Control of Gene Expression Pieces of Chapter 16 Pieces of Chapter 17 Pieces of Chapter 18

Objectives Understand the process of DNA replication Understand why DNA is synthesized from the 5’ end to the 3’end Recognize the function of telomeres Understand how protein structure and function are affected by genetic mistakes Be familiar with the kinds of mutations that may occur during replication of DNA Understand the role of an operon Be aware that gene expression can be regulated at many points from DNA to polypeptide synthesis

DNA Replication DNA replication is semiconservative in that each new molecule incorporates an old strand that serves as a template Requires many enzymes for assistance Few mistakes (~1/billion nucleotides)

Semiconservative Replication Early works tested several potential methods of replication Matthew Meselson and Frank Stahl determined that DNA replication was semiconservative

DNA Polymerase Enzymes called DNA polymerases are responsible for the assembly of DNA These enzymes convert nucleoside triphosphates into linked nucleotides through their action

Five principle proteins are used in the synthesis of DNA ****************

DNA Replication: The Process Origins of Replication: regions on the DNA where synthesis begins Synthesis occurs in both directions of the “bubble” along the replication fork (site of DNA elongation) 1.Helicase: responsible for unwinding the DNA –Topoisomerases prevent torque 2.Single-strand binding proteins: keep original complimentary strands separated

DNA Must Be Primed DNA Polymerase III is unable to replicate DNA directly and requires that the original DNA be primed 3.Primase makes the initial nucleotide (RNA primer) to which DNA polymerase attaches RNA primer is replaced with DNA nucleotides later by DNA Polymerase I

DNA Replication 4.Elongation of DNA is catalyzed by DNA Polymerase III and driven by the hydrolysis of phosphate groups from nucleoside triphosphates added to the 3’ end hydroxyl group of the growing molecule

DNA Strands are Antiparallel New DNA “grows” from 5’  3’ as DNA Polymerase III only adds nucleotides to the 3’ end of the DNA strand. Continuously synthesized piece is called the leading strand. Okazaki fragments, short pieces of discontinuously synthesized DNA, are formed and joined together by (5) DNA ligase to form the lagging strand of DNA

Leading Strand Synthesis

Lagging Strand Synthesis

Both Together

mm2WoWBshttp:// mm2WoWBs

Other things to consider DNA polymerase cannot synthesize the extreme ends of a DNA molecule Gradual shortening with each replication could lead to deletion of important information Telomerase adds many copies of TTAGGG nucleotide sequence (Telomere) to ends of DNA Telomerase is usually only found in germ cells and sex cells Presence in cancerous cells may lead to proliferation of tumors

Other things to consider Placement of mismatched nucleotides during synthesis is not rare and is repaired immediately by DNA Polymerase III. DNA polymerase I can repair “uncaught” mistakes through a mechanism called mismatch repair Excision repair takes place in DNA to repair damaged DNA (not related to replication) that could eventually lead to problems

Mutations: Changes in the genetic material of a cell Point mutations:chemical changes in just a single or a few base pairs in a gene –Base-pair substitutions: replacement of one nucleotide with another Silent Missense Nonsense –Insertion/Deletion: change in the number of nucleotide pairs Frame shift

Sickle Cell Disease

Controlled Expression: Operons Genes that are used together are often found associated (linked) on the same chromosome and may require a single promoter for transcription An Operator may regulate transcription by interaction with a repressor protein controlled through allosteric regulation An Operon is the entire stretch of DNA required for the synthesis of enzymes in a specific enzymatic pathway (Operator, promotor, and genes)

We have only scratched the surface of gene expression. Regulatory mechanisms may occur at many different stages from DNA to Protein -methylation: leads to inactivation of a gene -histone acetylation: ease of transcription -Transcription factors: enable transcription to occur -intron/exon regulation -modification of mRNA -degradation of mRNA: elimination is eminent -inhibition/activation of ribosome binding -processing of protein/assembly of subunits -ubiquitin enhances degradation