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 D eoxyribo n ucleic A cid 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

Double helix like a twisted ladder made of subunits called nucleotide. Has three parts: – deoxyribose (sugar) – phosphate group – nitrogen base 4 different bases: adenineguanine cytosinethymine 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

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.