Understanding DNA: Structure, Function, and Viruses

• DNA, or deoxyribonucleic acid, is referred to as "the genetic material” or “the molecule of heredity" and it determines the structure and function of living things.
• DNA stores genetic information and transmits it from one generation to the next, used as a blueprint for offspring cell construction.

Viruses and DNA

• Viruses contain DNA (or RNA) but aren't considered living because they can't reproduce without a host.
• Viruses inject DNA into host cells, forcing the host to reproduce the virus.
• Viruses are DNA (or RNA) surrounded by a protein shell.

DNA Structure and Reproduction

• DNA's structure is designed to facilitate the copying of its code during cell reproduction, which benefits DNA testing applications also.
• The chemical and physical structure of DNA is important for understanding genetic information.

Chemical Ingredients of DNA

• DNA is durable and can be stored for long periods of time under certain conditions.
• DNA comprises three chemical components: nitrogen-rich bases, deoxyribose sugars, and phosphates.
• These components combine to form nucleotides, which are the building blocks of DNA.
• Thousands of nucleotides form a single DNA molecule.

Chemical Composition

• Bases are made of carbon (C), hydrogen (H), nitrogen (N), and oxygen (O) atoms.
• The two purine bases in DNA are adenine and guanine; purines are compounds composed of two rings.
• Pyrimidines: The two pyrimidine bases in DNA are cytosine and thymine and have a single six-sided ring structure.
• The flat, ring-like structure of all four bases enables them to stack, compact, and strengthen the DNA molecule.
• DNA is three-dimensional and arranged in strands, giving it a linear shape (rope-like).

Nucleotides and DNA Composition

• The sequence, or order, of the bases carries the message of DNA that provides the information to produce corresponding proteins.
• Bases must attach to deoxyribose and a phosphate molecule to make a complete nucleotide.
• Deoxyribose is ribose sugar lacking one oxygen atom and is the chemical basis for RNA.
• The absence of an oxygen atom is the only difference between ribose and deoxyribose sugars at the 2' site.
• "Deoxy" means an oxygen atom is missing from the sugar molecule.
• Deoxyribose facilitates phosphate completion of each nucleotide, and it also contains a reactive group for chemical interactions.

Double Helix Assembly

• Nucleotides are the building blocks of DNA, and each nucleotide consists of one deoxyribose sugar, phosphate, and one of the four bases. Nucleotides join to make long chains. Then, matched pairs of nucleotides come together to form the long strands making a double helix.
• Strands of DNA always join using a pattern using a pre-existing strand.
• Thousands of nucleotides link to form DNA strands, connecting phosphates to sugars to form the sugar-phosphate backbone.
• The bases are parallel to each other and sugars and phosphates run perpendicular to the stack of bases.
• A long strand of nucleotides is called a polynucleotide strand.
• Scientists use a chemical numbering system to describe the ends of a polynucleotide strand.
• The phosphate end is the 5' end and the sugar end is the 3' end. The bonds between each phosphate and its sugars are collectively called a phosphodiester bond.

Base Pairing

• DNA strands are constantly seeking a complementary match.
• A complete DNA molecule has two side-by-side polynucleotide strands twisted together and bases attached in pairs in the center of the molecule. This has sugars and phosphates on the outside, forming a backbone.
• Strands pair with each other in opposite or antiparallel directions.
• Adenine always pairs with thymine, and cytosine always pairs with guanine and these are considered to complement each other.

Complementary Base Pairs

• Base pairs are complementary because of their structure and function where the strands attach with hydrogen bonds.
• G-C (guanine-cytosine) contains three hydrogen bonds.
• A-T (adenine-thymine) contains two hydrogen bonds.
• If the correct complementary base is not paired, then the DNA cannot bond correctly in an antiparallel orientation and that is why a purine will always pair with a pyrimidine.
• Because the two strands run in opposite directions, they pull causing the molecule to twist and form a spiral staircase
• One complete is approx. every ten base pairs
• The helical form of the DNA creates grooves on the outside of the molecule and the major groove lets the bases peep making the is accessible for the cellular information to be read.

Other DNA facts

• The genetic information in DNA is carried in the order of the base pairs which then determines protein structure.
• DNA uses a preexisting DNA strand as a pattern or template in replication.

Varieties of DNA

• DNA has the same bases and structure, but different sets carry out different genetic functions.

Nuclear DNA

• Nuclear DNA is in cell nuclei and carries out the majority of cell functions. It is what makes the physical traits of an organism (that is, physical phenotypes).

Mitochondrial DNA

• Animals, plants, fungi, and other eukaryotes all have mitochondria which has its own DNA that is different from that of nuclear DNA. Human mtDNA is circular and is short with 37 genes.
• These genes control cellular metabolism.
• All mtDNA is inherited from the mother due to the egg donating the the zygote.

Chloroplast DNA

• Plants have three sets of DNA: nuclear, mitochondrial, and chloroplast DNA Plants contain mitochondria where photosynthesis occurs.
• Chloroplast DNA molecules are circular and large. Contains only about 120 genes but most supply needed information used for photosynthesis.
• cpDNA, along with mtDNA, is transmitted to offspring in the cytoplasm of the seed.

Discovering DNA

First Discoveries

• In 1868, Johann Friedrich Miescher isolated DNA as a medical student.
• The substance then called nuclein was rich in phosphorus and acidic. It took 84 years for DNA to be recognized as the genetic material.

Early Experiments

• In 1928, Frederick Griffith recognized that bacteria could transfer something to transform harmless bacteria into deadly bacteria.
• A team of scientists led by Oswald Avery determined that, after Griffith's experiments, the "transforming principle" was DNA.

Viruses

• Alfred Chase and Martha Hershey worked with a virus called a bacteriophage (a virus that attacks bacteria). Hershey and Chase attached radioactive chemicals to track different parts of the "parent" bacteriophage to track the hereditary material.
• The results showed that the viruses injected only DNA into the bacterial cell to infect it. This confirmed that DNA was the hereditary material.

Chargaff's Rules

• Erwin Chargaff realized that DNA was the genetic material. He made the discovery that the amount of guanine always equaled the amount of cytosine. Likewise, the amount of adenine equaled the amount of thymine.
• These are now referred to as Chargaff's rules.

Franklin, Wilkins, Watson, and Crick

• Rosalind Franklin's data revealed that DNA was in the shape of a double helix.
• James Watson and Francis Crick worked together to build a model and figure out its structure. They deduced that structure was a double helix using Franklin's photo and Chargaff's work.
• But, Franklin was not properly recognized for her work.