DNA & RNA

Before We Knew about DNA Gregor Mendel – the “father of genetics” was a monk who, in the 1800’s, was the first person to hypothesize “factors” which were passed down, or inherited, from generation to generation.

Before We Knew about DNA But what were these “factors”? Many scientists were on the case. In the first half of the 1900’s, scientists Griffith, Avery, Hershey and Chase all conducted experiments contributing to the isolation of the DNA molecule.

Before We Knew about DNA In 1950, Erwin Chargaff conducted chemical experiments and concluded there was the same amount of guanine as of cytosine in DNA. In addition, there is the same amount of adenine as thymine in DNA. Thus establishing the pairing of the nitrogenous bases. This became known as Chargaff’s rule, and from this we know that: adenine always pairs with thymine, and that cytosine always pairs with guanine A=T and C=G

Before We Knew about DNA In the early 1950’s, Rosalind Franklin worked with X-ray photography to take the famous photo, showing the structure of DNA for the first time. Sadly, she died of radiation poisoning

Before We Knew about DNA Then in 1953, James Watson and Francis Crick took the work of all these scientists before them and built a model of the DNA molecule. Think “Whats in the Crick?”

Discovering DNA So how long have we known about DNA? If you count from Watson and Crick, only a little over a half century…. But Mendel had a pretty good idea long before that.

The Structure of DNA DNA: deoxyribonucleic acid Genetic material that contains the instructions for the functions of the cell Found in the nucleus of eukaryotic cells In prokaryotes, shaped as a ring DNA is a very long, thin molecule Shape is double helix – like a twisted ladder

Structure of DNA “railing” or “backbone” of ladder consists of 2 alternating parts : sugars, deoxyribose, and phosphate molecules Steps, or “rungs” of ladder consist of pairs of nitrogen bases: Adenine & Thymine Guanine & Cytosine

Structure of DNA Each nitrogen base “step” is connected to the railing at the sugar molecule Each sugar molecule is connected to its own phosphate molecule One nitrogen base, together with its sugar and phosphate backbone make one unit called a nucleotide Nucleotides are made up of three components: 1. deoxyribose (sugar) 2. phosphate 3. nitrogenous base

DNA structure Nitrogen bases: 1. Purines: molecules whose shape is 2 ringed -Adenine and Guanine 2. Pyrimidines: 1 ring -Thymine and Cytosine Adenine only bonds with thymine and guanine only bonds with cytosine Remember Chargaff’s rule? A=T and C=G

DNA structure This bonding is called base pairing The nitrogen bases are held together with loose hydrogen bonds These are usually shown as dotted lines

DNA Structure There are millions of “steps” in each of our 46 DNA molecules, in each of our cells’ nuclei. If you unraveled one single DNA molecule all the way and stretched it out, It would be about 5 cm long – that’s about 2 inches! How does all that DNA fit inside a nucleus?

DNA Structure Histones are proteins that the DNA molecule wraps itself around to make a smaller compact unit to fit in the nucleus.

DNA Replication p 334 DNA must replicate before cell can reproduce. Each new copy must be exactly the same as the original

DNA Replication – Big Picture Two complete strands will result, these will be exactly alike Each resulting strand has one “old” half and one “new” half

DNA Replication p 334 First step: DNA unwinds and enzymes “unzip” DNA into two separate strands by breaking the hydrogen bonds, exposing unpaired nucleotides

DNA Replication – p 334 Because nitrogen bases can only match in specific pairs, this allows exact copies to be made from each open half

DNA Replication Second step: free nucleotides join onto open strands to form two new strands of DNA, aided by enzymes called DNA polymerese. DNA polymerese: an enzyme that helps new nucleotides bond onto open strand of DNA during replication.

DNA Replication When DNA replicates, each strand works in opposite directions, from 5 prime to 3 prime ends.

The leading edge replicates in one smooth motion. The lagging strand adds nucleotides in pieces, working toward open end.

DNA Replication The segments produced on the lagging end are called Okazaki segments DNA ligase helps connect the Okazaki segments

RNA RNA: ribonucleic acid Nucleic acid which carries out protein synthesis Differences from DNA: different sugar (ribose), single strand, different base (no thymine, uracil instead)