovery of the DNA structure…

• James Watson and Francis Crick- 1953