CP Bio Chapter 12-13 DNA Function

CP Bio Chapter 12-13 DNA Function

History: Learning about DNA • Frederick Griffith 1928 • Tried to make a vaccine for pneumonia • Used mice and two strains of bacteria • - one harmless (“R type”) • - one caused pneumonia (“S type”) • Live R alone and dead S alone did not cause immune response • Mixed live R with dead S •  mice got sick and DIED

How did harmless bacteria turn into deadly? Live R (‘rough’) – no disease Live S (‘smooth’) – pneumonia Mix Live R and dead S - pneumonia

Griffith’s conclusion • Something from dead S cells transformed living R cells into living S cells

Avery, McLeod, and McCartey 1944 • What cellular substance changed Griffith’s bacterial cells from harmless to deadly? • Used a series of enzymes on bacterial cultures • - destroyed specific molecules in the cells • - carbs, proteins, lipids, DNA • Found that when DNA was destroyed, bacterial cells did NOT change • CONCLUSION: DNA is the substance that can change bacterial cells

The Hershey-Chase experiment 1952 • Background: Viruses enter cells and change them - make cells produce copies of virus. • Problem: Which part of virus enters cells? Is it the protein coat? Or the DNA? • Experiment: Grow bacteria, add phage virus tagged with radioactive isotope • - use sulfur  radioactivity in proteins (capsid) • - use phosphorus  radioactivity in DNA • Which enters bacterial cells?

Hershey-Chase Experiment Grow bacteria in culture with tagged phage. Virus infects bacteria Blender shakes phage loose from bacterial cells. Is radioactivity in the liquid (virus), or in the cells (bacteria)? Centrifuge separates cells from culture liquid

Which part enters cells? • When phage was tagged with phosphorus  bacterial cells became radioactive • Conclusion: DNA entered cells, but protein did not

Finding the structure of DNA Erwin Chargaff 1950 - base-pairing rules • Franklin & Wilkins 1952 • - X-ray pictures of DNA crystals • - showed double helix shape Watson & Crick 1953 discovered structure of DNA

DNA is a double helix • Two strands, held together by hydrogen bonds • Between complementary bases • four nitrogen bases: adenine, thymine, cytosine, guanine • Deoxyribose sugar

DNA replication: • For cell division • Starts in several places at once • Each original strand is template for a new strand • Proceeds until entire strands are duplicated • Copies stay together at centromere

Ch. 13 DNA Function – “Gene Expression” Flow of genetic information: DNA to RNA to protein • DNA (genotype) codes for proteins • Proteins make the phenotype • A gene is one section on a DNA molecule • Has instructions to make one polypeptide • CODE is the sequence of DNA bases

RNA • ONE strand • Ribose sugar • Uracil base (no thymine)

Three kinds of RNA Messenger RNA (mRNA) – carries code from DNA in nucleus to ribosome Ribosomal RNA (rRNA) – makes up ribosome, along with proteins Transfer RNA (tRNA) – carries one amino acid to ribosome and matches to mRNA code

Two stages in protein synthesis DNA Transcription RNA Translation Protein Figure 10.6A • The DNA of the gene is transcribed into RNA • The gene is translated into a polypeptide

RNA nucleotides RNA polymerase A C C A T T A U T C T G U G A C A U C C A C C A G A T T T A G G Direction of transcription Template Strand of DNA Newly made RNA Stage 1: Transcription – in nucleus • mRNA synthesis - writes the gene onto a messenger molecule Copy ONE side of DNA DNA unzips mRNA leaves nucleus

Transcription – writes DNA code onto mRNA

1. In the nucleus, the DNA unzips 2. RNA nucleotides line up along one strand of the DNA, follow base pairing rules 3. Messenger RNA (mRNA) is single strand, detaches from DNA 4. leaves the nucleus

RNA is processed before leaving the nucleus • Noncoding segments called introns are edited out • Coding segments called exons are spliced together • A cap and tail are added to the ends

Stage 2: Translation- in ribosome • 13.2 How is the genetic code read? • The “words” of the DNA “language” are written in sets of three bases – codons • Codons spell the amino acid sequence - primary structure of a protein

Strand to be transcribed T A C T T C A A A A T C DNA A T G A A G T T T T A G Transcription G U U U A G A U A A G U RNA Startcondon Stopcondon Translation Met Polypeptide Lys Phe Translating the genetic code

All organisms use the same code UUG

How RNA helps • 1. Ribosome attaches to mRNA • 2. Transfer RNA (tRNA) brings amino acids to ribosome

3. Begins at “start” codon 4. Amino acids are set in sequence, according to the code on mRNA 5. Ends at “stop” codon

Genetic information: DNA to RNA to protein Sequence of codons  primary structure of protein

Mutations can change the meaning of genes • change in the DNA base sequence • Errors in replication or by mutagens

Ch. 13- What turns genes ON and OFF? • In bacteria, control genes are next to code genes • Lac operon – gene is OFF when lactose absent • - ON when lactose present Repressor protein on DNA blocks RNA polymerase - Lactose removes repressor  transcription

Gene Control in Eukaryotes • Control genes are NOT near code genes • Many proteins interact to help mRNA form • Transcription Factors • Control genes may be on different chromosomes from coding genes

CP Bio Chapter 12-13 DNA Function