DNA Structure and Replica2on Structure of DNA | James Watson and Francis Crick (with Maurice Wilkins)—awarded the Nobel Prize in 1962 for the construc2on of the double helix model of DNA | Rosalind Franklin—used X-­‐
ray crystallography to produce an image of the double helix model of DNA DNA Structure Made of Nucleo<des —3 parts: 1. Phosphate group 2. Sugar (deoxyribose) 3. Nitrogen base *Two Nitrogen bases that bind together form a base pair DNA keeps its structure with a sugar (deoxyribose) & phosphate backbone DNA—Deoxyribonucleic Acid | DNA contains your genes { Genes are sec2ons of DNA that code for a certain trait (hair color, height, nose shape…) | DNA is double stranded in the form of a double helix (twisted ladder) | DNA contains four nitrogen bases, phosphate and deoxyribose DNA Nitrogen Bases 4 Nitrogen Bases | Purines (2 rings) { Adenine { Guanine (PUGA2) | Pyrimidines (1 ring) { Thymine { Cytosine Chargaff’s Rules of Base Pairing | Adenine (A) always binds with Thymine (T) { Two hydrogen bonds | Guanine (G) always binds with Cytosine (C) { Three hydrogen bonds Write the Complimentary DNA Strand | ATGCGGATACATCCGATCGATGCAATCATGACATA | TGACGGACACTTACAGATTCTGTGAGTCTCAACGG | GAGCATTAGCAGGACTGCACGATATTGTCCTAGTA | GTAGTAGTAGTAGTAGTAGTAGTAGTAGTAGTA DNA Replication
Definition: process that
copies the entire genome
(all of the DNA) of a cell
In humans, your cells will
replicate 3 billion
nucleotides in 6 hours!
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Replication Facts
| DNA has to be copied before a cell
divides
| Occurs in the nucleus of eukaryotic
cells
| DNA is copied during the S or
synthesis phase of interphase
| Remember, each of the 2 new cells
made during Mitosis will need
identical DNA strands at the
completion of the cell cycle
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Synthesis Phase (S phase)
| S phase during interphase of the
cell cycle
| Nucleus of eukaryotes
S
phase
DNA replication takes
place in the S phase.
G1
interphase
G2
Mitosis
-prophase
-metaphase
-anaphase
-telophase
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| Barbara McClintock is credited for the discovery of the replica2on process. | She was awarded the Nobel Prize for her work in 1983. 11
DNA Replication STEP 1
| The enzyme Helicase unwinds and
separates the 2 DNA strands by
breaking the weak hydrogen bonds
between the nitrogen bases
HELICASE 12
| The two DNA strands open forming
Replication Forks (Y-shaped region)
Parental DNA Molecule
Replication
Fork
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| As the DNA strand
opens at the replication
forks, Replication
Bubbles form
| Eukaryotic DNA will
have MANY replication
bubbles
| Prokaryotes (bacteria)
have a single bubble
Bubble
Bubble
Bubble
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REPLICATION STEP TWO | Complementary nucleo2des are added to each strand by the enzyme DNA Polymerase | New replica2on bubbles form un2l the en2re strand
is replicated. | DNA polymerase also proofreads the DNA errors, hopefully catching most of them 15
Proofreading New DNA
| DNA polymerase initially makes
about 1 in 10,000 base pairing
errors
| DNA Polymerase proofreads and
corrects these mistakes
| The new error rate for DNA that
has been proofread is 1 in 1 billion
base pairing errors
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REPLICATION STEP 3 * The enzyme Ligase
attaches all the replication
bubble fragments together.
* The new double helixes
are identical- each contains
one original template
strand and one copied
strand
* Replication is semiconservative since each new
DNA molecule is made of
one old strand and one new
strand.
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Semi-conservative Model of Replication
| Idea presented by Watson & Crick
| The two strands of the parental
molecule separate, and each acts as a
template for a new complementary
strand
| New DNA consists of 1
PARENTAL (original) and 1 NEW
DNA Template Strand
strand of DNA
Parental DNA Strands
New DNA Strand
New DNA Strand
DNA Template Strand
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SUMMARY OF REPLICATION STEPS ONE MORE TIME: | DNA is unwound with the help of the enzyme Helicase. It separates the hydrogen bonds between the nitrogen bases. | Each original strand of nucleo2des acts as a template (paaern) to make a new chain | The new strand is assembled as DNA polymerase matches free-­‐floa2ng nucleo2des w/complementary nitrogen bases in the replica2on bubble. DNA polymerase then proofreads and corrects errors. | DNA ligase links together the individual replica2on bubbles. 19
ENZYME
FUNCTION
Helicase
Unwinds helix by breaking
hydrogen bonds between
nitrogen bases
DNA Polymerase
*Builds new strands inside
replication bubble by attaching
nucleotides to their
complementary base
*Proofreads for mistakes
Forms bonds between
replication bubbles so DNA can
return to the helical form.
Ligase
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DNA Replica2on BIG IDEA | DNA is replicated during S-­‐phase of interphase | Semiconserva,ve replica,on—produces two copies of DNA, each containing one original strand and one new strand | The end result of DNA replica2on is two iden2cal strands of DNA (half old and half new) DNA Damage & Repair
| Chemicals & ultraviolet radiation
damage the DNA in our body cells
| Cells must continuously repair
DAMAGED DNA
| DNA polymerase and DNA ligase work
together to help replace damaged
DNA and bond the new nucleotides
together
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