DNA Structure & Replication Research PDF

DNA Unit 7 Friedrich Miescher First person to isolate DNA (it was called nuclein at the time)  Oswald Avery Alfred Hershey and Martha Chase DNA is responsible for heredity (transferring genetic information from one generation to the next) Befor...

DNA Unit 7 Friedrich Miescher First person to isolate DNA (it was called nuclein at the time)  Oswald Avery Alfred Hershey and Martha Chase DNA is responsible for heredity (transferring genetic information from one generation to the next) Before this, it was thought that proteins were responsible  Erwin Chargaff Discovered that the percentages of guanine and cytosine are roughly equal in ALL organisms Same thing is true for adenine and thymine Now called Chargaff’s Rules Rosalind Franklin X-rayed concentrated DNA samples Obtained Photo 51 (left) The x-shaped pattern indicated that the strands of DNA are twisted around each other like a coil Other clues suggested that the nitrogenous bases were near the center of the molecule  James Watson & Francis Crick Tried to understand the structure of DNA by building physical models out of cardboard and wire Twisted and folded in multiple ways but could not explain DNA’s properties until they got hold of Rosalind Franklin’s work (without her permission!) Just a few weeks after, they had determined the structure of DNA was a double helix The Structure of DNA DNA is an extremely long molecule made up of nucleotides Each nucleotide has 3 parts: ○ 5 carbon sugar - deoxyribose ○ Phosphate group ○ Nitrogenous base The Nitrogenous Bases Purines ○ Adenine and guanine ○ 2 rings in their structure Pyrimidines ○ Thymine and cytosine ○ 1 ring in their structure ○ Uracil only found in RNA  An Explanation of Chargaff’s Rules What holds the strands of DNA together? ○ Hydrogen bonds connect base pairs ○ 3 hydrogen bonds form between cytosine and guanine ○ 2 hydrogen bonds form between adenine and thymine ○ Base pairing also explained Chargaff’s Rules (why A = T and C = G)  DNA is Antiparallel Each phosphate/sugar backbone in a molecule of DNA runs in opposite directions → this is called antiparallel The 5’ end will always be a phosphate. The 3’ end will always be a sugar.  DNA and Chromosomes Prokaryotes have a single, circular DNA molecule found in the cytoplasm Eukaryotes have 1000x more, typically located in the nucleus as chromosomes Nucleus of a human cell contains more than 1 meter of DNA - HOW? ○ Eukaryotic chromosomes contain both DNA and protein wound closely together  DNA Replication The double helix structure of DNA explained how it was copied Because of base pairing, each strand has all the information needed to reconstruct the other half ○ Because of this, the strands are said to be complementary  Replicating DNA 1) DNA molecule unzips (separates into 2 strands) - the hydrogen bonds between base pairs are broken by HELICASE 2) Each separated strand serves as a template for the attachment of new complementary bases ○ DNA PRIMASE - directs polymerase ○ DNA POLYMERASE - builds the new strand by adding bases  Replicating DNA (continued) 3) Because DNA strands are antiparallel, and DNA polymerase can only work in the 5’ to 3’ direction, replication happens in pieces on what is known as the lagging strand These pieces must then be “glued” together using ligase 4) DNA Polymerase will proofread, and the result is 2 DNA strands identical to each other and the original molecule Each new DNA molecule has one original strand and one new strand - for this reason, we say that DNA replication is semi-conservative  DNA Replication: Prokaryotes vs. Eukaryotes Prokaryotes - Replication begins at a single point and proceeds in 2 directions until the entire chromosome is replicated. Eukaryotes - Replication occurs in hundreds of places. It proceeds in both directions until each chromosome is completely copied. The sites where replication begins are called replication forks.  Replication Practice Use the rules of base pairing to replicate the following DNA sequences: 1) TGCGGCATTAAG 2) AATGCCGTTAAT 3) CCGAAAGGTTC