Chapter 16: The Molecular Basis of Inheritance GENETICS PDF

**Chapter 16** ============== THE MOLECULAR BASIS OF INHERITANCE ================================== Evidence that DNA is genetic material Mendel =\> heritable factors Morgan =\> genes on chromosomes \>\>\>\>\>\>The search for the genetic material **Griffith and Transformation (see fig. 16.2)**...

**Chapter 16** ============== THE MOLECULAR BASIS OF INHERITANCE ================================== Evidence that DNA is genetic material Mendel =\> heritable factors Morgan =\> genes on chromosomes \>\>\>\>\>\>The search for the genetic material **Griffith and Transformation (see fig. 16.2)** In 1928, Griffith working with *Streptococcus pneumoniae* demonstrated that genetic material could be transferred from one bacterium to another. -Rough cells (no capsule) are not pathogenic -Heat killed smooth cells are not pathogenic -Heat killed smooth cells plus rough cells are pathogenic AND PRODUCE LIVE SMOOTH CELLS! The trait from the smooth cells was transferred to the rough cells, and the rough cells were **transformed.** \-\--**Transformation** But what was transferred?? -Chromosomes are composed of DNA and protein. -Protein is denatured by heat. -Maybe it's the DNA. Many doubters. \>\>\>Evidence that viral DNA can program cells. More evidence that DNA is the genetic material came from the study of **bacteriophages (see fig 16.3).** \-\--**bacteriophage (a.k.a. phage)** -Bacteriophages quickly reprogram the bacterial cells they infect. In 1952, Hershey and Chase discovered that the genetic material from bacteriophage T2 that reprograms the bacteria was DNA (see fig 16.4). ^35^S-phage protein or ^32^P-phage DNA -Infected bacteria with the labeled phage. -Separated the phage from the bacteria and looked for which label was Concluded that the DNA was the genetic material in a virus. BUT WHAT ABOUT EUKARYOTES?? \>\>\>Additional evidence that DNA is the genetic material Circumstantial evidence -In a eukaryotic cell, the DNA content doubles prior to mitosis -During mitosis the double DNA is divided equally between the two -Diploid cells have twice the DNA content as haploid cells -DNA is found to be less uniform than original thought In 1947, Chargaff used paper chromatography to separate the bases of the DNA from different species -Found that the base composition varied between species -There was a relationship between the bases \>\>\>\>\>\>Watson and Crick (and others) discovered the double helix Remember the structure of the monomer (nucleotide) and the polymer (single strand DNA) (see fig 16.5) Based of the experiments of Hershey and Chase, along with the circumstantial evidence and the work of Chargaff, DNA was accepted as the genetic material. Understanding the structure of DNA -Physical structure evidence Chargaff's rules Size of the bases Lead to the **double helix** model (see fig 16.7) -2 strands are antiparallel =\> A-T and G-C pairs, explained Chargaff\'s rules -10 bases pairs/turn **Replication: Meselson and Stahl** \>\>\>\>\>\>DNA replicates by using itself as a template (see fig 16.10). The double helix model with it's base pairing provides mechanism for replication =\> each strand can serve as a template This type of replication follows the **semiconservative model** (see fig. 16.11) old DNA and a strand of new DNA. 2 alternatives were possible -dispersed model =\> mixture of old and new in each strand Meselson and Stahl provided experimental support for the semiconservative model (see fig 16.12) -Labeled DNA with ^15^N (makes DNA denser) -DNA replication in regular ^14^N media -Both DNA double helixes have the same density -Second round of replication in regular ^14^N media -Two different densities of DNA -Can't be Dispersive Model **DNA: Polymerization with Triphosphate Nucleotides** \>\>\>\>\>\>The replication of DNA The process of DNA replication is conceptually easy, but reality of the process very complex. \>\>\>Where it starts The **origin(s) of replication (see fig 16.13)** -In prokaryotes there is only one, also only one circular chromosome. -In eukaryotes there are many per chromosome At each origin of replication the DNA separates to from a replication "bubble". At each end of that replication "bubble" is a **replication fork** where the new strands are being synthesized. \-\--**Replication forks** \>\>\>Elongation of the strand Two processes: **Synthesis of the new strands** strand separation (see fig 16.14) -enzymes called helicases unwind the helix -topoisomerase relieves the unwind strain -strands are kept apart by proteins synthesis of the new strands (see fig 16.15) -**DNA polymerase** -synthesis occurs in 5\' =\> 3\' direction -energy for the synthesis comes from the high-energy phosphate Because of the bidirectional nature of the replication one strand is synthesized in a continuous manner (**leading strand**) the opposing strand (**Lagging strand**) is synthesized in pieces (**Okazaki fragments**) \-\--**Leading strand** \-\--**Lagging strand** \-\--**Okazaki fragments** The Okazaki fragments of the lagging strand are joined together by another enzyme called **DNA ligase** \>\>\>Priming the reaction Because DNA polymerase can only add nucleotides on to a polymer, there must be a **primer** to start the reaction. \-\--**Primer** The leading strand only has to be primed once at each replication bubble, after that synthesis is continuous. The lagging strand requires many primers. The synthesis of theses primers is accomplished by another enzyme called **primase**