DNA: HEREDITARY MOLECULES OF LIFE SBI 4U, Ms.Zafar

Every time a cell divides, an exact copy of its genetic information must be passed on to each of the two new cells produced. This is a huge task. Copies of molecules that consist of 6 billion base pairs (the diploid set) must be made, and each base pair must be “matched”. At the rate of one base per second, this would take 190 years. However our cells can do it in minutes – with an error rate of less than one in a million! How is that possible?

What is DNA?  Deoxyribonucleic acid  Primary carrier of genetic information  DNA has the instructions necessary to build all the proteins an organism requires  Responsible for ensuring the genetic continuity of the species, and therefore the continuity of life

Chromosomes  DNA is passed from one generation to another in the form of chromosomes  Eukaryotes – chromosomes only visible during cell division  X-shaped objects  Size and number of chromosomes vary in species – ex: humans have 46, turkeys have 82, potatoes have 48 and the adder’s tongue fern has 1200!

Recall: Proteins  Composed of amino acids  how many kinds of amino acids do we have?  Many roles  physical and chemical  Ribosomes translate sequence in RNA to make amino acid chains/polypeptides  some of which will later become functional proteins

Heredity & DNA  Gregor Mendel: Austrian monk, who spent his spare time breeding the pea plant  Noticed that through the selective cross-breeding of common pea plants over many generations, certain traits show up in offspring without any blending of parent characteristics!  Example: pea flowers are either purple or white--intermediate colors do not appear in the offspring of cross-pollinated pea plants.

Heredity & DNA, continued …  Mendel knew that a heredity molecule existed, and we now know that hereditary molecule as DNA  All information that is needed for optimal cell functioning is coded in a cell’s DNA  DNA stored in nucleus for eukaryotes (some in mitochondria and chloroplasts)  DNA stored in cytoplasm for prokaryotes

Genes and Chromosomes  Genes: coding regions of DNA  contain instructions for building the proteins responsible for each inherited trait  Alleles: several versions of each gene that give a trait more than one potential physical appearance

Genes and Chromosomes, continued …  DNA stored as chromosomes in nucleus  in the chromosome, the DNA is wrapped around special proteins called histones to form a complex  Several of these complexes are bundled into coils to form thicker strands called chromatin  protects DNA and helps to reduce its volume

Genes and Chromosomes, continued …  Archae and bacteria do not have a nucleus  they contain less genetic material and DNA is free- floating  Bacterial chromosomes are usually found in loops  Smaller, accessory loops of DNA, called plasmids may also occur

The Genome  The entire complement of hereditary information that is contained in an organism is called its genome  It is the complete set of DNA, including all of the genes  In humans, a copy of the entire genome – more than 3 billion DNA base pairs – is contained in all cells that have a nucleus

Summary  DNA is the molecule that carries genetic information in all living things  A gene is a region of DNA that codes for the building of a particular polypeptide  Eukaryotic DNA is wound around histone proteins and organized into linear chromosomes. The chromosomes are found inside the nucleus of each cell

Summary, continued …  The genome of most eukaryotes is in the form of homologous sets of chromosomes in diploid organisms  Prokaryotic cells usually have a single chromosome, which is in the form of a loop of DNA and is not associated with histones. Most of the genome is stored in this chromosome, but smaller loops of DNA, called plasmids may also be present