 Genes carry information from one generation to the next  Genes determine the heritable characteristics of organisms  Genes can be replicated or copied exactly

 DNA is a nucleic acid ◦ It has monomers called  nucleotides  Each nucleotide is composed of three basic parts:  5 carbon sugar – deoxyribose  A phosphate group  A nitrogenous base

 There are four nitrogenous bases categorized into two groups  Purines: (two rings)  Adenine (A)  Guanine (G)  Pyrimidines: (one ring)  Cytosine (C)  Thymine (T)

 DNA can be considered to be like a ladder  The sugar and phosphate groups make up the backbone of the molecule, or the sides of the ladder ◦ They alternate along the sides sugar  The nitrogenous bases stick out of the sides of the sugar ◦ They make up the rungs of the ladder

 Realized the number of A’s equals the number of T’s and that the number of G’s equals the number of C’s  Base pairing rule A = TG = C Purines Pyrimidines

 Used x-ray diffraction to gather information about DNA structure ◦ Revealed that DNA is in helix form (coil or twisted ladder)

 Made 1 st 3-D model of a DNA molecule ◦ Used the work of Chargaff and Franklin to create this model  DNA structure is called a double helix, in which two strands are wound around each other  The two strands are connected by hydrogen bonds

 Chromosomes are tightly wound up DNA strands  Increased organization allows for the tremendous length of DNA to fit into the nucleus of the cell

 DNA is tightly wound around proteins called histones  8 histones + DNA = nucleosome ◦ These form chromatin  Chromatin tightly wound up makes a chromosome 

 Prokaryotic chromosomes are located floating in the cytoplasm of the cell (not in the nucleus)  They are circular in shape (not “X shape” like in eukaryotes)

 Replication occurs during interphase (S phase) of the cell cycle ◦ Same in prokaryotes and eukaryotes within  Occurs within the nucleus of the eukaryotic cell or in the cytoplasm of the prokaryotic cell

 Watson and Crick  Hypothesized that each strand of the original DNA molecule acted as a template for the creation of a new DNA molecule. Should work because of the base pairing rules

 DNA replication begins at a single point in the chromosome and proceeds in two directions

 DNA replication begins at hundreds of different locations on the same molecule ◦ It proceeds in both directions until everything is done  Place where separation of strands occurs and replication begins is called a replication fork

 Begins when an enzyme (helicase) binds to specific site on the DNA molecule called the origin  Unwinds DNA molecule to make a straight ladder  Also, unzips DNA strands by breaking hydrogen bonds  Occurs at several spots on the DNA

 Once unzipped, another enzyme (DNA polymerase III) binds to DNA strand and adds free DNA nucleotides ◦ DNA winds back up to double helix shape

 One original DNA strand turns into two identical copies of original ◦ Copies contain one strand of the original and one new strand  Mistakes can be made during this process ◦ Cell has an editor that fixes these mistakes

PdU