DNA Structure and
Function
I) DNA Structure
 Polymer
=
nucleic acid
 Monomer =
nucleotide (has
3 parts )
1)Sugar:
Deoxyribose 5
carbon ring
 results
in a 5’
and 3’ ends of
sugar.
DNA Structure Cont’d
2)Phosphate:
Connected to the 5’
end of sugar and will
bond to the 3’ end of
another deoxyribose
sugar on another
nucleotide
 Results in the
phosphate-sugar
backbone.
DNA Structure Cont’d
3) Nitrogen Base: (polar) 2 classes
–
Purines = 2 nitrogenous rings


–
Pyrmidines = 1 nitrogenous ring


–
Thymine (T): 2 H-bonds possible
Cytosine (C): 3 H-bonds possible
Purines bind to Pyrmidines that have the same
number of H-bonds



Adenine (A): 2 H-bonds possible
Guanine (G): 3 H-bonds possible
A–T/T–A
C–G/G–C
DNA has two strands that will be antiparallel
II) DNA Function
Replication: DNA copied to make more
DNA (s phase; sister chromatids)
- When finished each sister
chromatid has 1 old strand and 1
new strand = Semi-conservative
replication.
DNA Function: Replication
Process:
 Unwinding = Helix is uncoiled and Hbonds b/w bases are broken (unzipped).
– Done by enzyme Helicase

Base Pairing = new complimentary bases
are added to old DNA strand
– Done by DNA Polymerase

Joining = Phosphate of one new nucleotide
bonds to 3’ of other sugar.
Replication Cont’d
Problem: DNA polymerase binds (builds)
nucleotides 5’  3’
 What happens on the original 3’  5’ DNA
strand?
 Leading Strand: New 5’  3’ DNA
synthesized continuously towards
replication fork. (faster)
 Lagging Strand: on the old 5’  3’ DNA.
5’3’ DNA fragments built by many DNA
polymerases away from replication fork =
Okazaki fragments (slower)

– Enzyme ligase connects the fragments
together.
How Genes Work
>
3 million species discovered; differ
by genes (sequence of nucleotide
bases).
 How does a difference in base
sequences cause uniqueness of
species?
 DNA sequence of nucleotides 
sequence of amino acids  specific
proteins  specific varieties of
anatomy and physiology
Modern Ideas
Gene: sequence of DNA nucleotide
bases that codes for a product.
 Problem: DNA in nucleus (can’t
leave; too big).
 Protein made in ribosomes in
cytoplasm; how is the message sent?
RNA
Gene does not affect phenotype directly;
gene product affects the phenotype
 Nucleic acid
 Differences:
– Single stranded
– Ribose
– Uracil instead of Thymine

Types
– Messenger RNA (mRNA): sends code from DNA
to ribosomes
– Transfer RNA (tRNA): brings amino acids to
ribosomes
– Ribosomal RNA (rRNA): Part of Ribosomes
(along with proteins)
Gene Expression has 2 Steps
1.
2.
Transcription: DNA
template for RNA
formation; mRNA
then moves to the
cytoplasm.
Translation: mRNA
directs the
sequence of Amino
acids in a
polypeptide
(protein).
DNA Function: Protein Synthesis
Genetic Code:
20 amino acids build all of life’s proteins. Has to
be a system to code for them using only 4
bases on DNA
Codon: 3 consecutive bases on mRNA that code
for a specific amino acid.
Properties:
 Degenerate: Most amino acids have more
than 1 codon
 Has a beginning and end ( 1 start codon; 3
stop codons)
 Unambiguous: 1 code = 1 meaning
Universal: all living
things obey this code
Therefore this dates
back to 1 life on this
planet.
All living things are
related.
A tobacco plant expressing a
firefly gene. Because diverse
forms of life share a common
genetic code, it is possible to
program one species to produce
proteins characteristic of another
species by transplanting DNA. In
this experiment, researchers were
able to incorporate a gene from a
firefly into the DNA of a tobacco
plant. The firefly gene codes for
an enzyme that catalyzes a
chemical reaction that releases
light energy.
1) Transcription (nucleus)
Helicase unzips DNA (same as replication)
RNA Polymerase (not DNA Polymerase)
binds RNA nucleotides
 Also builds in the 5’  3’ direction
 Connects @ Promoter = segment on the
DNA that defines the start of a gene, the
direction of transcription.
 Terminator = segment of DNA that defines
the end of a gene. Stops RNA polymerase.
1) Transcription (nucleus)
New mRNA = Primary mRNA transcript
 Needs to be modified before it can leave
the nucleus.
1. Ends protected:
 5’ = modified Guanine (Cap)
 3’ = 150 – 200 Adines attached (poly A tail)\
 Inhibits degradation (friction)
2.
Introns removed; mRNA that is never
expressed
 Exons = RNA that is actually expressed
 Done by Splicesomes
3.
Mature mRNA transcript  Cytoplasm
2) Translation (cytoplasm)
Message in 1 language
(nucleic acid)
translated into
another language
(protein).
tRNA: single stranded,
but doubled back on
itself.
 Anticodon=3 bases
that complimentary
base pair to mRNA.
 Amino acid binds to
the 3’ end.
2) Translation
(cytoplasm)
rRNA: combined
w/variety of proteins
into subunits
 1 large & 1 small
subunit.
 Sm. subunit has site
for mRNA.
 Lg. subunit has 3
sites for tRNA
complexes. (E,P, and
A sites)
If lots of protein
product is needed
many ribosomes
may bind to 1
mRNA molecule =
Polyribosome
Translation Requires 3 steps
1.



Initiation: energy/enzymes required
Small subunit binds to mRNA @
AUG codon.
1st tRNA binds its anticodon
Large subunit binds to small so that
tRNA is in the P site.
Translation Requires 3 steps
2.





Elongation: energy/enzymes required
tRNA already in P site; proper tRNA arrives @ A
site.
Amino Acid from tRNA #1 is transferred and
bound (peptide) to new amino acid on tRNA #2.
Translocation occurs: ribosome moves down
mRNA thus shifting the tRNA’s so that 1st one is
now in E site and 2nd one is now in P site.
tRNA in E site leaves ribosome and A site is now
empty will bind the 3rd tRNA.
Process occurs until ribosome reaches stop
codon.
Translation Requires 3 steps
3.



Termination:
Occurs @ stop codon
Release factor (enzyme) cleaves
polypeptide from last tRNA which then
leaves P site.
Subunits dissociate.
Polypeptide may function on own,
become part of larger protein, and if
ribosome is on the ER then it gets
ready to leave the cell.
Coupled transcription and
translation in bacteria.
In prokaryotic cells, the
translation of mRNA can
begin as soon as the
leading (5′) end of the
mRNA molecule peels
away from the DNA
template.
Attached to each RNA
polymerase molecule is a
growing strand of mRNA,
which is already being
translated by ribosomes.
The newly synthesized
polypeptides are not visible
in the micrograph but are
shown in the diagram.