The Structure of DNA Ag Biology

What is DNA? Deoxyribonucleic AcidDeoxyribonucleic Acid Structure that stores hereditary materialStructure that stores hereditary material By 1950, most scientist accepted genes were made of DNA but did not know what it looked likeBy 1950, most scientist accepted genes were made of DNA but did not know what it looked like

Watson and Crick’s Model James Watson and Francis Crick discovered that the structure of DNA was important to determining how DNA serves as the genetic material.James Watson and Francis Crick discovered that the structure of DNA was important to determining how DNA serves as the genetic material. They found that DNA is a molecule that is a double helixThey found that DNA is a molecule that is a double helix Two strands twisted around each otherTwo strands twisted around each other

DNA Structure Each strand is made of nucleotidesEach strand is made of nucleotides A nucleotides are made of three parts:A nucleotides are made of three parts: 1.Phosphate 2.Five-carbon Sugar called deoxyribose 3.Nitrogen base

Nitrogen Bases There are four different kinds nitrogen bases that make up DNA:There are four different kinds nitrogen bases that make up DNA: 1.Adenine (A) 2.Guanine (G) 3.Thymine (T) 4.Cytosine (C)

Nitrogen Bases Adenine and Guanine are classified as purines (made of two rings of carbon and nitrogen atoms)Adenine and Guanine are classified as purines (made of two rings of carbon and nitrogen atoms) Thymine and Cytosine are classified as pyrimidines (made of a single ring of carbon and nitrogen atoms)Thymine and Cytosine are classified as pyrimidines (made of a single ring of carbon and nitrogen atoms)

Base-paring Rules Bases are always paired with a pyrimidine and purine.Bases are always paired with a pyrimidine and purine. Adenine is always paired with ThymineAdenine is always paired with Thymine Guanine is always paired with CytosineGuanine is always paired with Cytosine

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

How did this model come to be? Erwin ChargaffErwin Chargaff BiochemistBiochemist In 1949, he observed that the amount of adenine (A) always equaled the amount of thymine (T) and the amount of guanine (G) always equaled the amount of cytosine (C).In 1949, he observed that the amount of adenine (A) always equaled the amount of thymine (T) and the amount of guanine (G) always equaled the amount of cytosine (C). However, the numbers of each varied between organismsHowever, the numbers of each varied between organisms

How did this model come to be? Maurice Wilkins and Rosalind FranklinMaurice Wilkins and Rosalind Franklin Scientist from LondonScientist from London Developed high quality X-ray photographs of fibers of the DNA moleculeDeveloped high quality X-ray photographs of fibers of the DNA molecule Pictures suggested that DNA resembled a tightly coiled helixPictures suggested that DNA resembled a tightly coiled helix

How did this model come to be? Watson and Crick’sWatson and Crick’s From prior discoveries, Watson and Crick developed a 3-D model using tin and wireFrom prior discoveries, Watson and Crick developed a 3-D model using tin and wire

The Replication of DNA

Copying DNA In order the cells to divide, DNA must be copied by the process of DNA ReplicationIn order the cells to divide, DNA must be copied by the process of DNA Replication Watson and Cricks proposed that DNA serves as a template for building other strands.Watson and Cricks proposed that DNA serves as a template for building other strands. Occurs in 3 stepsOccurs in 3 steps

DNA Replication Step 1 Double helix unwinds with the help of the DNA helicasesDouble helix unwinds with the help of the DNA helicases DNA helicases are enzymes that break the hydrogen bonds that link the nitrogen bases.DNA helicases are enzymes that break the hydrogen bonds that link the nitrogen bases. Additional proteins are attached to each strand to keep the strands apart and from recoilingAdditional proteins are attached to each strand to keep the strands apart and from recoiling The two ends where the DNA separates are called replication forksThe two ends where the DNA separates are called replication forks

DNA Replication Step 2 DNA polymerases (enzymes) add complementary nucleotides to each strand at the replication forks.DNA polymerases (enzymes) add complementary nucleotides to each strand at the replication forks. Nucleotides are added according to the base pairs rule.Nucleotides are added according to the base pairs rule.

DNA Replication Step 3 Polymerase remain attached until the DNA is completely copied and the signal for detach is given.Polymerase remain attached until the DNA is completely copied and the signal for detach is given. Two DNA molecules form that are identical to the original DNA.Two DNA molecules form that are identical to the original DNA. Each is composed of an original and a new stand.Each is composed of an original and a new stand. The nucleotide pattern is identical to the original strands.The nucleotide pattern is identical to the original strands.

Checking for Errors Sometimes errors occur during this process when the wrong nucleotides are added to the strand.Sometimes errors occur during this process when the wrong nucleotides are added to the strand. DNA polymmerases “proofread” by:DNA polymmerases “proofread” by: Only being able to add nucleotides when the previous nucleotide is correctly paired with its baseOnly being able to add nucleotides when the previous nucleotide is correctly paired with its base Backtracking and removing incorrect nucleotides and inserting correct onesBacktracking and removing incorrect nucleotides and inserting correct ones This prevents most errors– only 1 error occurs in a billion nucleotides. This is called a mutation.This prevents most errors– only 1 error occurs in a billion nucleotides. This is called a mutation.

Multiple Replication Forks Replication does not begin at one end of the DNA strand and end at the other.Replication does not begin at one end of the DNA strand and end at the other. Since most eukaryotic cells have one long DNA strands in each chromosome, it would take too long to go from one end to the other.Since most eukaryotic cells have one long DNA strands in each chromosome, it would take too long to go from one end to the other. Human DNA is replicated in about 100 sectionsHuman DNA is replicated in about 100 sections Each sections has a starting pointEach sections has a starting point Since multiple forks work at the same time, the entire chromosome can be replicated in 8 hours.Since multiple forks work at the same time, the entire chromosome can be replicated in 8 hours.