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AW MAN, DNA
NUCLEOTIDES
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Three components: pentose sugar, phosphate group, nitrogen containing organic
base
o Guanine and cytosine (3 H bonds, more GC pairs in a DNA molecule, the more
stable it is)
o Adenine and Thymine (and Uracil which replaces thymine in RNA)
Components joined via condensation reaction to form a MONONUCLEOTIDE
Mononucleotides can join by further condensation reactions- via the pentose sugar
of one monomer and the phosphate group of another
o PHOSPHODIESTER BOND
o Releases water lmao
o Continued linking in this way = polynucleotide

RNA

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Single stranded, relatively short, pentose sugar = ribose¸ thymine is replaced by
uracil
mRNA transfers genetic info from DNA to ribosomes
u learn more about this later

DNA
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double stranded, twisted to form double helix¸10 base pairs = 1 full turn
pentose sugar = deoxyribose, adenine, thymine, guanine, cytosine
extremely long molecule, contains a fuck tonne of info
two strands are joined together with hydrogen bonds between the complimentary
base pairs
o because base pairs are complimentary, you will always have the same
quantity of cytosine as you do guanine in a DNA molecule
o although ratio of GC to AT can vary
o makes for nice questions ~

STABILITY OF DNA

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PHOSPHODIESTER BACKBONE protects the more chemically reactive organic bases
inside the double helix, so well that DNA usually passes generation to generation
without mutation

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HYDROGEN BONDS link the base pairs, forming bridges between the
PHOSPHODIESTER UPRIGHTS
There are OTHER INTERACTIVE FORCES between the base pairs that hold the
molecules together (base stackings)

ADAPTIONS OF DNA

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DNA is the hereditary material responsible for passing on genetic info from cell to
cell and generation to generation
It’s adapted to this role in several ways:
o V stable structure, normally passes generation to generation without
mutation
o Separate strands are only joined by hydrogen bonds- easy to break during
DNA replication and protein synthesis
o Large molecule, carries lots of info
o Backbone protecting base pairs protects genetic material from being
corrupted by outside chemical and physical forces
o Base pairing means that DNA can replicate and transfer info to mRNA

DNA REPLICATION

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Before cell division, nuclear division must take place
DNA must be replicated so both daughter cells have all the genetic material to make
all proteins and enzymes they need
Semi-conservative replication
Ie, each strand of DNA is used as a template to make another, called semi
conservative because half of the original DNA is built into the new DNA molecule
You need FREE NUCLEOTIDES, DNA POLYMERASE and a SOURCE OF CHEMICAL
ENERGY for this process to take place
o DNA helicase breaks hydrogen bonds between complimentary base pairs
(usually starting at A-T bonds because there’s only two, weaker)
o Causes two strands to split and unravel, both now act as templates
o Activated free nucleotides bind to complimentary bases
o Then are joined together in condensation reaction by DNA polymerase
(PHOSPHODIESTER bonds)
o DNA polymerase also checks for mistakes with bond pairing and corrects
most ~

EVIDENCE FOR SEMI-CONSERVATIVE REPLICATI ON

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Used bacteria
All bases contain nitrogen

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Bacteria incorporate nitrogen of their growth medium into their DNA
Grown on N15 for many generations, then N14, samples were removed after every
generation
DNA extracted, suspended in solution, centrifuged
Since the N14 DNA would have been lighter, and collect nearer the top of the tube
than the N15 DNA, they were able to analyse the composition of the new DNA
And say that after one generation, all of the DNA was a mix of N15 and N14
Supporting semi-conservative replication ~

TRIPLET CODE

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Every three base pairs is called a triplet, and codes for a specific amino acid
64 codes for 20 amino acids means that some amino acids have several triplets that
code for them
Possible that a mutation in the DNA won’t change the amino acid sequence of a
polypeptide
You’ll learn more about this later also ~



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