Discovery of DNA 1850’s Gregor Mendel discovered hereditary “factors”, but did not know about genes or chromosomes or DNA. 1928…Frederick Griffith showed genes were responsible for heredity DNA from dead bacteria was picked up by live bacteria (transformation) and killed mice

1928…Griffiths

Discovery of DNA 1944…Oswald Avery showed DNA was responsible for heredity. Repeated experiment, but destroyed molecules trying to find what it was that caused the transformation…what caused the dead bacteria to kill the mice Destroyed proteins, lipids, carbohydrates, RNA and still transformation occurred, so must be DNA Did again and destroyed DNA, no transformation occurred

Discovery of DNA 1952…Alfred Hershey and Martha Chase backed up Avery’s experiments Used bacteriophages viruses that attack bacteria Has a protein coat and DNA core

1952 Hershey and Chase Used sulfur and phosphorus … proteins have sulfur, DNA has phosphorus Found phosphorus so must be DNA

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 X-ray of DNA molecule 1953 James Watson and Francis Crick announced double helix shape (based on RF’s Xray).

DNA Function in cell DNA represents a code That code will be copied (transcription) The copy will leave the nucleus The copy will be decoded to make a protein (translation) The protein will run some function in the cell or body That’s how DNA in the nucleus is the ‘control center’ of the cell

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 nucleotides. Has three parts: deoxyribose (sugar) phosphate group nitrogen base 4 different bases: adenine guanine cytosine thymine Bases match each other (complementary), A matches with T, G matches with C Order of bases called the base sequence

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…Chargaff’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) by DNA ligase DNA polymerase continues until whole strand is copied. Since two new strands have 1 original and 1 new, it’s called semi-conservative 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.

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 There are also mutations to chromosomes, but we’ll talk about all of this in the next unit.