Control of Gene Expression

1. Control of Gene Expression Pieces of Chapter 16 Pieces of Chapter 17 Pieces of Chapter 18 Pieces of Chapter 19

2. 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

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

4. DNA Replication • 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)

5. DNA Replication • Elongation of DNA • Catalyzed by DNA Polymerase and driven by the hydrolysis of phosphate groups from nucleosides (a nucleoside is a nucleotide with three phosphate groups)

6. DNA Strands are Antiparallel • New DNA grows from 5’3’ as DNA Polymerase only adds nucleotides to the 3’ end of the DNA strand (leading strand) • Okazaki fragments, short pieces of synthesized DNA, are formed and joined together by DNA ligase to form the lagging strand of DNA

7. DNA must be Primed • DNA Polymerase is unable to replicate DNA directly and requires that the original DNA be primed • Primase makes the initial nucleotide (RNA primer) to which DNA polymerase attaches • RNA primer is replaced with DNA nucleotides later

8. Protein Summary • Additional to Primase, DNA Polymerase and ligase proteins are 2 others • Helicase: responsible for unwinding the DNA • Single-strand binding proteins: keep original complimentary strands separated

10. Other things to consider • Placement of mismatched nucleotides during synthesis is not rare and is repaired by DNA Polymerase 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

11. Other things to consider • RNA 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

12. 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

13. 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)

14. 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