DNA – structure and component (12-1)
How does DNA control the function of genes?
Genes: carry info from one generation to the next
Influence the heritable characteristics
Easily copied
DNA = polymer made of monomers = nucleotides
Shaped as a double alpha helix
Nucleotide = 5 carbon sugar(deoxyribose), phosphate group(PO4-)and
nitrogenous base (adenine and guanine = purines – double ring or thymine and
cytosine are pyrimidines – single ring)
Backbone of DNA chain = phosphate – sugar alternating - phosphate of one
nucleotide is attached to the sugar of the next nucleotide etc
Polynucleotide strand has DIRECTIONALITY 5’ end (end that starts with phosphate
group ) to the 3’ end (with the OH group on the sugar)
The “rungs” of the DNA ladder = paired nitrogenous base
Always one purine with one pyrimidine: AT and CG
However, the sequence of nitrogenous bases can be joined tog in any order on a
single side of the double helix
CHROMOSOMES AND DNA REPLICATION
Vocabulary:
Chromatin: in eukaryotic organisms the tightly packed DNA and protein(histones)
Nucleosomes: beadlike structure comprised of a handful of histones wrapped
with the tightly coiled DNA to form a thick fiber
1
DNA REPLICATION
Structure of DNA helps explain how DNA can be copied/replicated
Each strand of the double helix has the info to construct the other half because of
the specific base pairing
DNA strands = complimentary
DNA replication occurs at HUNDREDS of places along the strand, occurring in
BOTH DIRECTIONS until the strand is completely replicated
REPLICATION FORKS: sites where separation and replication take place
(www.scilinks.org
webcode cbn 4122) also: PHSchool.com webcode cbp 4122
DUPLICATING DNA
REPLICATION – before cell division – DNA is copied
DNA molecule separates into two strands
Each strand acts as a template for ordering nucleotides into a NEW
complimentary strand (base pairing rules)
Where there was one double stranded DNA molecule now there are 2
double stranded DNA molecules, each an exact Replica
If original strand has bases TACGTT new strand becomes ATGCAA – each
strand of the “unzipped” DNA molecule is replicating itself
Our cells have 46 DNA molecules – one double helical molecule/chromosome
6 billion base pairs – our cells can take a few hours to copy all of this DNA!
How Replication Occurs
2
ENZYMES! And regulatory molecules(more proteins)
1. replication begins at sites called “origins of replication” – short stretches of
DNA with a specific sequence of nucleotides
2. Proteins that initiate DNA replication recognize the sequence and attach
3. strands separate once protein attaches creating a replication “bubble”
4. Replication proceeds in BOTH directions
5. eukaryotic chromosome has hundreds – thousands replication origins
6. replication “bubbles” fuse – speeding up replication process
7. replication fork at each end of “bubble”
8. Helicases – enzymes that unwind the double helix at replication forks
9. single strand binding proteins bind to unpaired DNA
10. Primer – a short chain of RNA is the initial nucleotide chain produced –
enzymes that synthesize DNA can’t initiate the synthesis of a
polynucleotide – PRIMASE synthesizes PRIMER by using the DNA template
11. the new DNA strand will start from 3’ end of RNA primer
12.DNA polymerase(11) – joins individual nucleotides to RNA primer to
produce a DNA molecule complimentary to the “parent” strand AND
“proofreads” for mistakes in replication
How does the antiparallel arrangement of the double helix affect replication?
DNA polymerases can add nucleotides ONLY to the 3’ end of primer
A new DNA strand can elongate only in the 5’3’ direction
(remember each double helix has a strand that begins with 5’ and its
complimentary strand that begins with 3’ – so if the polymerase can add
3
nucleotides to the 3’ strand only - then the new complimentary strand is
starting with 5’ and leading to the 3’ of its other end SEE DIAGRAM
DNA strand moving from 5’3’ = leading strand
DNA strand moving away from replication fork also moves in 5’  3’ direction but
on the other side of the “bubble” from the replication fork = lagging strand
Lagging strand is synthesized in fragments = OKAZAKI FRAGMENTS
DNA ligase joins the Okazaki fragments into a continuous strand
QUALITY CONTROL
1. DNA polymerases proofread each nucleotide – removes incorrect
nucleotides – synthesis resumes
2. Mismatch repair – enzymes remove and replace errors in replication
3. nucleotide excision repair – damaged DNA – an enzyme – nuclease cuts out
the damaged portion – new nucleotides fill in the “gap”
Ends of chromosomes = telomeres = special nucleotide sequences
Hundreds of 6 nucleotide sequences comprise a telomere
Telomeres protect the genes of a chromosome
During continual replication the telomeres shorten ultimately leading to
death of a cell
Germ cells(egg/sperm) have telomerase which prevents the shortening of
the telomere - cancer cells also have telomerase 
ONE GENE ONE PROTEIN – Protein synthesis
4
5