DNA Structure & History

DNA Background DNA stands for Deoxyribonucleic Acid. DNA is often called the blueprint of life. The way that DNA controls the cell (and your body) is by determining what proteins to make and how they are built. This process is called Protein Synthesis. Why is DNA called the blueprint of life?

DNA Background DNA is found in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. When viewed under a microscope, the DNA appears as long, dark strands or coils called chromosomes A chromosome is a tightly coiled strand of a long piece of DNA. (Like a spool of string) Why is DNA called the blueprint of life?

DNA Structure DNA is a polymer that is incredibly long. The monomer of DNA is the nucleotide. The average human chromosome (long coiled strand of DNA) has 260 million nucleotides. Because of the incredible length of a strand of DNA, it is necessary to coil it into a chromosome structure to fit it within the cell’s nucleus. Each cell contains about 2 meters (6 feet) of DNA if it were stretched out into a long strand. The combined length of DNA in the human body would stretch to the moon and back 170 times. Why is DNA called the blueprint of life?

DNA Structure To understand DNA’s overall structure, you must understand the monomer structure (nucleotides). A nucleotide has 3 parts Phosphate group Pentose sugar Nitrogen base In drawings, the phosphate is usually round and symbolized with a letter P. The pentose sugar should be drawn as a pentagon and usually labeled S, sugar, or pentose sugar. The nitrogen bases are Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

Nucleotide Structure Copy the drawing in the space provided on your notes. Base shown is Adenine. Label the Phosphate, Sugar, and Base

Nucleotide Structure The ribose sugar is a carbohydrate and is called a pentose sugar because it contains 5 carbon atoms, labeled 1’ through 5’. The name of the entire DNA molecule comes from the ribose sugar. Deoxyribose Is missing an oxygen atom Ribose Has an oxygen atom Deoxyribose is the sugar used in DNA (Deoxyribonucleic acid), and ribose is the sugar used in RNA (Ribonucleic acid). The 5’ carbon connects to the first phosphate and the 3’ end connects to the second phosphate.

Nucleotide Structure The phosphate is a compound that is built around a phosphorous atom. The phosphate’s job is to help hold the DNA molecule together. The phosphate plus the pentose sugar is referred to as the backbone of DNA. Backbone Phosphate

Nucleotide Structure There are 2 types of nitrogen bases: Purines and Pyrimidines. Any of the bases can be used in a nucleotide The Purines are called “double ring” bases and the Pyrimidines are called “single ring” bases. The bases are usually abbreviated as a single letter (A, T, C, or G) (U for Uracil comes later).

DNA Structure The completed nucleotide monomer (Phosphate, Sugar, Base) bonds with many other nucleotides in order to form the entire DNA molecule (polymer). The phosphate-sugar backbone is formed when the 3’ carbon on the sugar grabs a phosphate from another nucleotide. Once each backbone (side) is assembled, the nitrogen bases join together.

DNA Structure The nitrogen bases join together with hydrogen bonds. (drawn with dashed lines) For hydrogen bonding, purines only bond with pyrimidines and vice versa. Once the nucleotide has bonded with other nucleotides to make the backbone and hydrogen bond, it has become part of a DNA polymer.

DNA Structure The Purines only bond with Pyrimidines. This is due to the number of available bonds on each nitrogen base. Adenine and Thymine have 2 hydrogen bonds available while Guanine and Cytosine have 3.

DNA Structure The sugar-phosphate backbone is in an anti-parallel direction. The 5’ and 3’ ends of the backbone are on opposite sides. Each side of a DNA molecule is actually running in the opposite direction. All DNA molecules are built from 5’ to 3’.

DNA Structure The 5’ to 3’ direction makes the bases assemble in opposite directions. Adenine pairs with Thymine (A-T) Guanine pairs with Cytosine (G-C) Copy the four nucleotide segment into the space provided in your notes.

DNA Structure Once the entire strand of DNA has been assembled, it coils into chromosomes. DNA in eukaryotes is found coiled in chromosomes in the nucleus DNA in prokaryotes is found coiled in the cytoplasm (nucleoid). Why is DNA called the blueprint of life?

DNA Structure The classic shape of a fully assembled DNA molecule is a double helix. Sometimes we will call it a “twisted ladder”. DO NOT call DNA a spiral staircase! (that is RNA and is another unit) DNA twists because the chemical bonds within the molecule twist it like a rope. Why is DNA called the blueprint of life?

DNA Structure Discovery & History Watson and Crick (1953) were the first scientists to discover the structure of DNA. They coined the term “Double Helix” to describe the twisted ladder shape of DNA and also named the new molecule DNA after the deoxyribose sugar within it. Watson and Crick’s discovery was based on the work of several other scientists.

Frederick Griffith (1879-1941) In 1928, while working on a vaccine for pneumonia, Griffith discovered that when harmless bacteria and a virulent (disease-causing) bacteria were mixed some of the harmless bacteria became virulent, he called this transformation.

Click the Picture for an animation.

Oswald Avery (1877-1955) Avery continued Griffith’s experiments and tried to find out what substance caused the transformation, he determined it was DNA that caused the transformation. Avery demonstrated that an injection of DNA into a living cell could change the offspring of that cell.

Erwin Chargaff In 1949, Chargaff discovered that the amount of Adenine in in a cell always equaled the amount of Thymine & the amount of Guanine always equaled the amount of Cytosine. When he discovered this, the complimentary base pairing rules were formed. They are sometimes called Chargaff’s Rules. Chargaff’s Rules: A pairs with T (A-T) G pairs with C (G-C) Complimentary bases pair together (A goes with T and C goes with G)

Rosalind Franklin In 1952, just prior to Watson and Crick’s discovery, Franklin used x-rays to find that a DNA molecule resembled a tightly coiled spring (helix).

Franklin’s Experiment

Watson and Crick In 1953, using the discoveries of the other scientists, Watson and Crick, completed the first model of DNA. They also found that the bases were joined by hydrogen bonds. Named the new molecule DNA after the deoxyribose sugar within it. Described the shape of the molecule as a double-helix.

DNA Structure & Function