Intro DNA and DNA Replication 2012

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The Cell

• Smallest unit of life

• Compose all living things

• The “nucleus” (one of

many organelles) containshereditary genetic

information the cell needs

to exist and reproduce

- most cells organise

genetic information into

chromosomes



Chromosomes

• Chromosomes - organise all

the information that an

organisms genetic material

contains (note - mitochondrial

DNA)

• 23 pairs in humans

•2n = 46 – diploid (somatic cells)

n = 23 – haploid (gametes)- each chromosome pair contains

one from mother and one from

father



Genes

•

Each chromosome contains

100s to 1000s of information

blocks called genes – position

on chromosome called locus

(loci) – different forms of a

gene - alleles

• Each gene is the blueprint for

a specific protein in the body –

expressed via genetranscription then translation

of mRNA

• gene expression is regulated



• DNA is a Double Helix - like a twisted

rubber ladder made from three main

components

• Sides of the ladder are composed of

alternating sugar and phosphate pieces

• Each “rung” of the ladder is made up of

two complementary bases

– A bound to T

– C bound to G

• DNA is put together in chunks called

NUCLEOTIDES

– Each nucleotide has a sugar, PO43- and base



• A nucleotide is held together

with strong molecular bonds

• Oxygens provide the “hooks”necessary to bind to other

NTP’s

• We identify where these

Oxygens are by labeling the

carbons of the deoxyribose

sugar

1’

2’ 3’

4’

5’

(see, no oxygen at 2’…)



• Each of the components of the

nucleotide binds to another

nucleotide to form a singleDNA strand

– Since Oxygen easily forms two

strong bonds (phosphodiester

bridge), the phosphate and sugars both bind most strongly (covalent

bond) – enzyme DNA ligase and

results in the release of water.



• Each of the components of the

nucleotide binds to another

nucleotide to form a singleDNA strand

– The Hydrogens on the base bind

less strongly with an Oxygen or

Nitrogen from another base to bind one strand to another



Adenine & Guanine = Purines

Cytosine & Thymine = Pyrimidines



• Finally we have the characteristics of

double stranded DNA

• STRONG bonds between the

deoxyribose sugars and phosphates

• WEAK bonds between the Hydrogen

and Oxygen or Nitrogen (weaker

between A and T than C and G)

• Twisting of the two anti-parallel

strands to form a double helix

• Heat it up, which bonds break first?...



• DNA is a replicator because of the

specific complimentary base pairing

– Since only one of the bases can pair with only one other of the other bases,

each strand is a negative of the other

(complimentary)

– The process of pulling the strands apartfrom each other and replacing each

single strand with its complement is

called REPLICATION

• We start with some ds DNA



• DNA Polymerase

comes on the scene

• UNWINDS the DNAupstream

• UNZIPS the DNA

• This site is known as

the replication bubble



• Complementary Bases begin adding

into both sides of the ds DNA

• A binds with T, C binds with G (no

other possibility because of the shape

of the bases!)

• The DNA Polymerase precedes the paired bases and clips out any that are

already added ahead of it (on the 3’

end) with an exonuclease activity



• Finally you have 2

identical copies of ds

DNA• The final job of the

Polymerase is to

Proofread the NTP’s

after they are added(back on the 5’ end)

and to clip out any that

are incorrectly paired



• Unwinds DNA at the replication fork

• Unzips DNA (breaks apart Hydrogen Bonds)

• Cuts out nucleotides attaching upstream on the 3’ end

• Proofreads back on the 5’ end and cuts out improperly

paired bases



Restr iction Enzymes

Enzyme that cuts DNA at specific

sequences

Recognize and binds to 6-8 nucleotide

stretch



Gel Electrophoresis

• Gel with different sized pores

agarose and acrylamide are common materials

• Load DNA samples into wells at top of gel

• Run electric current through the gel

• DNA moves due to negative charge

• Smaller bands run “faster”

Intro DNA and DNA Replication 2012

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