DNA: Chapter 12 October 2014
Goals for the Unit Iden>fy the substance of Genes Explain DNA Structure Sequence and explain the steps of DNA Replica>on
Iden>fying Substance of Genes In 1928, Frederick Griffith tried to develop a vaccine against Streptococcus pneumoniae (causes pneumonia) He discovered that S strain (smooth) produced disease and R strain (rough) did not He conducted experiments to determine what hereditary material could be passed on to new cells and cause a safe strain to become harmful
Ini>al bacteria: Griffith s Experiment Experiment part 1:
Following experiments:
Avery s Experiment Oswald Avery (1940s) wanted to determine if Griffith s hereditary material was a protein, carbohydrate, lipid, or nucleic acid Used protease (RNAase & DNAase) Enzymes- breakdown protein, RNA & DNA
Avery s Experiment con>nued Found that cells without protein & RNA were s>ll able to transform safe bacteria strain into harmful bacteria Cells without DNA could not do this. DNA was responsible for change
Hershey-Chase Experiment Martha Chase & Alfred Hershey (1952) Set out to test Avery s experiment- - whether DNA or protein was the hereditary material viruses transfer when entering bacteria. Concluded that DNA is the hereditary molecule in viruses.
Hershey and Chase grew viruses in cultures containing radioac>ve isotopes of phosphorus- 32 (P- 32) sulfur- 35 (S- 35)
If they found radioac>vity from S- 35 in the bacteria, it would mean that the virus s protein coat had been injected into the bacteria. If they found P- 32 then the DNA core had been injected.
Nearly all the radioac>vity in the bacteria was from phosphorus P- 32, the marker found in DNA. Hershey and Chase concluded that the gene>c material of the bacteriophage was DNA, not protein.
Role of DNA Storing gene>c informa>on Copying gene>c informa>on Transmiang gene>c informa>on Same as how you share a book
Storing Genes control paberns of development - instruc>ons that cause a single cell to develop into an oak tree, a sea urchin, or a dog must somehow be wriben into the DNA of each of these organisms.
Copying Before a cell divides, it must make a complete copy of every one of its genes, similar to the way that a book is copied To many scien>sts, the most puzzling aspect of DNA was how it could be copied Once the structure of the DNA molecule was discovered, a copying mechanism for the gene>c material was soon put forward.
The loss of any DNA during meiosis might mean a loss of valuable gene>c informa>on from one genera>on to the next. Transmiang When a cell divides, each daughter cell must receive a complete copy of the gene>c informa>on. Careful sor>ng is especially important during the forma>on of reproduc>ve cells in meiosis.
Structure of DNA
What is DNA? Deoxyribonucleic acid Located in the nucleus of cells It is a nucleic acid made up of nucleo>des (monomers) Basic components Sugar- Deoxyribose Phosphate Group Nitrogen- containing bases (4) Adenine & Guanine (purines) Thymine & Cytosine (pyrimidines)
DNA Structure 1953: James Watson & Francis Crick described the structure of DNA Watson and Crick relied on work of Rosalind Franklin & Maurice Wilkins: photographs of DNA Double Helix (spiral staircase) Rungs = nitrogenous bases with hydrogen bonds Sides of ladder = sugar and phosphate backbone Each strand is direc>onal (5 and 3 end) Within a double helix, the strands run an>parallel (in opposite direc>ons)
Bonding of DNA Hydrogen bonds hold the nitrogen- containing bases together ( steps ) Complementary base pairing (Chargaff s Rules) Adenine & Thymine bond together Cytosine & Guanine bond together Hydrogen bonds hold the 2 ladder sides together Phosphodiester bonds hold the phosphate groups and the deoxyribose sugars together ( handrails of the ladder )
Chromosomes and DNA Replica>on Length of DNA: The nucleus of a human cell has >1 meter of DNA How does this much fit? Chromosomes- Compacted DNA which is coiled around proteins
Bell Work: 10/16/13 Fill in the Chart Base A C G T % of Total DNA 22
DNA Replica>on Copying the Code
What is DNA replica>on? Process by which DNA is copied in a cell before a cell divides by mitosis, meiosis or binary fission (bacteria) Semi- conserva<ve replica<on Each new DNA molecule keeps (conserves) one original strand and gains one new strand
Steps of DNA Replica>on 1. Separa>on of strands 2. Addi>on of complementary nucleo>des 3. Two iden>cal molecules are a result
Step 1 DNA molecule unwinds & unzips Helicase (enzyme) separates DNA strands by breaking the hydrogen bonds Replica>on fork forms Y- shaped region that results when the 2 strands separate
Replica>on Fork
Step 2 DNA polymerase (enzyme) adds complementary nucleo>des that are already in the nucleus Hydrogen bonds form between the nitrogenous bases Covalent bonds (phosphodiester bonds) bond the sugar and phosphate groups together Replica>on occurs in two different direc>ons
DNA polymerase ac>on
Step 3 DNA polymerase finishes adding nucleo>des and leaves the DNA molecule Two separate and idenacal DNA molecules are produced that are ready to move to new cells in cell division
Errors in DNA Replica>on Replica>on is usually a very accurate process About 1 error for each 1,000,000,000 base pairs added Repair enzymes that proofread Muta>ons can occur MutaAon: change in the nucleo>de sequence of a DNA molecule
Prokaryo>c vs. Eukaryo>c Replica>on Prokaryo>c cells: starts from a single point and proceeds in two direc>ons un>l the en>re chromosome is copied. Eukaryo>c cells: replica>on can begin at hundreds of places on the DNA molecule, proceeding in both direc>ons un>l each chromosome is completely copied.
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