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Discovery of DNA. 1850’s Gregor Mendel discovered hereditary “factors”, but did not know that DNA was what genes were made of . 1928…Frederick Griffith showed genes were responsible for heredity 1944…Oswald Avery showed DNA was responsible for heredity. - PowerPoint PPT Presentation
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Discovery of DNA
• 1850’s Gregor Mendel discovered hereditary “factors”, but did not know
that DNA was what genes were made of.
• 1928…Frederick Griffith showed genes were responsible for heredity
• 1944…Oswald Avery showed DNA was responsible for heredity.
• 1952…Alfred Hershey and Martha Chase backed up Avery’s experiments
Discovery of DNA
• 1949 Edwin Chargaff noticed that A&T and G&C appear almost
equally in DNA, no matter what the organism!
• Early 1950’s…Rosalind Franklin took first Xray of DNA molecule
• 1953 James Watson and Francis Crick announced
double helix shape (based on RF’s Xray).
DNA Structure
• Deoxyribonucleic Acid• Makes up chromosomes• Chromosomes have small areas
called genes• Genes are a section of DNA that is a
code for a protein that does some function in your body (more about this in next unit)
DNA Structure
DNA Structure• Double helix like a twisted ladder made of
subunits called nucleotide. Has three parts:– deoxyribose (sugar)– phosphate group – nitrogen base
• 4 different bases:adenine guaninecytosine thymine
• Bases match each other (complementary), A matches with T, G matches with C
• Order of bases called the base sequence
• (DNA overview)
DNA Replication
• DNA Copies itself during the S phase of the cell cycle, so daughter cells have complete copy of all genes.
• DNA Helicase (an enzyme) “unzips” the DNA molecule by breaking the hydrogen (weak) bonds between nitrogen bases.
• Area where “unzips” is called replication fork. • DNA polymerase (another enzyme) matches
base pairs with their complement (A with T, G with C…Chagraff’s Rule) on both strands at once. The base pairs are already in the nucleus.
DNA Replication
DNA Replication
• Because one strand is upside down (antiparallel), one strand is copied as one whole piece (leading strand) and one is copied in chunks (Okazaki fragments) and pieced together (lagging strand)
• DNA polymerase continues until whole strand is copied. Since two new strands have 1 original and 1 new, it’s called semi-conservative replication.
• (DNA Replication)
Prokaryotic DNA
• In Prokaryotes (bacteria), DNA is a ring and replication starts at one point and goes around both ways until it’s done.
• Thus there will be only two replication forks.
• In eukaryotes, replication will start in many places, so there are multiple replication forks.
• (Replication Forks)
Mistakes
• Mistakes in copying DNA can be (and often are) made, but are usually caught by DNA polymerase and fixed.
• If not fixed, called mutation, and the base sequence changes.
• Sometimes this is good and leads to a new adaptation (evolution)
• Sometimes its bad (cancer, other genetic disorders)
Mistakes
• (Mutation)• There are also mutations to chromosomes, but
we’ll talk about all of this in the next unit.