01:694:301 Lecture Notes - Lecture 4: Dna Replication, Nucleic Acid Double Helix, Intron

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Chapter 4‘DNA, RNA, and the Flow of Genetic Information’
DNA, RNA, and Protein Synthesis
Focus on basic idea of the working of DNA and RNA, and the basic facts of protein
synthesis.
o You should earn the definitions of transcription, translation, and replication.
! Replication – DNA synthesis; DNA forms double helix
Each strand of helix serves as template for another DNA replication.
! Transcription – DNA is not the direct template for protein synthesis. Instead
DNA is copied into RNA (mRNA specifically), the info carried intermediates in
protein synthesis
! Translation – the synthesis of proteins according to instructions given by
mRNA templates
DNA and RNA are polynucleotides.
o Flow info depends on genetic code;
o Flow of info relates to both sequence of bases along nucleic acid chain carries
genetic info and the sequence of amino acids
! A sequence of 3 bases " codon specifies an amino acid
! Mosaic of nucleic acid sequences " introns and exons (both transcribed)
Introns are cut out to newly synthesized RNA molecules, leaving mature
RNA with continuous exons
As with proteins in order to understand the polymer, we have to look at the monomer
o Here the monomer are nucleotides
o Each nucleotide has a ‘base’, a sugar, and a phosphate
! Both sugar and phosphate make up ‘backbone’ of DNA
Sugar in DNA is deoxyribose
o Lack 2’ hydroxyl group
Phosphates form phosphodiester bridges (negative charge less susceptible to
hydrolytic attack – resistant)
o Refer pg. 110 for nomenclature.
Please learn to draw structures of A, C, T, and G either alone or as part of a
nucleotide
You should also understand the simplified “stick and P” diagrams show in Fig. 4.7.
o The top of the stick represents carbon one of the sugar, which is where the ‘base’
attaches.
o The bottom of the stick is carbon 5, where the phosphate is linked by an ester bond
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DNA’s Structure
The understanding of DNA took a quantum leap when Watson and Crick
determined the structure of DNA double helix in 1953.
o For the first time it was clear how DNA could be precisely copied during cell
replication
o Double-helical structure aids in replication
KNOW the properties of the double helix (pg. 114-115) and be able to draw AT and
GC base pairs. Bases pairs are held together by hydrogen bonding
o Watson and Crick concluded the following:
! Two helical polynucleotide chains coil with ‘right-handed’ screw sense;
antiparallel (opposite polarity)
! Backbone (sugar and phosphate) lies outside; bases (ATCG) lie inside helix
! Bases perpendicular to axis; adjacent bases 3.4 Å; every 34 Å helical structure
repeats (every 10 bases); each base rotated 36° degrees from one below it
! Helix diameter = 20Å
o Stacking of base pairs stabilizes helix
! Double helix stabilized hydrophobic effect (bases hydrophobic (inside) vs
polar surface exposed to water)
! Stacked base pairs attracted by vdW forces further stabilize helix
! Base stacking favored by rigid 5-membered rings of backbone sugars
o DNA also has different structures " B form vs. A form
! B form is narrower and bases perpendicular; A form wider and bases tilted
! RNA helices are typically in DNA A form (bind fewer H2O molecules;
dehydration favors this form)
! Z DNA – left handed helix; backbone zigzagged
Double Helix and Transmitting Genetic Info
Understand that the two chains in the double helix are antiparallel, so if one strand
reads 5’ ATG 3’, the other strand is not 3’ TAC 5’ but 5’ CAT 3’;
o DNA chain has directionality (polarity); base sequence is always written 5’ " 3’
Meselson and Stahl confirmed Crick’s speculation about how DNA replication works
o The sequence of bases of one strand of the double helix precisely determines the sequence of
the other strand
o It is a semi-conservative process, the two stands separate, and a new strand is
‘grown’ to match each of the older strands.
o You should understand the experiment that proved this (pg. 119)
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