• Watson and Crick – 1st to propose structure of DNA.

• Requires precise transmission during replication.

http://www.ncbe.reading.ac.uk/DNA50/Resources/wc1993.gif

• Prior to Watson, Griffith tested transmission.

• Griffith - injected live bacterial strains into mice.

• Mixed R strain of bacteria (harmless) with heat-killed S strain (harmful) and injected it.

• After mouse died, removed strain from mouse.

• Substance eventually found to be DNA - supported by injecting bacteria into viruses.

• Viruses consist of DNA (sometimes RNA) enclosed by protective coat of protein.

• To replicate - virus infects host cell; takes over cell’s metabolic machinery.

• Viruses that specifically attack bacteria - bacteriophages (phages)

http://www.monografias.com/trabajos5/virus/Image164.gif

• Transformation - change in genotype and phenotype due to assimilation of foreign substance (now DNA) by cell.

http://www.swbic.org/products/clipart/images/bacteriophage.jpg

• Hershey and Chase labeled protein and DNA - injected them into bacteria.

• Hershey and Chase concluded that DNA, not protein, is responsible for transmission.

• DNA doubles prior to mitosis.

• 1940’s - DNA made of bases (adenine, thymine, cytosine, guanine)

• Also known that sugar of one nucleotide attached to phosphate of another - forms backbone of DNA.

• Chargaff’s rules - even amount of thymine and adenine. (and guanine and cytosine)

• Watson 1st to figure DNA in helix shape + specific distance between nucleotides.

• Partnered with Crick – came up with double stranded model of DNA - double helix.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 16.5

• Found purine (A, G) has to pair with pyrimidine (T, C) to achieve distance needed.

• Knew A - T (2 H bonds), C - G (3 H bonds

• Each gene found to have unique sequence of nitrogen bases - DNA strands not all the same.

http://academy.d20.co.edu/kadets/lundberg/dna_wallpaper/dna800x600.jpg

• Each strand of DNA can be template to make more DNA.

• Cell copies DNA - each strand forms as template to determine new complementary bases.

• Nucleotides pair in complementary fashion, one by one.

• Semiconservative replication - each DNA molecule has one parent strand and one daughter strand.

• Even though process is amazingly quick, only about 1 in a billion nucleotides copied wrong.

• Proteins and enzymes also part of process, not just nucleotides.

http://www.bio.miami.edu/dana/250/nucleotides.jpg

• Origins of replication - where replication begins.

• Bacteria - 1 site - replication is bubble moving along DNA.

• Eukaryotes - many origins of replication on each chromosome.

• Origin sites - DNA strands separate forming replication “bubble” with replication forks at each end.

• Elongation of DNA catalyzed by 1DNA polymerase.

• Polymerase adds complementary bases to growing strand of new DNA.

• 2Helicase - untwists double helix of DNA at replication fork.

• 3Single-strand binding proteins help keep strands apart from one another during replication.

• Strands of DNA - antiparallel.• Sugar-phosphate backbones

run in opposite directions.• Each end of strand labeled

either 5’ end or 3’ end.

• Nucleotides only be added to 3’ end.

• DNA strand can only elongate from 5’ end to 3’ end.

• Replication fork - problem - system because strands run in opposite directions (antiparallel)

http://www.mie.utoronto.ca/labs/lcdlab/biopic/fig/11.16.jpg

• 1 parent strand (leading strand - one that runs 3’ to 5’) used as template to keep complementary strand continuous.

• Other strand (lagging strand - one that runs 5’ to 3’) copied from fork in small segments - Okazaki fragments.

http://www.biology.arizona.edu/molecular_bio/problem_sets/nucleic_acids/graphics/repfork1.gif

• Fragments “glued” together by 4DNA ligase to form backbone (made of sugar and phosphate) of single DNA strand.

• Polymerase adds nucleotides to strands, cannot start whole process.

• Done by a piece of RNA - primer.

http://www.biologie.uni-hamburg.de/b-online/library/bio201/primase.jpg

• Once primer formed, polymerase adds DNA nucleotides to growing daughter strand of DNA.

• After, 5DNA polymerase (different) replaces original RNA with new complementary DNA nucleotides - no RNA left in strand.

• Replication fork, leading strand copied continuously into fork from single primer.

•Lagging strand copied away from fork in short segments, each requiring new primer.

• Original errors in reading of template occur.

• Enzyme (DNA polymerase) removes mistake and replaces it.

• Some things can alter DNA outside of body.

http://library.thinkquest.org/C0123260/basic%20knowledge/images/basic%20knowledge/DNA/polymerase%201.jpg

• X-rays, UV rays can alter DNA after replication.

• Mistakes can be fixed after DNA synthesis - cell continually monitors DNA.

• 1Mismatch repair - special enzymes fix incorrectly paired nucleotides - happens in certain types of cancers.

http://www.sinauer.com/cooper4e/sample/Figures/Chapter%2006/highres/CELL4e-Fig-06-24-0.jpg

• 2Nucleotide excision repair -nuclease cuts out segment of damaged strand.

• Xeroderma pigmentosa (genetic disease) cannot go through process.

• Disease prevents person from going in sun - UV rays interfere with DNA replication (more susceptible to skin cancer - can’t fix mistakes)

http://162.129.70.33/images/xeroderma_pigmentosa_2_040620.jpg

• Ends of DNA strand can break down from constant replication.

• Ends of chromosomal DNA molecules – telomeres - special nucleotide sequences.

• Telomeres protect genes from being eroded through multiple rounds of DNA replication.

• When telomeres shorten, telomerase uses piece of RNA to lengthen telomere.

• Telomerase has life span to certain tissues or organism.

• Important for telomerase to be in gamete cells so they can pass long telomeres on to zygote.

• Active telomerase in body cells can be responsible for cancer cells because cells keep dividing.

• Proteins - link between genotype (what DNA says) and phenotype (physical expression)

• Beadle and Tatum – 1st to make connection between genes and enzymes that carry out genes (bread mold experiments)

http://fig.cox.miami.edu/~cmallery/150/gene/17x3.jpg

• Bridge between DNA, proteins - RNA.

• RNA similar to DNA - sugar ribose; contains uracil instead of thymine.

• RNA single-stranded.

http://gibk26.bse.kyutech.ac.jp/jouhou/image/nucleic/rna/rna_bb_st.gif

• Nucleotides found in DNA and RNA - code - determines order of amino acids.

• 2 steps - transcription and translation.

• Transcription - DNA serves as template for complementary RNA strand.

http://www.ktf-split.hr/glossary/image/nucleotide.gif

• Transcription produces mRNA strand (messenger RNA).

• Translation uses mRNA sequence to determine order of amino acids - creates polypeptide.

http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/graphics/Transcription.02.GIF

• Bacteria - transcription and translation occur at once.

• Eukaryotes, most transcription occurs in nucleus, translation occurs at ribosome.

• Before primary transcript can leave nucleus - modified during RNA processing before enters cytoplasm.

• Genetic code - triplet code - series 3 nitrogen bases that code for specific amino acid.

• 64 possible combinations of nitrogen bases.

• Only 20 amino acids = each amino acid has more than 1 code.

http://www.dls.ym.edu.tw/lesson/gen.files/codon.jpg

• 61 of 64 codes specific to an amino acid.

• Other 3 - stop codons - determine when process stops.

• Specific code that signals start of translation - also codes for amino acid.

• Start begins correct reading frame of polypeptide.

• Transcription, 1 DNA strand - template strand, provides template for ordering sequence of nucleotides in RNA transcript.

• Translation, blocks of 3 nucleotides, codons, decoded into sequence of amino acids.

• Possible to take genetic code of 1 organism, place it into another - nearly universal.

• Firefly gene for luminescence transplanted into tobacco plant.

• Bacteria can be inserted with specific genes to synthesize genes in large amounts.

Synthesis and Processing of RNA

• mRNA transcribed from template of original gene.

• RNA polymerase separates DNA strands and bonds RNA bases along complementary strand.

• Bases can only be added to 3’ end.

http://www.csu.edu.au/faculty/health/biomed/subjects/molbol/images/7_9.jpg

• Specific sequences determine where transcription starts and where it ends.

• Promoter sequence – initiates; terminator ends.

• Presence of promotor determines which strand of DNA helix is template.

• Proteins (transcription factors) recognize promotor region (TATA box) and bind to promotor.

http://www.nslij-genetics.org/pic/promoter.gif

• After binding, RNA polymerase binds to transcription factors.

• RNA polymerase starts transcription.

• RNA polymerase moves along - nucleotides added to 3’ end.

• Single gene can be transcribed simultaneously by several RNA polymerases at a time.

• Growing strand of RNA trails off from each polymerase.

• RNA splicing - removal of large portion of RNA molecule because most eukaryotic genes and RNA transcripts have long noncoding (introns) stretches of nucleotides between coding regions (exons)

http://ghs.gresham.k12.or.us/science/ps/sci/ibbio/chem/nucleic/chpt15/introndeletion.gif

• RNA splicing removes introns, joins exons to create mRNA molecule with continuous coding sequence.

• Splicing done by spliceosome.

• Translation - cell interprets codons along mRNA molecule.

• Transfer RNA (tRNA) transfers amino acids from cytoplasm’s pool to ribosome.

• Ribosome adds each amino acid carried by tRNA to growing end of polypeptide chain.

• tRNA links mRNA codon with amino acid.

• tRNA arriving at ribosome carries specific amino acid at 1 end, has specific nucleotide triplet, anticodon, at other.

• Anticodon base-pairs with complementary codon on mRNA.

http://bioweb.uwlax.edu/GenWeb/Molecular/Theory/Translation/ribosome.jpg

• tRNA synthesized like other forms of RNA.

• Once in cytoplasm, each tRNA used repeatedly to pick up and drop off that amino acid.

• Anticodons recognize more than one codon.

• Rules for base pairing between 3rd base of codon and anticodon relaxed (wobble).

http://www.geneticengineering.org/chemis/Chemis-NucleicAcid/Graphics/tRNA.jpg

• Each ribosome has 3 binding sites for tRNA molecules.

•P site holds tRNA carrying growing polypeptide chain.

•A site carries tRNA with next amino acid.

• Discharged tRNAs leave ribosome at E site.

http://nobelprize.org/educational_games/medicine/dna/a/translation/pics/translation2.gif

• 1Initiation brings together mRNA, tRNA with 1st amino acid.

• 2Elongation - each amino acid added to previous one.

• 3 steps of elongation continue codon by codon to add amino acids until polypeptide chain completed.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 17.18

• 3Termination - 1 of 3 stop codons reaches A site.

• Release factor binds to stop codon, breaks bond between polypeptide and tRNA in P site - frees polypeptide.

• 2 types of ribosomes active in process.

• 1Free ribosomes suspended in cytosol synthesize proteins in cytosol.

• 2Bound ribosomes attached to endoplasmic reticulum.

Fig. 17.21

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Bacteria and eukaryotes have differences in details of processes.

• Eukaryotic RNA polymerases differ from prokaryotic; require transcription factors.

• Differ in how transcription terminated.

• Ribosomes also different. • Prokaryotes can transcribe and

translate same gene simultaneously.

• Mutations - changes in genetic material of cell (or virus).

• Chemical change in 1 base pair of gene causes point mutation.

• Occur in gametes or cells producing gametes - may be transmitted to future generations.

http://staff.jccc.net/PDECELL/evolution/mutations/mutypes.gif

• If it results in replacement of pair of complementary nucleotides with another nucleotide pair - base-pair substitution.

• Can have little or no impact on protein function (silent mutations).

http://fajerpc.magnet.fsu.edu/Education/2010/Lectures/26_DNA_Transcription_files/image008.jpg

• Missense mutations - code for different amino acid.

• Nonsense mutations - code for “stop” - leads to malfunctioning protein.

Fig. 17.24

Copyright © Pearson Education, Inc., publishing as Benjamin Cummings

• Insertions and deletions - additions or losses of nucleotide pairs in gene.

• Unless these mutations occur in multiples of 3 - cause frameshift mutation.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 17.24

• Mutations can occur in many ways - during DNA replication, DNA repair, or DNA recombination.

• Mutagens - chemical or physical agents that interact with DNA to cause mutations (high-energy radiation - X-rays UV light).