The Structure, Function, and Replication of DNA
By Mrs. Ingram
Academic Biology
Hello,
I am Professor DogNA. Today, we are going to be learning about the structure, function, and replication of DNA.
There is a textbook link and page numbers provided on each page for additional information.
At the end of each page, there is a question for you to answer.
In the 1920's, a scientist named Frederick Griffith was researching how bacteria caused disease when he discovered transformation.
Griffith mixed heat-killed, disease causing bacteria with live, non-disease causing bacteria. This resulted in the non-disease causing bacteria to change into disease causing bacteria. An unknown "factor" was causing this change. Griffith's discovery prompted more scientists to search for the "factor" in transformation eventually leading to the discovery of DNA.
Textbook Link: Go to page 288; DNA 12-1
Oswald Avery expanded on Griffith's discovery of transformation and tested each of the four different macromolecules: carbohydrates, lipids, proteins, and nucleic acids. By the process of elimination, he found nucleic acids to be the "factor" that transformed non-disease causing bacteria into disease causing bacteria. He narrowed his discovery down to the nucleic acid now known as DNA.
Textbook Link: Go to page 289; DNA 12-1
Later in the 1950's, two scientists named Alfred Hershey and Martha Chase wanted to confirm Avery's conclusion that the transformation "factor" was DNA. They used bacteriophages (viruses) that infect bacteria. They attached markers to the virus protein coat and virus DNA to follow the route of virus infection. While the protein coat was not found inside the bacteria, the DNA was and confirmed it was the "factor" of transformation.
Textbook Link: Go to page 290; DNA 12-1
Also in the 1950's, a woman scientist named Rosalind Franklin used x-ray diffraction to see the first images of the shape of DNA. Her DNA x-ray images became the source for the discovery of the double helix by two other scientists.
Textbook Link: Go to page 291; DNA 12-1
In 1953, James Watson and Francis Crick were shown Franklin's x-rays of DNA and developed the first DNA model showing a double helix structure. The structure is held together by hydrogen bonds. Watson and Crick were awarded the Nobel Prize for their DNA structure discovery and model.
Textbook Link: Go to page 291; DNA 12-1
DNA is called Deoxyribonucleic Acid. DNA contains many segments called genes which code for proteins. These gene segments carry the information from one generation to the next. That is why you look like your family members.
The DNA structure looks like a twisted ladder called a double helix. The sides, called the backbone, are made up of 5 carbon sugars and phosphate groups. The rungs of the ladder are made up of nitrogen bases and hydrogen bonds.
Textbook Link: Go to page 292, 12-1
DNA is a long polymer molecule made up of units or building blocks called nucleotides. Each nucleotide is made up of three basic parts: a 5 carbon sugar called deoxyribose, a phosphate group, and a nitrogen base.
There are four kinds of nitrogen bases: adenine, thymine, cytosine, and guanine.
Textbook Link: Go to page 292; DNA 12-1
The rule of base pairing, discovered by a man named Chargaff, states that the nitrogen bases adenine always binds with thymine, and cytosine always binds with guanine.
So, the % of adenine in a DNA molecule will always equal the % of thymine, and the % of cytosine will always equal the % of guanine.
The nitrogen bases are held together by hydrogen bonds which can be easily broken during DNA replication or protein synthesis.
Textbook Link: Go to page 292; DNA 12-1
Each nitrogen base is either a single or double ring structure. Adenine andguanine are comprised of double rings called purines. Cytosine and thymine are comprised of single rings called pyrimidines. A purine always binds with a pyrimidine. Consequently, the % of purines equals the % of pyrimidines.
Textbook Link: Go to page 292, 12-1
Prokaryotes, also known as bacteria, have DNA in a single, circular chromosome located in the cytoplasm since they do not have a nucleus.
Eukaryotes have their DNA inside their nucleus in the form of individual chromosomes which varies in number from one species to the next.
Textbook: Go to page 295-299; 12-2
Eukaryotic DNA is tightly packed together in the nucleus to form chromatin. DNA's double helix is coiled around proteins called histones. A bunch of histones bound together form a nucleosome. The nucleosomes then coil tightly to form supercoils.
Textbook Link: Go to pages 295-299, 12-2
DNA Replication
Before a cell divides during mitosis, it must copy its DNA so each of the two new daughter cells have the same number of chromosomes as the original cell they came from.
First, an enzyme called helicase must break the hydrogen bonds and open up the DNA molecule. The two strands unwind and each serves as a template for attaching the new complimentary bases. DNA polymerase, another type of enzyme, wraps around each strand to attach the new bases creating a new strand of DNA. Each new strand will form hydrogen bonds with the complimentary nitrogen bases of the original strands forming two new double helixes or DNA.
Textbook Link: Go to pages 295-299, 12-2
This concludes your on-line lesson on the structure, function, and replication of DNA.
Please complete the worksheet provided by your teacher. You may refer to pages in this on-line book to help you answer the questions. Turn in the worksheet when you are finished.
If others are still working, you should watch the following video link:
DNA video link