History and Structure of DNA

Thomas Hunt Morgan (1904) Discovered that genes are on chromosomes, but didn’t know if it was the protein or DNA part of the chromosomes

Fredrick Griffith (1928) Discovered Transformation 1.He injected a mouse with a pathogenic bacteria – the mouse died. 2.He injected a mouse with a non-pathogenic bacteria – the mouse lived. 3.He injected the mouse with dead pathogenic bacteria – the mouse lived. 4.He injected the mouse with dead pathogenic bacteria and live non-pathogenic bacteria – the mouse died. Its blood contained live pathogenic bacteria.

The bacteria took in DNA from the pathogenic bacteria, and incorporated it. He did not know what was transferred, he called it a “transforming factor.”

Avery, McCarty & MacLeod (1944) Repeated Griffith’s experiment to determine what the “transforming factor” was: DNA or protein. They combined live non-pathogenic bacteria with dead pathogenic bacteria. -Removed protein, bacteria is still pathogenic. -Removed RNA, bacteria is still pathogenic. -Removed DNA, bacteria is NOT pathogenic.

Alfred Hershey and Martha Chase (1952) Used bacteriophage (virus that infects bacteria) - Tagged bacteriophage proteins with radioactive sulfur. - Tagged bacteriophage DNA with radioactive phosphorus.

Bacteria had radioactive phosphorus, not sulfur – DNA is the “transforming factor” not proteins.

Erwin Chargaff (1947) Compared the nucleotide make up of the DNA of various species. - A = T and G = C -This is called “Chargaff’s Rule” -Differences are caused by the differing ratios of AT:GC -This suggests evolutionary relatedness.

Rosalind Franklin (1953) Rosalind Franklin took an x-ray diffraction photograph of DNA – called Photo 51 Maurice Wilkins shared this data with Watson and Crick.

James Watson and Francis Crick (1953) Using data from Chargaff and Franklin, Watson and Crick discovered the double helix structure of DNA.

DNA Structure Nucleotide strand is always equal length - Purine always pairs with Pyrimidine Purine – double ring structure (adenine and guanine) Pyrimindine – single ring structure (cytosine and thymine)

The building blocks of DNA are nucleotides. Nucleotides are made up of a phosphate-sugar backbone bound to a nitrogenous base. Nitrogenous bases are bonded to their pair by a hydrogen bond. A&T are bonded by 2 hydrogen bonds G&C are bonded by 3 hydrogen bonds

DNA Orientation The carbons in the sugar are numbered. - The nitrogenous base is attached to the 1’ carbon. - The phosphate is attached to the 5’ carbon. - The 3’ carbon bonds to the phosphate on the next nucleotide. We will use 5’ and 3’ when we discuss direction of DNA replication and transcription.

Anti-parallel - The 2 complementary DNA strands run in opposite directions.

Chromosome Structure

Chromosomes are made of chromatin. Chromatin is the combination of DNA and histones. It is found inside the nucleus of eukaryotic cells.

Chromatids A chromosome can be made of one chromatid or two chromatids (after duplication). Two chromatids are held together in a narrow region called a centromere.

Chromatids Each chromatid contains one DNA molecule, which is made up of a series of genes. Each gene is one unit of inheritance (humans have about 30,000 different genes!). The gene for a particular characteristic is always found at the same position, or LOCUS.

Alleles - Forms of genes. A gene codes for a trait like handedness. Possible forms of this gene (alleles) are left- handed and right-handed.

Karyotype Photograph of individual chromosomes – cut and arranged in order with homologous pairs.

DNA Comparison Double-stranded Circular One chromosome In cytoplasm No histones Supercoiled DNA Double-stranded Linear Usually 1+ chromosomes In nucleus DNA wrapped around histones (proteins) Forms chromatin Prokaryotic DNAEukaryotic DNA

Plasmid Plasmid – a small, circular piece of double- stranded DNA Plasmids can replicate independently of the rest of the DNA and do not have all the information required for survival, just additional information. Most often found in bacteria, but can also be found in other organisms