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Lecture

BIO120H1 Lecture Notes - Genetic Code


Department
Biology
Course Code
BIO120H1
Professor
Belinda Chang

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Lecture 2 Intro to Nucleic Acids & Proteins
Nucleic Acid Chains
1. DNA is synthesized from deoxyribonucleoside triphosphates, otherwise known as:
dNTPs things that are used to make this linear structure. 2 phosphates get
removed during this process.
2. RNA is synthesized from ribonucleoside triphosphates, or: NTPs
3. Nucleotides are linked by: phosphodiester bonds (through phosphate groups)
Base Pairing
-DNA is double stranded while RNA is single stranded in order for
replication & transmission of info contained in DNA strand, it needs to be
double-stranded. Basis of exactly how those strands together is mediated
through process called base pairing ensures that base opposite of it in
opposing strand is specific base b/c not all bases are complementary to each
other.
1. Holds DNA double helix together
2. A-T – 2 H-bonds
G-C – 3 H-bonds
H-bonds form, helping to keep 2 strands together
Strands run anti-parallel
3 forces that keep DNA together:
1. H-bonds – dipole-dipole interactions with each other
2. Van der Waals are induced dipoles (slight variation in positive &
negative charge) clouds of e- that when they come near each other
induce dipoles in each other.
3. Hydrophobic interactions tendency to avoid water stacking b/w bases
which are very hydrophobic particularly ring structure tend to like to
stack in certain way.
DNA Structure
- Info is always read 5’ to 3’ DNA is composed of 2 antiparallel
strands. Has ladder-like structure. Same info present on 1 strand is on
other b/c of complementarity of bases & this redundancy is extremely
important for preservation of info.
-DNA structure is organized into double helix ladder twists
around itself forming major groove & minor groove. Interactions with
turns of helix make DNA more stable.
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Review Qs
1. Strands in double helix are antiparallel (go in opposite direction)this means that 1 strand is 5’ 3’ while
its partner is 3 5’
2. What are 2 ends of DNA strand composed of? 5’ = P, 3’ = OH
Sequence of 2 strands are: complementary. Strands can be unzipped (reversiblecan be reannealed).
This is important for: DNA replication (making copy of DNA), protein synthesis (i.e. making RNA copy 
protein)
Unzipping Helix
1. Heating denatures double-stranded DNA by: disrupting H-bonds b/w bases (specifically those H-bonds
mediating base pairing holding 2 strands together)
2. Temp at which DNA denatures is called: TM
3. Denaturation of DNA is reversible process
Review Qs
1) Why is TM of DNA rich in G-C base pairs higher than TM of DNA rich in A-T base pairs? G-C have 3 H-
bonds; A-T have 2
2) Why do high temp organisms such as bacteria around undersea vents have G-C rich genomes? DNA would
denature
Intro to Protein Structure
Ex:
Primary (sequence)
Secondary (local folding)
Tertiary (long-range folding)
Quaternary (multimeric organization)
Supramolecular (large-scale assembly)
- AA sequence
- a-helix, B-sheet
- 3D structure
- protein assemblies
Protein Structure
1. Proteins are composed of AA
2. AA side-chain or R group is variable & determines type of AA (only part of
entire AA that variesanalogous to base in nucleotide)
3. 4 major categories of AA: basic (positively charged AA), acidic (negatively
charged), uncharged polar (1s with hydroxyl groups on them), non-polar
(hydrophobic AA)
Mutational steps b/w codons
1) What is min # of mutational steps b/w AA? - # mutations b/w codons for different AAs
2) How many mutational steps required to get from a codon for proline to one for cysteine, minimum? 2 (CCG 
UGC)
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