DNA STRUCTURE, FUNCTION AND REPLICATION MARÍA CRISTINA SALVADOR COLEGIO TERRANOVA QUITO - ECUADOR

DNA STRUCTURE DNA (deoxyribonucleic acid) is an incredibly long molecule composed of nucleotides. A polymer of nucleotides is called a nucleic acid. Examples of nucleic acids are RNA (ribonucleic acid) and DNA.

NUCLEOTIDES A nucleotide consists of three parts: a sugar, a phosphate group and a nitrogen-containing ring structure called a nitrogenous base. The sugar can be a ribose or a deoxyribose The nitrogenous bases are of two types: Purines: adenine (A) and guanine (G) Pyrimidines: cytosine (C) thymine (T) and uracyl (U). The nucleotides are joined within by covalent bonds.

DNA The molecule of DNA is composed of two polymers of nucleotides, coilded around each other, in a structure called the double helix. One strand is oriended in the direction 5’- 3’ and the other is oriented in the direction 3’- 5’. Alternating sugar and phosphate molecules form a backbone for each strand.

DNA The strands are complementary since a purine always bonds with a pyrimidine (A – T with two hydrogen bonds and C – G with three hydrogen bonds) and anti- parallel because they run in opposite directions. In the chain of DNA (or RNA), the sequence of aminoacids of one strand determines the sequence on the other strand. This feature of DNA is what makes it capable of self replication and allows it to serve as a guide for the production of RNA.

HOW DO WE KNOW ABOUT THE STRUCTURE OF DNA? When x rays pass through a strand of DNA they bend or diffract like water waves passing through a space between two rocks. The X ray shown the figure was produced by Rosalind Frankin’s working group at King’s College, London. Later, Crick and Watson deduced from this image some important details about the structure of DNA and created the model that we use and know.

Compare in the following table the structure of RNA and DNA CharacteristicDNARNA Type of sugar.. Nitrogenous Bases Type of chain..

EXERCISES: WORK ON EXERCISES OF PEARSON’S BIOLOGY BOOK PG. 65

DNA REPLICATION When a cell prepares to divide, the DNA content of the cell doubles in a process called DNA replication. This process ensures that there is an exact copy of each DNA molecule when cell division occurs. The two strands of the double helix will separate and serve as a guide or template for the creation of a new strand.

The result will be two DNA molecules composed of an original strand and a newly synthesized strand. For this reason, DNA replication is referred to as being semi-conservative. The base sequence on one DNA strand will determine the base sequence on the other strand since they are complementary. DNA REPLICATION

MOLECULES INVOLVED IN THE PROCESS OF DNA REPLICATION INCLUDE: Enzymes: helicase and DNA polymerases among others Free nucleotides (nucleoside triphosphates) The original molecule of DNA

THE PROCESS OF REPLICATION OCURRS IN THE FOLLOWING SEQUENCE: Helicase unwounds the DNA molecule (template) Free nucleotides attach to their complementary base pairs. DNA polymerase III forms covalent bonds between two adjacent nucleotides and moves forward in an “fashion”direction until the whole chain has been replicated The same happens in the other strand but in an opposite “antifashion”direction

As a result of this process, two strands are produced that are identical to each other. No molecule of DNA is completely new. Every DNA molecule after replication consists of a strand that was old paired with a strand that is new. DNA replication is described as a semiconservative process.

BIBLIOGRAFÍA Allot, A., & Mindorff, D. (2010). IB Biology Course Companion. New York: Oxford University Press. Allot, A., & Mindorff, D. (s.f.). IB Biology Course Companion 1st ed. Obtenido de Actual Book Pages: html Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Biology Standard Level. New Jersey: Pearson Education.