BASIC MOLECULAR GENETIC MECHANISMS Introduction
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BASIC MOLECULAR GENETIC MECHANISMSIntroduction:
nucleic acids. (1)contain the information for determining the amino acid
sequence & the structure and function of proteins
(1)part of the cellular structures:
-select & align amino acids in the correct order ( polypeptide chain)
(3) catalyze chemical reactions e.g
formation of peptide bonds between amino acids duringprotein synthesis.
Electron micrograph of DNA (green arrow) being transcribedinto RNA (red arrow). [O. L. Miller, Jr., and Barbara R.Beatty, Oak Ridge National Laboratory.]
DNA: Contains information required to build the cells, tissues
-exact replication of DNA assures genetic continuity from generation toGeneration. - information stored in DNA arranged in hereditary units= genes,
-Transcription:DNA into RNA
RNA: Three distinct roles in protein synthesis.
-Messenger RNA (mRNA) carries the instructions from DNA that specify the correct order of amino acids
-Assembly of amino acids into proteins by translation
-The information in mRNA is interpreted by tRNAwith the aid of rRNA
-Correct amino acids brought into sequence by tRNAs, linked by peptide bonds.
central dogma—of molecular biology
Sourse: Text2: P.101-108Objectives:
N.A structure -N.A : Linear Polymer with End-to-End Directionality
- Native DNA a Double Helix of ComplementaryAntiparallel Strands
-DNA Can Undergo Reversible Strand Separation - Many DNA Molecules Are Circular
-Different Types of RNA Exhibit Various Conformations Related to Their Functions
KEY CONCEPTS
■ DNA &RNA are long, unbranched polymers of nucleotides, consist of a phosphorylated pentose linked to organic base a purine or pyrimidine.
■ Adjacent nucleotides in a polynucleotide linked byphosphodiester bonds.
The entire strand has a chemical directionality:
3 end with a free OH or phosphate group (5 end) ■ Natural DNA (B DNA) contains two complementary antiparallel polynucleotide strands
- wound together into a regular right-handed double helix with the bases in-side and sugar-phosphate backbones outside.
• Base pairing between strands and hydrophobic interactions between adjacent bases in same strand stabilize native structure.
■ bases in NA interact via hydrogen bonds.
standard Watson-Crick base pairs G·C, A·T(in DNA), and A·U (in RNA).
Base pairing stabilizes the native three-dimensional structures of DNA & RNA.
■ Binding of protein to DNA can deform helical structure, causing local bending or unwinding of DNA molecule.( dense packing of DNA in chromatin)
Alternative Forms of DNA:
•B form: Most DNA in cells is a right-handed helix: The x-ray diffraction the stacked bases : 0.36 nm apart helix makes complete turn/ 3.6 nm 10.5 pairs per turns Strands form two helical grooves major groove& minor groove - base within these grooves accessible for DNA binding proteins •Low humidity:, e crystallographic B DNA changes to A form; RNA DNA& RNA-RNA helices in cells/ in vitro. •Z form: Short DNA composed of alternating purine pyrimidine( Gs and Cs) adopt left-handed helix.
evidence suggests t Z DNA may occur in cells(function unknown)
•Ttriple-stranded DNA:
-formed when synthetic polymers of poly(A)&(U) mixed in the test tube. OR stretches C and T residues in one strand& A and G residues in the other form a triple-stranded
-do not occur naturally in cells(useful as therapeutic agents).
DNA Can Undergo ReversibleStrand Separati:
Concepts:
■ Heat causes DNA strands to separate (denature).
melting temperature Tm of DNA increases with percentage of G·C base pairs.
separated complementary nucleic acid strands renature.
Uunwinding & separation of DNA strands= denaturation, or “melting,”(in vitro) Increasing temperature---- increase molecular motion------- breaks
hydrogen bonds & forces stabilize double helix----------strands separate, driven apart by repulsion of Negatively deoxyribose-phosphate
. Near denaturation temperature, a small increase in temperature causes
loss of weak interactions holding strands together along the entire length of
the DNA molecules--------change in the absorption of ultraviolet (UV) light
melting temperature Tm at which DNA strands separate, factors : 1 Molecules contain a greater proportion of G·C pairs require higher temperatures to denature ?
-------- these base pairs more stable than A·T pairs?
2- ion concentration decrease--- Tm decrease
, negatively charged phosphate groups covered by positive ions ,----decrease ions------increase repulsive force
Agents destabilize hydrogen bonds e.g formamide or urea------ lower Tm. extremes of pH denature DNA at low temperature. At low (acid) pH, bases become protonated ------ positively charged----- repellingeach other. At high (alkaline) pH ------bases lose protons ------negatively charged----- repelling.Lowering temperature, increasing ion concentration or neutralizing the pH causes the two complementarystrands to reassociate into a perfect double helix---= renaturation dependent: time,DNA concentration, and concentration. . Denaturation and renaturation of DNA basis of hybridization
Many DNA Molecules Are Circular
Prokaryotic DNAs, viral DNAs, mitochondria& chloroplasts,= : circular. two strands in circular DNA forms closed structure without free ends----------!Uunwinding of circular DNA during replication------- DNA twists back on itself(OVERWOUND)-------- forming supercoils
Bacterial and eukaryotic contain topoisomerase I,= relieve OVERWOUND in DNA during replication
topoisomerase I binds to DNA at random sites & breaks phosphodiester bond in one strand = a nick----------------------------------- broken end winds around the uncut strand-------to loss of supercoils --------- same enzyme ligates two ends of broken strand
topoisomerase II= breaks in both strands of ds DNA ------ topoisomerase II relieve overwound & link two circular DNA
Eeukaryotic DNA is linear: long loops of DNA fixed within chromosomes ,---------- supercoils could occur during replication-------------- topoisomerase I in eukaryotic Relieves overwound in DNA.
Different Types of RNA Exhibit Various Conformations Related to Their Functions
RNA structure similar to DNA except:?
hydroxyl group on C2 of ribose:
1--RNA more chemically labile than DNA 2- provides a chemically reactive group---- takes part in RNA-mediated catalysis.-------- RNA is cleaved into mononucleotides by alkaline solution but DNA not.
Most RNAs SS and exhibit conformations ------ permit RNA carry out specific functions in cell.
Simplest secondary structures in SS RNAs formed by pairing of complementary bases. “Hairpins” formed by pairing of bases within ≈5–10 nucleotides of each other
& “stem-loops” by pairing of bases separated by >10 to several hundred nucleotides.
These simple folds cooperate form complicated tertiary structures = “pseudoknot.”
, tRNA molecules adopt three-dimensional architecture in solution----protein synthesis
rRNA have three-dimensional structures & with flexible links in between.
Secondary and tertiary structures recognized near ends of mRNA
Some RNAs have catalytic activity = ribozymes Ribozymes stabilized via association with proteins. Some ribozymes catalyze splicing! Where?Some RNAs carry out self-splicing.
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