DNA Replication 

DNA or Proteins

There was a debate to whether DNA or proteins were the genetic material until the 1940s. It seemed more likely that it was proteins, since there are more proteins than nucleic acids.

==Frederick Griffith== helped to discover that DNA was the genetic material with his pathogenic/non-pathogenic bacteria experiment. When he mixed heat-killed pathogenic bacteria with live non-pathogenic bacteria, the mice injected with it lived. He discovered that something must have been passed between the bacteria, calling it ==transformation==. ==Oswald Avery, Maclyn McCarty, and Colin MacLeod== discovered that it was DNA.

==Bacteriophages== are viruses that infect bacteria. ==Alfred Hershey and Martha Chase== found that DNA is what phages used by tagging protein with radioactive sulfur and tagging DNA with radioactive phosphorus (because only proteins contain sulfur and only DNA contains phosphorus). Only the DNA entered the bacteria, and the protein remained in the extra liquid.

==Erwin Chargaff== analyzed the base composition of DNA from different organisms and found that the amount of each base is different in different organisms, proving that DNA can account for genetic variation, even with only four bases. He also discovered that the amount of A=T and C=G.

DNA Structure

==Watson and Crick== used photos taken by ==Rosalind Franklin== to determine that DNA was helical, contradicting ==Linus Pauling==’s three-stranded model. Franklin was the first to determine that the ==sugar-phosphate backbone was on the outside of the DNA molecule and the bases were in the middle==. Watson and Crick then made models until they figured out the structure.

==Purines== have two rings, and ==pyrimidines== have one ring. A-T has two bonds and C-G has three bonds.

Template Strands

The correct model is the ==semi-conservative== model, in which each daughter molecule will have one strand from the parental molecule and one new strand. This is opposed to the ==conservative model==, where the parent strands come back together after being a template for the daughter strand, and the ==dispersive model==, where all the strands of the daughter molecules are partially parental and partially new. This was discovered by ==Meselson and Stahl.==

Replication Mistakes

Although DNA polymerase proofreads DNA, sometimes it misses and incorrect pair. In ==mismatch repair==, other enzymes proofread and fix base pairs. In ==nucleotide excision repair, nuclease== cuts out the mismatched base and DNA polymerase or ligase fill the gap.

The Ends of DNA

The ends of DNA cannot be replicated because once the primer for the 5’ end of the daughter fragment is removed, DNA polymerase can’t attach to anything to replace those bases. The ends are called ==telomeres==.

In germ and cancer cells, ==telomerase== catalyzes the lengthening of the telomeres.

Genes=Proteins

The first school of thought was the one gene, one enzyme hypothesis, stating that only one gene coded for only one enzyme. This changed to one gene one protein after scientists realized not all proteins are enzymes. Later, this became one gene one polypeptide.

The template strand is chosen based off of the orientation of the enzyme that transcribes genes. It will always be the same for the same gene, but it could be different strands between genes.

The coding strand is the non-template strand since it is identical to the mRNA strand.

==Codons are read from 5’ to 3==’.

RNA Processing

The 5’ cap aids in ribosomal recognition. The poly-A tail is for protection - a longer poly-A tail = a longer lifespan = more proteins can be made

The mRNA enters the ribosome starting with the 5’ end