The Structure of DNAThe Structure of DNA

Ag BiologyAg Biology

What is DNA?What is DNA?

• Deoxyribonucleic AcidDeoxyribonucleic Acid• Structure that stores hereditary Structure that stores hereditary

materialmaterial• By 1950, most scientist accepted By 1950, most scientist accepted

genes were made of DNA but did genes were made of DNA but did not know what it looked likenot know what it looked like

Watson and Crick’s ModelWatson and Crick’s Model

• James Watson and Francis Crick James Watson and Francis Crick discovered that the structure of DNA discovered that the structure of DNA was important to determining how was important to determining how DNA serves as the genetic material. DNA serves as the genetic material.

• They found that DNA is a molecule They found that DNA is a molecule that is a that is a double helixdouble helix• Two strands twisted around each otherTwo strands twisted around each other

DNA StructureDNA Structure

• Each strand is made of Each strand is made of nucleotidesnucleotides

• A nucleotides are made of three A nucleotides are made of three parts:parts:1.1. PhosphatePhosphate

2.2. Five-carbon Sugar called deoxyriboseFive-carbon Sugar called deoxyribose

3.3. Nitrogen baseNitrogen base

Nitrogen BasesNitrogen Bases

• There are four different kinds There are four different kinds nitrogen bases that make up nitrogen bases that make up DNA:DNA:

1.1. Adenine (A)Adenine (A)

2.2. Guanine (G)Guanine (G)

3.3. Thymine (T)Thymine (T)

4.4. Cytosine (C) Cytosine (C)

Nitrogen BasesNitrogen Bases

• Adenine and Guanine are Adenine and Guanine are classified as classified as purinespurines (made (made of two rings of carbon and of two rings of carbon and nitrogen atoms)nitrogen atoms)

• Thymine and Cytosine are Thymine and Cytosine are classified as classified as pyrimidinespyrimidines (made of a single ring of (made of a single ring of carbon and nitrogen atoms)carbon and nitrogen atoms)

Base-paring RulesBase-paring Rules

• Bases are always paired Bases are always paired with a pyrimidine and with a pyrimidine and purine.purine.• Adenine is Adenine is alwaysalways paired paired

with Thyminewith Thymine• Guanine is Guanine is alwaysalways paired paired

with Cytosinewith Cytosine

Hydrogen BondingHydrogen Bonding

• The bases are connected with their The bases are connected with their pair by hydrogen bonding which also pair by hydrogen bonding which also keeping the two strands of DNA keeping the two strands of DNA together. together.

How did this model come to How did this model come to be?be?

• Erwin Chargaff Erwin Chargaff • BiochemistBiochemist• In 1949, he observed that the amount of adenine In 1949, he observed that the amount of adenine

(A) always equaled the amount of thymine (T) and (A) always equaled the amount of thymine (T) and the amount of guanine (G) always equaled the the amount of guanine (G) always equaled the amount of cytosine (C). amount of cytosine (C).

• However, the numbers of each varied between However, the numbers of each varied between organismsorganisms

How did this model come to How did this model come to be?be?

• Maurice Wilkins and Rosalind Maurice Wilkins and Rosalind FranklinFranklin• Scientist from LondonScientist from London• Developed high quality X-ray Developed high quality X-ray

photographs of fibers of the DNA photographs of fibers of the DNA moleculemolecule

• Pictures suggested that DNA Pictures suggested that DNA resembled a tightly coiled helixresembled a tightly coiled helix

How did this model come to How did this model come to be?be?

• Watson and Crick’sWatson and Crick’s• From prior discoveries, Watson and From prior discoveries, Watson and

Crick developed a 3-D model using tin Crick developed a 3-D model using tin and wireand wire

The Replication of DNAThe Replication of DNA

Copying DNACopying DNA

• In order the cells to divide, In order the cells to divide, DNA must be copied by the DNA must be copied by the process of process of DNA ReplicationDNA Replication

• Watson and Cricks proposed Watson and Cricks proposed that DNA serves as a template that DNA serves as a template for building other strands.for building other strands.

• Occurs in 3 stepsOccurs in 3 steps

DNA Replication Step 1DNA Replication Step 1

• Double helix unwinds with the help of Double helix unwinds with the help of the DNA helicasesthe DNA helicases• DNA helicases are enzymes that break the DNA helicases are enzymes that break the

hydrogen bonds that link the nitrogen hydrogen bonds that link the nitrogen bases.bases.

• Additional proteins are attached to Additional proteins are attached to each strand to keep the strands apart each strand to keep the strands apart and from recoilingand from recoiling

• The two ends where the DNA separates The two ends where the DNA separates are called are called replication forksreplication forks

DNA Replication Step 2DNA Replication Step 2

• DNA polymerases DNA polymerases (enzymes) add (enzymes) add complementary complementary nucleotides to each strand nucleotides to each strand at the replication forks.at the replication forks.• Nucleotides are added Nucleotides are added

according to the base pairs according to the base pairs rule.rule.

DNA Replication Step 3DNA Replication Step 3

• Polymerase remain attached until the Polymerase remain attached until the DNA is completely copied and the DNA is completely copied and the signal for detach is given. signal for detach is given.

• Two DNA molecules form that are Two DNA molecules form that are identical to the original DNA.identical to the original DNA.• Each is composed of an original and a new Each is composed of an original and a new

stand.stand.• The nucleotide pattern is identical to the The nucleotide pattern is identical to the

original strands.original strands.

Checking for ErrorsChecking for Errors

• Sometimes errors occur during this process Sometimes errors occur during this process when the wrong nucleotides are added to the when the wrong nucleotides are added to the strand.strand.

• DNA polymmerases “proofread” by:DNA polymmerases “proofread” by:• Only being able to add nucleotides when the previous Only being able to add nucleotides when the previous

nucleotide is correctly paired with its basenucleotide is correctly paired with its base• Backtracking and removing incorrect nucleotides and Backtracking and removing incorrect nucleotides and

inserting correct onesinserting correct ones

• This prevents most errors– only 1 error occurs in This prevents most errors– only 1 error occurs in a billion nucleotides. This is called a mutation.a billion nucleotides. This is called a mutation.

Multiple Replication ForksMultiple Replication Forks• Replication does not begin at one end of the Replication does not begin at one end of the

DNA strand and end at the other.DNA strand and end at the other.• Since most eukaryotic cells have one long Since most eukaryotic cells have one long

DNA strands in each chromosome, it would DNA strands in each chromosome, it would take too long to go from one end to the other. take too long to go from one end to the other.

• Human DNA is replicated in about 100 Human DNA is replicated in about 100 sections sections • Each sections has a starting pointEach sections has a starting point• Since multiple forks work at the same time, the Since multiple forks work at the same time, the

entire chromosome can be replicated in 8 hours.entire chromosome can be replicated in 8 hours.