Nucleic Acids and Protein Synthesis

CHAPTER 8

DNA

Biologists call the program of the cell the ‘genetic code’

Genetic refers to anything related to heredity

The genetic code is the way in which cells store the program that they seem to pass from one generation to the next.

DNA is the molecule that carries this genetic code.

How they found out that is was DNA that was involved in heredity.

Fredrick Griffith’s Pneumonia/Mice Experiments- 1928 Was working with two different strains of

bacteria that cause pneumonia -one that was disease causing (we’ll call it strain D- for disease) and one that didn’t (we call that strain N for normal or not disease causing.

Strain D – culture with smooth edges Strain N- culture with ragged edges

What’s a culture you ask- look see

Bacterial Cultures

So what did he do and how do the mice play a part?

D strain Injected mice – they died Heat killed the strain- injected mice- they lived

N strain Injected mice – they lived

Mixed heat killed D with N strain and then injected mice.

What do you think happened next?

They Died! But why?

If the disease-causing D strain had been heat killed, it should not have been a problem for the mouse- so what had happened? And to make matters worse, when he grew out cultures from the dead mice, there was smooth disease-causing colonies instead of the rough non-disease causing ones he had started out with.

Griffith’s Theory

Griffith theorized that when the heat- killed D strain and N strain were mixed together, some factor had transferred from the heat killed strain to the live ones thus transforming the N strain into an active D strain. But what was this transforming factor?

I wonder what it was?

Summary of Griffith’s experiment

1944 – new guys keep at it

A new group in 1944 at the Rockefeller Institute in New York City decided to repeat Griffith’s work and see if they could figure it out.

Who? Oswald Avery Maclyn McCarty Colin MacLeod

Handsome fellas, huh?

So what did THEY do?

They made an extract, or juice from the heat-killed bacteria and tried to destroy whatever it was that was causing the disease by using a bunch of different enzymes.

It kept killing mice until they got to the DNA destroying enzyme.

Added enzyme to heat-killed D strain

They tested it one at a time to see which type of enzyme would destroy the ‘factor’- one that destroyed proteins, or lipids, or carbohydrates, or ribonucleic acid or deoxyribonucleic acid.

It kept killing mice until they got to the DNA destroying enzyme.

Oh happy mice!

This time the mice lived.

So DNA was the transforming factor!

What did we learn from all this mousing around?

DNA (deoxyribonucleic acid) is the nucleic acid that stores and transmits the genetic information from one generation to another.

DNA carries the genetic code.

But scientists are very skeptical so a bunch of other scientists had to prove it again with other experiments before it was fully believed.

That figures!

Okay so here’s what they did in a nutshell

Who? Alfred Hershey & Martha Chase (no he had nothing to do with chocolate)

When? 1952 Where? Some lab in America What? did a bunch of experiments with viruses

called bacteriophages to show that is the DNA in the virus that was transmitted into the bacteria. ( a virus with the name of a bacteria in it- why would

someone call it that? Why? Well, because the word means bacteria-killer

and this type of virus kills only bacteria. Oh.

How?

They put radioactive tags on different parts of the virus and the only part that entered a bacteria was the DNA, so that’s the only thing that was involved in killing the cell.

So, finally, the Hershey-Chase experiments added to the Avery & Co. work showed conclusively that DNA was the molecule that carried the genetic code.

The Structure of DNA

So the next thing to do was figure out what DNA looked like and how it worked.

The scientists could figure out the chemical involved- that part was easy, but how those chemicals were put together and how it worked was another thing.

So let’s look at what they knew.

Parts of DNA

DNA is a polymer formed from units called nucleotides.

Each nucleotide is a molecule made up of three basic parts: A 5-carbon sugar called deoxyribose A phosphate group A nitrogenous base

A bit more detail on nitrogenous bases

Purines

Adenine A

Guanine G

There are four different nitrogenous bases –

Adenine and Guanine are double-ring bases that belong to a chemical group called purines,

Thymine and cytosine are single ring bases that belong to a group of chemical compounds called pyrimidines

A and T are always paired together while G and C are always paired together

X-ray evidence

By the 1950’s scientists new what chemicals made up DNA but not what it looked like or how it worked

A British scientist, Rosalind Franklin and her associate Maurice Wilkens were able to successfully x-ray a strand of DNA.

It wasn’t really conclusive in itself but it provided just the information that Watson and Crick needed.

Who are they?

Building a DNA model With information from

Franklin, two young scientists, Francis Crick and James Watson figured out how DNA looked and worked.

It was a double helix structure and Franklin’s x-ray showed them that it twisted.

Features of double helix

Looks like a twisted ladder Sugar & phosphate forms the hand-rails Base pairs form the rungs Very specific base pairing –

A-T C-G

Held together by weak hydrogen bonds that can easily be separated

Last notes about Watson, Crick, Wilkens and Franklin

Watson, Crick, Wilkens won a Nobel prize in 1962.

Franklin had died in 1958 and never got credit for her accomplishment.

Watson and Crick are still working in Genetics- Watson headed up the Human Genome Project that recently decoded all 3 billion ‘rungs’ of human code.

DNA Replication

Before a cell divides it must make an exact copy of all of its DNA so that each new cell has its own copy.

This duplication process is called DNA replication.

DNA replication or DNA synthesis is carried out by a series of enzymes.

The enzymes separate or ‘unzip’ the two strands of the double helix, insert the appropriate nucleotide matching the bases and produce covalent sugar-phosphate bonds to join it all together.

RNA

Ribonucleic acid (RNA) is the nucleic acid that acts as a messenger between DNA and the ribosomes and carries out the process by which proteins are made from amino acids.

There are three different types of RNA mRNA – messenger RNA tRNA- transfer RNA rRNA- ribosomal RNA

RNA structure

Similar to DNA but with these differences Only single

stranded Contains ribose

sugar instead of deoxyribose

Uses the base Uracil instead of the base Thymine A-U instead of A-T

Making A Protein-Protein Synthesis

1. The DNA in the nucleus carries the code for a particular protein somewhere in its 3 billion ‘steps’ of code.

2. An enzyme (RNA polymerase, if you want to know) knows where the code is and when we want to make that protein, will go to the part on the DNA and ‘unzip’ it

3. mRNA starts to form from loose RNA nucleotides hanging out in the nucleus. The RNA will line up as the compliment to the DNA (A-U; T-A;C-G;G-C) until it makes a strand that represents the protein.

4. The single sided strand of mRNA will then leave the nucleus and go out into the cytoplasm where it finds a ribosome.

5. The DNA joins back up (zips up)

6. The mRNA now attaches to a ribosome that ‘reads’ the code alerting another type of RNA (tRNA) as to what the code says.7. The tRNA has three special bases (anti-codon) that will match up with three bases on the mRNA (codon)8. When it comes across a particular codon, it is like saying, “go and get such and such amino acid and bring it here” which the tRNA does.8. The amino acids are connected together and the protein is released.9. The mRNA falls apart into separate nucleotides and go back into the nucleus to await the next call for a protein. 10. And voila- protein!