Molecular Basis of Inheritance

Chapter 16

DNA as Genetic Material

• Watson and Crick – 1st scientists to propose structure of DNA - responsible for transmission of traits from 1 generation to next.

• Requires precise transmission occurs.

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

• 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.

http://www.virtuallaboratory.net/Biofundamentals/lectureNotes/AllGraphics/Griffith.jpg

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

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

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

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

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

• Viruses that specifically attack bacteria - bacteriophages (phages)

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

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

• Hershey and Chase concluded that injected DNA of phage provides genetic information that makes infected cells produce new viral DNA and proteins, which assemble into new viruses.

• DNA doubles prior to mitosis.

• By 1940’s - discovered that DNA was made of bases.

• Adenine, thymine, cytosine, and guanine.

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

http://fig.cox.miami.edu/~cmallery/150/gene/BasePairing.gif

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

• Watson 1st to figure out that DNA was 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 that purine has to pair with pyrimidine to achieve distance needed.

• Knew that adenine always paired with thymine and cytosine always with guanine.

• Discovered that bases were held together by hydrogen bonds.

• Adenine forms 2 hydrogen bonds

only with thymine; guanine forms 3 hydrogen bonds only with cytosine.

• 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

DNA Replication and Repair

• Watson and Crick discovered each strand of DNA can make 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.

• Happens by semiconservative replication - each DNA molecule has one parent strand and one daughter strand.

• Idea later supported through bacteria studies.

• This

• 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 process begins.

• Bacteria - 1 site - replication looks like 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 DNA polymerase.

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

• 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 DNA 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.• Primase links ribonucleotides

that are complementary to DNA template into primer.

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

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

• Later - different DNA polymerase replaces original RNA with new complementary DNA nucleotides - there is no RNA left in strand.

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

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

• At 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 and UV rays can alter DNA after replication.

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

• Mismatch 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

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

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

• Disease prevents person from going in sun because UV rays interfere with DNA replication.

• More susceptible to skin cancer because they cannot fix mistakes.

http://162.129.70.33/images/xeroderma_pigmentosa_2_040620.jpg

• Limitations in DNA polymerase create problems for linear DNA of eukaryotic chromosomes.

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

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

Fig. 16.18

• Ends of eukaryotic 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 not present in all cells - life span to certain tissues or organism itself.

• Important for telomerase to be present 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.