DNA STRUCTURE AND CENTRAL DOGMA.pdf

RECOMBINANT DNA 202 DEOXYRIBONUCLEIC ACID STRUCTURE Dr Danisile Tembe, SLS Room 05-105, Tel: 031 260 7586, Email: [email protected]  DNA? DNA is a genetic molecule that...

RECOMBINANT DNA 202 DEOXYRIBONUCLEIC ACID STRUCTURE Dr Danisile Tembe, SLS Room 05-105, Tel: 031 260 7586, Email: [email protected]  DNA? DNA is a genetic molecule that store and transmit genetic information Made of polynucleotide covalently joined through 3’-5’ –phosphodiester bonds.  DNA DISCOVERY First identified in the late 1860s by Swiss chemist Friedrich Miescher Isolates DNA from Salmon sperm Several scientists carried out a series of research efforts Revealed additional details about the DNA 1944, Avery, MacLeod and McCarty discovered DNA could transform from pathogenic bacteria to non pathogenic  DNA DISCOVERY Hershey and Chase showed that DNA was a genetic material In 1953, Watson and Crick proposed a structure for DNA Following a DNA x-ray image by Rosalind Franklin  DNA COMPOSITION ❖ ERWIN CHARGAFF’s RULE Uncovered additional details of the structure/composition of DNA 1950 1. The composition of DNA varies between species Particular in the amount of bases (A, C., T, G) E.g. 32.8% of sea urchin DNA contain the base A, while only 24.7% bacterium E.coli contain A  DNA COMPOSITION ❖ ERWIN CHARGAFF’S RULE 2.  DNA COMPOSITION A DNA is composed by a repetition of three components:  DNA COMPOSITION DNA is composed of different nucleotides Different nitrogenous bases (A, C, T, G)  NITROGENOUS BASES Nitrogenous bases are grouped into 2 : Purines Pyrimidines  STRUCTURE OF BASES Purines: A & G Each have two linked rings and are called Purines  STRUCTURE OF BASES Pyrimidines: C, T and U Thymine and Cytosine each have a single ring and they are called Pyrimidines  STRUCTURE OF SUGAR Attachment site for phosphate group Attachment https://opentextbc.ca/biology/chapter/9-1-the-structure-of-dna/ site for base Nucleic Acids have 2 kinds of pentose (sugar) – Ribose and Deoxyribose DNA contains the sugar deoxyribose while RNA contains ribose  STRUCTURE OF SUGAR Due to structural difference in sugar components, the structural units of DNAs are called deoxyribonucleotides or deoxyribonucleoside While that of RNAs are called ribonucleotides or ribonucleoiside  STRUCTURE OF SUGAR Deoxyribonucleotides and Ribonucleotides BASE NUCLEOSIDE NUCLEOTIDE SHORT ADENINE Deoxyadenosine (DNA) Deoxyadenosine (DNA) dAMP (DNA) Adenosine (RNA) Adenosine-5’-phosphate (RNA) AMP (RNA) GUANINE Deoxyguanosine (DNA) Deoxyguanosine (DNA) dGMP (DNA) Guanosine (RNA) Guanosine-5’-phosphate (RNA) GMP (RNA) THYMINE Deoxythymidine (DNA) Deoxythymidine (DNA) dTMP (DNA only) CYTOSINE DEOXYCYTIDINE Deoxycytidine (DNA) dCMP (DNA) Cytidine (RNA) Cytidine-5’-phosphate (RNA) CMP (RNA) URACIL Uridine URIDINE-5’-PHOSPHATE UMP(RNA only)  PHOSPHODIESTER BONDS JOINS NUCLEOTIDES IN NUCLEIC ACIDS 5’ Terminus A bond that covalently joins the nucleotides units in DNA and RNA is called Phosphodiester bond 5’ All the phosphodiester linkages in a polynucleotide chain have the same orientation (5’ – 3’) oligonucleotides - short nucleic acid containing Phosphodiester 50 or fewer nucleotides 3’ 3’ Terminus  PHOSPHODIESTER BONDS JOINS NUCLEOTIDES IN NUCLEIC ACIDS 5’ Terminus A longer nucleic acid containing more 5’ nucleotides is polynucleotide Each polynucleotide has a 3 prime end and a five prime end. Phosphodiester 3’ 3’ Terminus  DNA IS A DOUBLE HELIX DNA consist of two complementary strands of nucleotides Twisted together (double helix) The two strands runs in opposite direction (antiparallel)  DNA IS A DOUBLE HELIX The two strands of DNA are joined together by the hydrogen bonds between nitrogenous bases on the opposite strands  DNA IS A DOUBLE HELIX Nitrogenous bases are paired in a specific combination: Adenine forms two hydrogen bonds with Thymine, and Guanine forms three hydrogen bonds with Cytosine Only combination resulting in a uniform diameter of a double helix  DNA IS A DOUBLE HELIX The strictness of these pairing rules results in a complementary nature of a double helix  DNA IS A DOUBLE HELIX The twist of the helix forms two unequal surfaces Called major grooves and minor grooves Major groves forms where the backbones are far apart Minor grooves occurs where the backbones are close together This is where most interactions with proteins or other nucleic acids occur Lehninger principles of Biochemistry, Fifth Edition, 2008. @W.H. Freeman and Company  IMPORTANT CHEMICAL BONDS IN DNA STRUCTURE 1. Covalent bond: bond formed between the sugar of one nucleotide and the phosphate of an adjacent nucleotide Covalent bond is stronger than a hydrogen bond  IMPORTANT CHEMICAL BONDS IN DNA STRUCTURE 2. Hydrogen bond: Responsible for holding nitrogenous bases in the double helix 3 H between C and G and 2 H between A & T IMPORTANT CHEMICAL BONDS IN DNA STRUCTURE 3. Phosphodiester bond: Acts as the connection between two nucleotides or adjacent nucleotides in the DNA strand DNA FORMS  THREE FORMS OF DNA STRUCTURES There are 3 helical forms of DNA A form: First discovered form, is favored in many solutions that are relatively devoid of water. B form: The Watson and Crick structure, is the most stable structure and commonly occurring DNA form. Standard point of reference in any study of properties of DNA Z form: no clear function of z form THREE FORMS OF DNA STRUCTURES  Form A B Z Helix direction Right handed right left Base pr/turn 10.9 10 12 Major Groove Narrow and deep Wide and deep Flattened Minor Groove Wide and shallow Narrow and deep Narrow and deep Overall morphology Short Longer Elongated wide thinner thinner Diameter 2.2 nm 2 nm 1.8 nm Base configuration anti anti Alternate Anti/syn  HELIX DIRECTION A – Right handed B - Right handed Z – Left handed Major G – Narrow & Deep Major G – Wide & Deep Major G – Flattened Minor G – Wide and Shallow Minor G – Narrow and deep Minor G – Narrow and deep  BASE PAIRS PER TURN A – 10,9 /11 B -10 Z – 12  DNA DENATUATION DNA can be denatured Process where a double stranded DNA is separated into 2 single strands Several experiments helped to establish that 2 strands of DNA unwinds when DNA solution is heated  DNA DENATUATION Disruption of both hydrogen bonding between paired bases on opposite strands causes the double helix to unwind 2 single strands  DNA DENATUATION E.g. in a well isolated solution, double stranded DNA is high viscous at 7.0 pH and room temperature, when such solution is exposed to extremes of pH or temperature above 80 ֯C, its viscosity degrease indicating that double helical DNA undergone a physical change Due to denaturation or melting of the double helical strand  DNA DENATUATION Denatured structure can be restored With slow cooling and annealing DNA can be reformed If denaturation conditions are removed, the helix rewinds  Central dogma The flow of genetic information from DNA to RNA to protein  Central dogma The flow of genetic information from DNA to RNA to protein This information was proposed by Fransic Crick after they discovered the structure DNA contains information needed for synthesis of proteins MRNA - messenger that carries that info to the ribosome  Central dogma At least three types of RNA are needed: 1. Ribosomal RNA (rRNA): catalyses translation 2. Messenger RNA (mRNA): carries protein message 3. Transfer RNA (tRNA): deliver correct amino acid to ribosome when needed during translation Other functional RNAs: microRNAs (miRNA): silencing of the gene Short interfering RNAs (siRNA): degradation of mRNA  DNA structure https://www.youtube.com/watch?v=o_-6JXLYS-k Central dogma video https://www.youtube.com/watch?v=9kOGOY7vthk