DNA - BIOL 101 Unit 5A - 2024

DNA PART 1: History & ID as genetic material  Evidence that DNA is the genetic material 1920s: scientists knew that chromosomes carried genetic info But which component of chromosomes was it? DNA or protein?  Evidence from e...

DNA PART 1: History & ID as genetic material  Evidence that DNA is the genetic material 1920s: scientists knew that chromosomes carried genetic info But which component of chromosomes was it? DNA or protein?  Evidence from experiments Frederick Griffith English physician, 1920s Wanted to develop a vaccine against pneumonia Bacterium: Streptococcus pneumoniae Loading… He worked with 2 strains of the bacterium: The “S” strain formed smooth colonies, and was deadly when injected into mice The “R” strain formed rough colonies and was harmless  Griffith continued For his vaccine work, Griffith heat-killed the S strain bacteria and injected them into mice The mice were fine Next, he injected a mixture of dead S bacteria and living R bacteria To his surprise, the mice died Their blood contained many living S bacteria!  Conclusion Some “factor” in the dead S bacteria was able to transform the harmless R strain into harmful S bacteria Since the new S bacteria could reproduce, the Loading… factor had to contain hereditary information We now know that the gene for a cell capsule was transferred from S to R The capsule protects against the host’s immune system  Evidence from experiments Oswald Avery wanted to identify the transforming factor Griffith had discovered Candidates: DNA, RNA and protein Used cell extracts (contents of dead cells) from the S strain and transformed R into S He then added enzymes to different cell extracts to destroy each candidate individually, then tested the extract for transforming ability  Avery continued Results: destroying DNA in the sample prevented the transformation from R into S Destroying RNA or protein had no effect on the transformation Conclusion: DNA is the transforming factor And thus must carry genetic information!  Transformation in eukaryotic cells Today, we can transfer specific DNA into cells in the lab, on purpose This is useful for research, agriculture and has many potential medical applications  Transgenic organisms Adding DNA to a fertilized egg cell results in a whole organism containing the artificially added gene(s) Such an organism is transgenic, and will pass on the added genes to its offspring A useful way to modify organisms Crops with higher yields, disease resistance, more nutrients Transgenic lab animals help identify genes linked to disease, developmental disorders, etc. Transgenic humans?  Determining the structure of DNA Chemical studies Biochemists found out that DNA consisted of nucleotides Sugar (deoxyribose) Phosphate One of 4 bases (A, C, T, G) Erwin Chargaff determined that the amounts of the bases were related: Amount of A = amount of T Amount of G = amount of C  Determining the structure of DNA X-ray diffraction studies Requires a crystallized strand of DNA No moving pieces X-rays are passed through the crystal The diffraction pattern gives info about the positions of individual atoms Loading… Rosalind Franklin determined that DNA was a double helix Bases in the middle, with a sugar-phosphate backbone on the outside  Determining the structure of DNA James Watson and Francis Crick built a physical model Double helix with bases in the middle Not on the outside! Strands must run antiparallel for bases to match up A paired with T, C paired with G YE NO Nobel Prize S Watson & Crick’s Model DNA PART 2: Structure & Replication  DNA structure Double-stranded helix Sugar-phosphate backbone, with bases in the middle Hydrogen bonds hold the bases together A bonds with T C bonds with G The strands run anti-parallel They are “read” (and replicated) in opposite directions  Antiparallel strands DNA strands have a direction Based on the position of the sugars Each strand has a 5’ end and a 3’ end The oxygen on the sugars always points toward the 5’ end The two strands run in opposite directions: the 3’ end of one strand is the 5’ end of the other  Antiparallel Strands The ‘message’ in the DNA is always read from the 5’ to the 3’ end 5’ end is the “beginning” of a gene 3’ end is the “end” of a gene  Reading a gene 5’ to 3’  Chromatin DNA is a LONG molecule The DNA in one human cell nucleus, if unwound, would be 6 feet long! It has to be compacted in order to fit into cells Binds to various proteins DNA + the proteins it is attached to = chromatin DNA inside of cells is always in the form of chromatin Always has proteins attached to it  Chromosomes The DNA in each cell nucleus is not one long piece It’s divided into multiple pieces Chromosomes Humans have 46 chromosomes in each cell nucleus Together, they contain all the genetic information for growing that person One full copy of the human genome One cell can be used to make a copy (clone) of the person!  DNA Replication DNA replication is semi-conservative: each original strand serves as a template for a new strand The two new DNA molecules each have one old and one new strand Meselson and Stahl demonstrated this in 1958  DNA Replication It starts at specific chromosome locations called origins of replication Step 1: An enzyme called helicase “unzips” the double helix Breaks hydrogen bonds between base pairs Separates the two DNA strands This exposes the bases  DNA Replication Step 2: Once the bases are exposed, a primase enzyme adds 5-10 RNA nucleotides to start the new partner strand RNA primer The nucleotides are complementary to the DNA bases  DNA Replication Step 3: An enzyme called DNA Polymerase starts adding new DNA nucleotides to the RNA primer Building the new DNA strand Complementary to the original strand It will keep doing this until it gets to the end of the strand Notice it starts on the 5’ end and adds to the new strand on the 3’ end  The antiparallel problem Polymerases can only add nucleotides to the 3’ end of the new, growing DNA strand The original DNA strands run in opposite directions 3 So one new strand has to be ’5 synthesized in the “wrong” direction ’ Solution: the problematic strand is synthesized one piece at a time, away from the replication fork Needs multiple RNA primers Allows the polymerase to add nucleotides to the 3’ end  Okazaki fragments The strand that is synthesized normally is called the leading strand The strand that is formed in pieces is the lagging strand The short pieces are called Okazaki fragments at_1304_leading_lagging.mp4  Finishing up… To finish replication, the RNA primers are removed and replaced with DNA nucleotides An enzyme called ligase connects the Okazaki fragments Forms bonds between adjacent nucleotides