DNA ReplicationDNA Replication
How does each cell have the same DNA?How does each cell have the same DNA?
How is a prokaryote different than a How is a prokaryote different than a eukaryote?eukaryote?
Chromosome
E. coli bacterium
Bases on the chromosome
Prokaryotic Chromosome Structure
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Eukaryotic DNA
• Eukaryotes have 1000 times the amount of DNA as prokaryotes, and its more complex
• DNA is found in the nucleus in chromosomes (the number of chromosomes varies widely of different species)
• DNA is very long!... but it is highly folded packed tightly to fit into the cell!– For example, a human cell contains more than 1 meter
of DNA made of more than 30 million base pairs!
Eukaryotic Chromosomes
• Contain DNA and proteins called histones• Tightly packed DNA and proteins form
chromatin• During mitosis, the chromatin condenses to
form tightly packed chromosomes
Figure 12-10 Chromosome Structure of Eukaryotes
Chromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix
Section 12-2
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Watson & Crick again
• Earlier: Discovered double helix of DNA• Then: DNA can be copied or replicated,
because each strand of the DNA double helix has all the information needed to reconstruct the other half by way of base pairing– The strands are complementary!
DNA Replication• The process of making a copy of the DNA• Occurs inside the nucleus of the cell• Occurs when the cell is going to divide so
each resulting cell will have a complete set of DNA
• During DNA replication, the DNA separates into two strands, then produces two new complementary strands following the rules of base pairing.
• Each strand serves as a template, or model, for the new strand.
• Replication occurs in both directions• The site where separation occurs is called
the replication fork
DNA Replication• The two strands of DNA unwind or “unzip”
breaking the hydrogen bonds and separating. Then each strand becomes the guide or “template” for the making of a new strand.
• A protein called an enzyme called DNA polymerase breaks the nitrogen base bonds and the two strands of DNA separate, polymerizes individual nucleotides to produce DNA and “proof reads” the new DNA.
• The bases on each strand pair up with new bases found in the cytoplasm
• Then the sugar and phosphate groups form the sides of each new DNA strand
• Each new DNA molecule contains an original DNA strand and a new DNA strand
Figure 12–11 DNA ReplicationSection 12-2
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Growth
Growth
Replication fork
DNA polymerase
New strand
Original strand DNA
polymerase
Nitrogenous basesReplication fork
Original strand
New strand
STEP 1• Two original strands of DNA
separates by unwinding• This occurs through the
action of an enzyme that breaks the hydrogen bonds between the strands
• The two areas on either end where the DNA separates is the REPLICATION FORK
Replication fork
STEP 2
• At the replication fork, DNA polymerase moves along the DNA Strands adding nucleotides
• As DNA polymerase moves along two strands of DNA form
Step 3
• DNA Polymerase continues until all the nucleotides have been added
• Two new identical molecules of DNA are formed
• Another enzyme connects all the pieces of DNA together
Proofreading• At the end of
Replication, DNA polymerase goes through the DNA to make sure there are no errors
• This prevents mistakes in the DNA sequences
• Mutations- Change in the sequence of the DNA