DNA and RNA Chapter 12, Page 287 What is DNA? How was it discovered? How do genes work? What are they made of? Are they single molecules? Polymers?
Deoxyribonucleic acid (DNA) First discovered in 1869 by Johann Friedrich Miescher (1844-1895), a young Swiss chemist studying in Germany. Collected pus (white blood cells), which have very large nuclei. Read more: http://www.answers.com/topic/history-of-dna- structure-and-function#ixzz1HpeKEEv8 http://www.answers.com/topic/history-of-dna- structure-and-function#ixzz1HpeKEEv8
From these, he purified a new compound, which he termed “nuclein." Miescher showed that nuclein was a large molecule, acidic, and rich in phosphorus. One of his students renamed the compound “nucleic acid”.
In 1885 the German biologist Oskar Hertwig (1849-1922) suggested that nucleic acid might be the hereditary material, based on its presence in the nucleus and the growing belief that the nucleus was the center of heredity.
Griffith and Transformation In 1928, Frederick Griffith, British scientist, was trying to figure out how certain bacteria caused pneumonia.
-He isolated 2 different strains of bacteria from pneumonia in mice (one which caused pneumonia; the other harmless). - Each grew well on culture plates. - The disease causing strain grew smooth colonies while the harmless one has rough edges.
-When Griffith injected mice with bacteria from smooth colonies, they died. -When he injected them with the rough-edged bacteria, they did not get sick at all. -He thought that the disease-causing bacteria was releasing a poison. He heated a sample to kill the bacteria and injected the mice. The mice survived.
Transformation Next, Griffith mixed the heat-killed formerly disease-causing bacteria with the live harmless ones and injected the mixture into the mice. The mice developed pneumonia and many of them died. Somehow the heat-killed bacteria had passed their disease-causing ability to the harmless strain.
Griffith named this process transformation because one strain of bacteria (the harmless one) had been transformed by the other (disease-causing). The transformation was passed onto the offspring of the bacteria. He knew that it happened but he didn’t try to explain why.
Avery and DNA (p. 288) 1944 Biologists at Rockefeller Institute in New York led by Canadian, Oswald Avery Repeated Griffith’s work to try to discover which molecule in the heat-killed bacteria was responsible for transformation.
Made an extract from the juice of the heat- killed bacteria. Using enzymes, his team destroyed the proteins, lipids, carbohydrates, one by one, in the extract. After they destroyed each one, they injected the extract into the mice. They died of pneumonia. Transformation had still occurred.
Then they destroyed the DNA and injected the extract into the mice. The mice survived. DNA was the transforming factor. Avery and the other scientists discovered that DNA stores and transmits the genetic information from one generation of an organism to the next.
Hershey Chase Experiments 2 Americans 1952 Trying to confirm that DNA contained hereditary material Worked with bacteriophages (Remember DNA or RNA core surrounded by protein)
They knew that bacteriophage were injecting genetic information into the bacterium, reproducing and eventually destroying the bacterium. There was debate on whether it was the DNA core or the protein coat that actually entered the bacterium.
Radioactive Markers To determine if the DNA or the protein entered the cell to infect it, they grew viruses in cultures containing radioactive isotopes of phosphorous and sulfur. Proteins contain almost no phosphorous and DNA, almost no sulfur.
If the sulfur was found in the bacteria after the virus “infected” it, the protein would have infected the bacteria. If the phosphorus was found, the DNA would have infected it. The genetic material that infected the bacteria was DNA.
The Structure of DNA (deoxyribonucleic acid) DNA is a long polymer made up of monomers called nucleotides.
Each nucleotide is made up of three basic components: – a 5-carbon sugar called deoxyribose – a phosphate group – a nirtrogenous base (nitrogen base)
There are four kinds of nitrogen-containing bases in DNA: 1.adenine ) ) Purines 2. guanine ) rings 3. cytosine ) ) Pyrimidines 1.thymine ) 1 ring
The backbone of a DNA chain is formed by the sugar and phosphate groups of each nucleotide. The nitrogen bases stick out sideways from the chain. The nucleotides can be joined in any order (as long as T pairs with A; C with G) so any sequence is possible.
Rosalin Franklin Franklin British 1950’s Purified DNA and stretched out the fibers in a thin glass tube so that most of them were parallel. X-rayed them and recorded the patterns on film.
Franklin and X-Ray Evidence X shape pattern shows DNA twisted around each other, a shape known as the helix Angle of the X suggests 2 strands Nitrogen bases are near the center of the molecule What do you mean, you can’t see it!!
The Double Helix Crick – British physicist Watson – American biologist 1953 in England Using Franklin’s x-rays, identified: The double helix in which 2 strands are wound around each other. Watson and Crick
Discovered that hydrogen bonds could form between certain (A & T, C & G) nitrogenous bases and provide just enough force to keep them together. They called this principle base pairing.
The exposed phosphate end of a nucleotide is called the 5’ end (said “5 prime). An exposed deoxyribose is the 3’ end (stated 3 prime).
Notice how one strand of DNA begins with a 5’ and ends with a 3’ and the complementary stand begins with a 3’ and ends with a 5’. Each strand of the DNA double helix “runs in the opposite” direction, antiparallel.
Chromosomes In each human haploid cell, there are almost 2 meters of DNA. Proteins called histones coil the DNA tightly to make sure that it fits into the nucleus. See page 229 in your text.
– DNA-wrapped histones (balls of protein) form nucleosomes. - Nucleosomes pack together to form a thick fiber which is coiled and looped, chromatin.
During most of the cell cycle, these fibers are dispersed throughout the nucleus so that they are hard to see. At the beginning of mitosis, the fibers are drawn together and become more visible. Then they coil tightly into chromosomes
Before a cell divides, it duplicates or copies its DNA in a process called replication.
What happens during replication? In most prokaryotes, DNA replication begins at a single point and continues until the whole chromosome is replicated. In larger eukaryotic chromosomes, DNA replication occurs at hundreds of places. The sites where separation and replication occur are called replication forks.
Enzymes DNA replication is carried out by a host of enzymes. Enzymes are biological catalysts or assistants. They consist of various types of proteins that work to drive the chemical reaction required for a specific action or nutrient. Enzymes can either launch a reaction or speed it up.
Unzipping of DNA Helicase is an enzyme capable of breaking the hydrogen bonds in the DNA double helix. It unzips the strands. Helicase does this at the beginning of replication.
Helicase usually begins at a bond between adenine and thymine. These bonds are easier to split because there are only 2 hydrogen bonds. Cytasine and Guanine have 3.
Once the DNA strands are separated, DNA polymerase brings the correct free- floating nucleotides to join to each strand.
DNA Polymerase The enzyme DNA polymerase joins the nucleotides to the strand. In this way, two strands are made from the one original strand.
DNA is read in a 3’ to 5’ direction. Therefore, polymerase attaches the free-floating nucleotides in that direction. The new strand is complementary to the “template”. The new strand is therefore being synthesized in a 5’ to 3’ direction.
One branch or prong is called the leading strand. The polymerese reads it in a continual 3’ to 5’ direction. The other prong is called the lagging strand. It is directionally opposite from the leading strand
This means that the polymerase has to attach nucleotides leading away from the replication fork. Therefore the nucleotides must be synthesized in short strands.
These short strands are called Okazaki fragments.
The Okazaki fragments are then joined by another enzyme called DNA ligase
In each cell, 1000 nucleotides are synthesized per second.
Enzymes are constantly moving up and down the DNA molecule---unzipping, bringing in the correct bases, checking for errors and cutting them out and replacing them with correct bases. This checking for errors and fixing them is called proofreading and is done by polymerase.