DNA STRUCTURE

HISTORY OF HEREDITARY MATERIAL Discovery of Nucleic Acids - Friedrich Miescher, 1869 Proteins Produce Genetic Traits - Archibald Garrod, 1909 Genetic Material can Transform Bacteria - Frederick Griffith, 1931 (Campbell animation)

DISCOVERY OF THE STRUCTURE OF DNA More Evidence: The Genetic Material is DNA - Alfred D. Hershey and Martha Chase, 1952 Erwin Chargaff, 1940’s and early 50's- % base pair composition A=T C=G M.H.F. Wilkins and Rosalind Franklin, early 50’s : X-ray Crystallography James Watson and Francis H.C. Crick, 1953

HERSHEY-CHASE EXPERIMENT From the 1800’s, scientist were convinced chromosomes played a role in hereditary material Since chromosomes are made of both proteins and DNA, both were contenders as genetic material Proteins were favored since it seemed more logical that the diversity of proteins were responsible for the diversity seen in species Used viruses to ascertain whether DNA or protiens were the genetic material Since DNA contains phosphorus and proteins contain sulfur these levels were measured in infected bacteria.

ROSALIND FRANKLIN Known for her work in x-ray crystallography (x-ray diffraction) Helped determine the overall structure of DNA Watson and Crick used her research and were accredited for the discovery of DNA structure (awarded Nobel prize) Not awarded Nobel prize as she died before getting recognized

STOP! Who was accredited with the discovery of the structure of DNA? WATSON AND CRICK

GENETIC INFORMATION Genetic information is stored in molecules called nucleic acids. There are 2 types of nucleic acids DNA: deoxyribonucleic acid Double stranded RNA: ribonucleic acid Single stranded

STOP! What is the difference between RNA and DNA? Single stranded vs double stranded

NUCLEOTIDES Nucleotides are the building blocks of nucleic acids A single nucleotide consists of: A pentose sugar A phosphate group A nitrogenous base Components are held together with covalent bonds

NITROGENOUS BASE  Also known as an organic base, or a nitrogen base  5 different bases: ◦ Adenine (A) ◦ Cytosine (C) ◦ Guanine (G) ◦ Thymine (T) – only found in DNA ◦ Uracil (U) – only found in RNA

STOP! What is a nucleotide made of? What bonds hold them together? What are the 5 different bases? What’s different about uracil?

PURINE Nitrogenous Bases with a double ring structure Adenine and Guanine

PYRIMADINE Nitrogenous bases with a single ring structure Cytosine, Thymine, and Uracil*

PENTOSE SUGAR  In RNA, the sugar is “ribose”  In DNA, the sugar is “deoxyribose”  (The difference is the presence or lack of oxygen on the 2 nd carbon)

NUMBERING SUGARS

STOP! What is the difference between deoxyribose and ribose In what structures are they found respectively?

NUCLEOTIDE STRUCTURE In a single nucleotide, the 1 st carbon of the pentose sugar is covalently bonded to the nitrogenous base. The 5 th carbon of the pentose sugar is covalently bonded to the phosphate group The 3 rd carbon of the pentose sugar is covalently bonded to the phosphate group of the next nucleotide in the chain. This bond is called a phosphodiester bond.

NUCLEOTIDE 1 st carbon 5th carbon 3 rd carbon

DNA STRUCTURE Remember DNA, is double stranded The 2 strands of DNA are complimentary The nitrogenous bases of 2 complimentary nucleotides hydrogen bond to each other to create the double strand

Sugar- phosphate backbone Nucleotide bases

COMPLIMENTARY BASE PAIRING Adenine always bonds to Thymine Cytosine always bonds to Guanine REMEMBER…. AT a G ood C lass Adenine –Thymine Guanine- Cytosine *Uracil is not found in DNA only RNA and binds with Adenine

Covalent Bond Phosphodiester bond (Covalent Bond) Hydrogen Bond

DNA STRUCTURE  There are 2 H bonds between Adenine and Thymine  There are 3 H bonds between Guanine and Cytosine  DNA forms a double helix  The helix is created by H-bonds between non- consecutive nucleotides

STOP! What base pairs bond together How many H-bonds for A-T? How many for G-C? How many would you expect uracil to make with a potential nucleotide?

DNA STRUCTURE  The 2 DNA strands will each have a phosphate at the end of one strand, and a sugar at the opposite end.  The end that has a phosphate is referred to as the “5 prime end” (5’)  The end that has a sugar is referred to as the “3 prime end” (3’)  The 2 strands are ANTIPARALLEL (because their 3’ and 5’ terminals are at opposite ends)

BONDS 5’ 3’ 5’ Hydrogen bonds- between nitrogenous bases Phosphodiester - between phosphate and adjacent Sugar Covalent - between sugar and nitrogenous bases + End Phosphate and sugar

STOP! What are 2 structural features of DNA? Double helix Anti-parallel Complimentary

Phosphodiester Covalent STOP!! 5’ 3’ Covalent? Phosphodiester? Nucleotide? Where’s 5’? Where’s 3’? Covalent

DNA ORGANIZATION DNA is extremely long If you took the DNA of a single cell and stretched it out into one long double helix, it would measure 1.8m in length If fits into a cell because it is very tightly packed – which also keeps it organized!

DNA ORGANIZATION  Just like thread is spun around a spool to keep it organized, DNA is coiled around a group of eight proteins called histones.  The complex of histones and DNA is called a NUCLEOSOME

NUCLEOSOME It takes 200 nucleotides to form a nucleosome The histone are positive, the DNA is negative – so they are strongly attracted! Nucleosomes consist of DNA wrapped around eight histone proteins and held together by another histone protein.

NUCLEOSOMES Histone proteins: 8 histone proteins (4 types, 2 of each type) inside each nucleosome 1 histone protein outside each nucleosome, which functions to organize and hold the nucleosome together

DNA ORGANIZATION  A series of nucleosomes coil into chromatin fibres  The chromatin fibres then coil to form a supercoil  The supercoiled chromatin is what makes up a chromosome  A chromosome is one unbroken double-stranded DNA helix

NUCLEOSOME FUNCTION Not only do nucleosomes keep DNA organized, they also prevent transcription When DNA is organized in a nucleosome, the promoter region is inaccessible so transcription cannot take place Transcription is when DNA is used as a template to produce an RNA strand. For this to occur, enzymes will alter the shape of the nucleosome and the enzyme RNA polymerase must attach to the 3’ end of a DNA strand.

STOP! What is a nucleosome? What is a histone? How many histone in a nucleosome? Describe DNA structural arrangements in order from least densely packed to most dense

DNA CODING Some parts of the DNA contain more genes than other parts. The gene-rich portions are rich in G and C while the junk DNA is rich in A and T. The light bands on chromosomes are gene-rich regions. A stretch of DNA sequence often repeats several times in the total DNA of a cell. An entire telomere, about 15 kb, is constituted by thousands of the repeated sequence "GGGTTA".

Highly Repetitive Sequences 10-25% of eukaryotic DNA consists of sequences of 5 to 10 nucleotides repeated 100,000 to 1,000,000 times. This type of DNA probably does not code for proteins. A large proportion of this type of DNA is found at the tips of the chromosomes and at the centromere.

Prokaryotes contain uninterrupted DNA Eukaryotes have coding regions interrupted by non- coding regions Genes are discontinuous Introns-intervening sequences Exons- coding (expressed)