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DNA replication

the process of making a copy of DNA

What's the point of DNA replication?

To provide genetic information for the next cell.

DNA helicase

an enzyme that unwinds and unzips the DNA (double helix) during DNA replication

Replication fork

a Y shaped point that results when the the 2 strands of DNA (double helix) seperates so that the DNA molecule can b seperated

DNA polymerase

an enzyme that catalyzes the formation of the DNA molecule, it adds nucleotides to exposed nitrogen bases.

When does DNA replication occur?

During the synthesis (S) phase.

STEP 1 of DNA replication

DNA helicases unwind the DNa strands by breaking hydrogen bonds that link complementary nitrogen bases. When seperated proteins attach to each strand to keep them apart.

Are hydrogen bonds weak or stong? Why?

Weak. It makes it easier for the DNA to unattach and break apart, making the DNA relication faster.

STEP 2 of DNA replication

Two new double helix's are formed.

How does step 2 occur?

At the replication forks polymerases move along each strand, adding nucleotides to he nitrogen bases (according to base pairing rules).

STEP 3 of DNA replication

The process in step 2 ccurs until all of DNA has been copied and polymerases are signaled to detach. It produces 2 DNA molecuels composed of 1 new and 1 original strand.(nucleotide sequences are identical)

What checks for errors?

DNA polymerases, they have a "proofreading" role

How does the DNA polymerases check for errors and fix them?

it can only add nucleotides to a growing strand if the previous nucleotide is correctly paired to its complementary base, if there is a mismatched nucleotide, it back tracks and replaces the incorrect pairing with the right one.

Double helix

the spiral-staircase structure characteristic of the DNA molecule


a submit consisting of a sugar, a phosphate, and a nitrogen base

Where is the nucleotide found?

In a nucleic acid chain


a five-carbon sugar that is a component of DNA nucleotides

Base-pairing rules

it states that cytosine pairs with guanine and adenine pairs with thymine in DNA. Adenine pairs with uracil in RNA

complementary base-pairing

a characteristic of nucleic acids in which the sequenceof bases on one trands is paired with the other sequence of bases on a seperate strand.


nitrogen bases made off two rings of carbon and nitrogen atoms

2 examples of purines

Adenine and Guanine


nitrogen bases made of a single ring of carbon and nitrogen atoms

2 examples of pyrimidines

Cytosine and Thymine

James watson, and Fancis Crick

pieced together model of DNA structure, They found out that DNA molecules are composed of double helixes and each strand is made up of linked nucleotides

Chargaff's observtions

for each organism studied, the amount of A always equaled T and the amount of G always equaled C, the amount of each base varied between different organisms

X-ray diffraction

Scientists used this to study the structures of molecules

The way x-ray diffraction works

x-rays bounce off the object and are scattered in a patteren onto a piece of film

How can scientists determine structure of the molecule?

By analyzing the patterns

Maurice Wilkins an Rosalind Frankiln

developed high-quality X-ray diffraction pictures of DNA strands, suggestng that it resembled a tightly coiled helix and was composed of two or three strands of nuleotides

Wton an crick determined...

that purine on one strand of DNA always goes with a pyrimidine on the opposite strand

Pairing between bases

adenine forms 2 hydrogen bonds with thymine, cytosine forms 3 hydrogen bonds with guanine


circular DNA found in bacteria

(true or false) Repliation begins at one end of DNA molecule and ends at the other end


Prokaryote DNA repliaton

2 replicated forks that begin at a single point and the replicating forks move away from eachother until they meet the opposite side of the DNA circle

Eukaryotic cells

each chromosome contains a single long strand of DNA

length of DNA makes it a challenge

the replication of a typiclal human chromosome with one pair of replication forks spreading rom a single point takes 33 days

Human chromosome replication

each chromo is replicated in 100 sections that are 100000 nucleotides long with a different starting point

An entire human chromome can be replicated in...

8 hours

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