Lecture 12: DNA and RNA
Chapter Reading 12.1, 12.2 pp 257-264
DNA (deoxyribonucleic acid); one of the nucleic acids hereditary material in humans and most organisms.
DNA is a polymer. The monomers of DNA are nucleotides.
- DNA found in the cell nucleus where it’s called nuclear DNA
- DNA can also be found in the mitochondria, mitochondrial DNA or mtDNA
- Nucleotides are monomers of DNA
- Stored as a code made up of four chemical bases
- Each nucleotide consists of a 5 carbon sugar (deoxyribose), a nitrogen containing base
attached to the sugar and a phosphate group.
1.Nucleotides; building blocks of DNA and 2.Sugar Backbone; only DNA, in RNA it 3.Phosphate
RNA. will just be ribose sugar. group; other
Made up of 3 parts -RNA has a hydroxyl-OH group, but repeating part
-4 different types of nucleotides found in missing in DNA). of the DAN
-Deoxyribose sugar of DNA has 5 backbone.
DNA, only differ in nitrogenous bases. carbons and 3 oxygen’s.
Nitrogenous bases; lots of nitrogen
- Guanine -The OH groups on the 5’ and 3’ carbons -Phosphate
- Cytosine link the phosphate groups to form the attached to
- Adenine DNA backbone. sugar
Ribose is RNA molecule in
- Thymine place of the –
*these come in 2 different types. OH group on
The bases are heterocyclic, aromatic rings the 5’ carbon.
Purine Bases Pyrimidine Bases
2 rings 1 ring
Will be G and A in Will be C and T in
*In RNA Uracil replaces thymine (no –
RNA; single strand, with strands folding
to compact themselves into the tight
space of the cell.
- There are 4 nitrogenous bases (4 types, A, C,G, T) (adenine, cytosine, guanine, thymine)
- A base is a purine (2 rings) or pyrimidine (1 ring) (adenine, cytosine, guanine, thymine)
- A base + a pentose sugar = nucleoside.
• if the sugar is a deoxyribose,it is a deoxyribonucleoside (deoxyadenosine, deoxycytidine,
- A nucleoside + phosphate = deoxynucleotide
• ( deoxyadenosine 5'-monophosphate, deoxycytidine 5'-diphosphate, etc.)
- There are 5'-monophosphates, 5'-diphosphates, 5'-triphosphates (deoxyadenosine 5'-
monophosphate (dAMP), deoxadenosine 5'-diphosphate dADP), deoxadenosine 5'-triphosphate
DNA is said to be antiparallel; said to flow in one direction on one side and opposite on the other side.
- Always flows from 3’ end to 5’ end
- Addition of nucleotide only in the 3’ end
The backbone will always be the same = phosphate deoxyribose backbone.
- Purines bond to pyrimidines through hydrogen bonding (very weak)
- However covalently bonded to other nucleotides AT CG
- Latter said to be twisted - DNA is found in the nucleus
- Tightly packaged
- Packaged form of DNA=Chromosomes
- Chromosomes made up of tightly coiled DNA many times around proteins called histones
(protein balls specific protein involved in DNA structures) The basic structural forms for DNA and RNA:
Type of Organism Genetic Material Organization
Bacterium dsDNA(s) circular Protein/DNA nucleoid
Eukaryote dsDNA linear Protein/DNA chromosone
Mitochondrion dsDNA circular Protein/DNA
Virus (no nucleus or Ss or ds DNA circular or Protein/DNA
Ss or ds RNA circular or
Viroid (infectious agents for ssRNA circular None?
plants) -RNA without a
*Most life forms are single cells, and most are bacterium
In early time many believed that proteins were the most likely candidate for the hereditary material.
- The appeared to have greater opportunity for information coding than nucleic acids.
- Proteins contain 20 types of amino acids vs nucleic acids only have 4 different nitrogenous
bases for coding.
Experiment What did they do? Conclusion
Griffiths -Griffith was trying to make vaccine to prevent pneumonia Concluded some
experiment infections in epidemics that occurred in world. molecules released when
- Studied the conversion of no virulent R form of bacterium S cells were killed could
streptococcus pneumonia to a virulent s form. S form had shiny, change living non virulent
smooth capsule around it. R had no capsule have rough R cells genetically to
appearance. Injected it into mice and determined how mice were virulent S form. He called
affected. this the transforming
1. Mice injected with S cells, die. Live S cells in blood. principle and process of
2. Mice injected with R cells, mice live. No R cells in blood. R is the genetic change
non virulent. Capsule is responsible for the virulence of S strain. transformation.
3. mice injected with heat killed S cells. Mice live, no S cells live
in the blood. Live S cells needed to be virulent. Out of the 4 largest
4. Mice injected with heat killed S cells and live R cells. Live S macromolecules,
cells in their blood, show living R cells can be converted to carbohydrates, proteins,
virulent s cells with some factor from dead S cell. lipids and nucleic acids, he
thought the complexity and
So living R cells had the ability to make a polysaccharide structure of the nucleic
capsule from the dead S cells- they had changed- transformed. acids or proteins makes
them candidates for ones
to carry information and
cause this transformation.
Avery- Made experiment to identify chemical nature of the transforming Concluded that the
Macleod- principle that can change non virulent R cells to virulent S cells. transforming principle was
McCarty -worked with bacteria indeed DNA due to the fact
-used heat to kill S cells. when DNA was destroyed
-treated the macromolecules with enzymes that break down by the enzymes the S cells
each of the three hereditary molecules, DNA, RNA and protein. no longer changed the R
-when they destroyed RNA and proteins they saw NO effect. The cells to virulent form.
extract S cells still transformed R cells to virulent.
-when they destroyed DNA, no transformation occurred.
Chargaff’s Used harsh chemicals to degrade DNS into four bases than Adenine = Thymine
Rule quantified the amount of each. Cytosine = Guanine
- The number of adenine bases is equal to the
amount of thymine bases e.g. A = 30%
- Number of cytosine is equal to guanine bases so T = 30% - The proportion of A:T and C:G are relevant for both
*Chargaff’s rule does not apply to RNA. RNA is found as single
stranded molecule. He states that DNA helicases contain equal
molar ratios of A and T and C and G. In DNA these base pairs
complement each other but in RNA only forms local helicases
which means it doesn’t necessarily have equal ratios.
Hershey and Tried to answer question is DNA or protein the genetic material? So the DNA phosphorus
Chase Used phage (bacteriophage, virus infect bacteria, they are DNA was found within the
or RNA surrounded by a protein coat, cannot reproduce except progeny phages, indicating
within a host cell, when infects cell uses cell’s resources to that DNA was the genetic
produce more virus cells) that only consisted of DNA and RNA. material. In the proteins
-made 2 batches, DNA or protein labelled with radioactive the sulfur was found in the
material. phage coat but not the
1. infected e.coli with radioactive 32P or 35S with phage T2. infected cell or progeny
Phages labelled with either S (protein) and P (DNA). phages.
2. Separate culture E.coli infected the radioactively labelled
3. Bacteria were then mixed in blender, it was then analyzed.
4. Progeny phages analyzed for radioactivity.
Result: The Sulfur (protein) phage coat lacking DNA had no
radioactivity within the cell, so no proteins in the cell, and no
radioactivity on the progeny phages. The phosphorus, was not in
the phage coat but was within the cell and the progeny phages
also had it.
Fiber Xray diffraction is xray beam is directed at molecule in form of Used xray diffraction to
Diffraction regular solid, in form of crystal. conclude DNA had a
Experiment -They used sample of DNA molecules that had been pulled out helical structure.
(Wilkins and of fiber. Patterns indicated the DNA were cylindrical and about 2
Franklin) nm in diameter, major separations between spots showed an x
shaped figure of the dots, means that DNA has a helical
Watson a crick proposed the famous double helix model for DNA combing all the experiments.
Double helix model: two sugar phosphate backbones are separated from each other. The bases extend in
to fill this central space. Whenever A occurs in the strand, T is on the opposite side. This feature of the
DNA is called complementary base paring. One strand is said to be complimentary to the other.
Hydrogen Bonding: The base pairs are stabilized by hydrogen bonds, two between A and T and 3 b/w C
and G. The hydrogen bonds along the base pairs repeated along the double helix holds the two strands