DNA Structure

Frederick Griffith In 1928, Frederick Griffith wanted to learn how certain types of bacteria produce pneumonia Griffith injected mice with disease-causing bacteria, the mice died of pneumonia Griffith injected mice with harmless bacteria, the mice didn’t get sick Griffith thought that the disease-causing bacteria might produce a poison

Frederick Griffith Griffith mixed heat-killed disease-causing bacteria with harmless bacteria and injected the mixture into mice To his amazement, the mice developed bacteria and many died He found that the lungs of those mice who died were not filled with harmless bacteria, but of disease-causing bacteria

Frederick Griffith The heat-killed bacteria had passed their disease-causing ability to the harmless strain Griffith coined this process transformation because one strain of bacteria had been changed into another Griffith hypothesized that when the two types of bacteria were mixed, some factor was transformed from the heat-killed cells into the live harmless bacteria

Frederick Griffith Griffith discovered that the ability to cause disease was inherited by the transformed bacteria’s offspring, the transforming factor was a gene

Oswald Avery In 1944, a group of scientists repeated Griffith’s work Avery and his colleagues made an extract from the heat killed bacteria and treated the extract with enzymes that destroyed proteins, carbohydrates, lipids, and other molecules Since transformation still occurred, these molecules were not responsible for the transformation

Oswald Avery Avery and other scientists repeated the experiment, this time using enzymes that targeted DNA When they destroyed the nucleic acid DNA, transformation did not occur Avery and other scientists discovered that the nucleic acid DNA stores and transmits the genetic information from one generation of an organism to the next

The Hershey-Chase Experiment In 1952, American scientists Alfred Hershey and Martha Chase collaborated in studying viruses One type of virus that infects bacteria is known as a bacteriophage Bacteriophages are composed of a DNA or RNA core and a protein coat When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it

The Hershey-Chase Experiment Hershey and Chase reasoned that if they could determine which part of the virus entered the infected cell, they would learn whether genes were made of protein or DNA Viruses were grown in cultures containing radioactive isotopes of phosphorus-32 ( 32 P) and sulfur-35 ( 32 S) Proteins contain almost no phosphorus and DNA contains no sulfur

The radioactive substances were used as markers If 32 S was found in the bacteria, it would mean that the viruses’ bacteria had been injected into the bacteria If 32 P was found in the bacteria, then it was the DNA that had been injected The Hershey-Chase Experiment

Marked viruses were mixed with bacteria, after a few minutes viruses injected their genetic material Viruses were separated were from bacteria, bacteria was tested for radioactivity Nearly all the radioactivity in the bacteria was from phosphorus, the marker found in DNA Hershey and Chase demonstrated that DNA, not protein, functions as the phage’s genetic material The Hershey-Chase Experiment

The Components and Structure of DNA DNA is made up of nucleotides Nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base

There are four kinds of nitrogenous bases in DNA Adenine and Guanine are two nitrogenous bases that belong in a group of compounds known as purines Cytosine and Thymine are known as pyrimidines Purines have two rings in their structures, whereas pyrimidines have one ring The Components and Structure of DNA

Adenine, guanine, cytosine, and thymine are four kinds of _________ bases in DNA. Nitrogenous

The phosphate of one nucleotide is attached to the sugar of the next nucleotide in line The result is a “backbone” of alternating phosphates and sugars The Components and Structure of DNA

Erwin Chargaff discovered that the percentages of guanine and cytosine bases are almost equal in any sample of DNA The same thing is true for the other two nucleotides, adenine and thymine The observation that [A]=[T] and [G]=[C] became known as Chargaff’s rules The Components and Structure of DNA

According to Chargaff’s rules, the percentages of ______are equal to thymine and the percentages of ______ are equal to guanine in the DNA molecule. The Components and Structure of DNA adenine cytosine

Describe the components and structure of a DNA nucleotide. DNA is made of nucleotides. Each nucleotide has three parts: a 5-carbon sugar, a phosphate group, and a nitrogenous base The four nitrogenous bases are adenine and guanine (purines), cytosine and thymine (pyrimidines).

The Components and Structure of DNA Francis Crick and James Watson were trying to understand the structure of DNA by building 3 dimensional models of the molecule Watson was shown a copy of Rosalind Franklin’s X-ray pattern of DNA

Using clues from Franklin’s pattern, Watson and Crick had built a structural model that explained the puzzle of how DNA could carry information, and how it could be copied Watson and Crick’s model of DNA was a double helix in which two strands were wound around each other The Components and Structure of DNA

Explain how Chargaff’s rules helped Watson and Crick model DNA. The Components and Structure of DNA The two stands of DNA are held together by hydrogen bonds between certain bases-A and T, and G and C-which explained Chargaff’s rules.

The Double Helix The two strands are held together by hydrogen bonds between the nitrogenous bases, which are paired in the interior of the double helix Hydrogen bonds can form only between certain base pairs-adenine and thymine, and guanine and cytosine. Base pairing principle that bonds DNA can form only between adenine and thymine and between cytosine and guanine

DNA and Chromosomes Eukaryotic chromosomes contain both DNA and proteins, packed together to form chromatin Eukaryotic chromosomes contain DNA wrapped around proteins called histones (globular proteins that DNA tightly coils around to form chromosomes)

DNA Replication The parent molecule has two complimentary strands of DNA The first step in replication is separation of the two DNA strands Each parent strand now serves as a template that determines the order of nucleotides along a new complementary strand Each parent stand produces two new complimentary strands following the rules of base pairing

DNA Replication What is the complimentary strand of bases for a strand with the bases TACGTT?  ATGCAA

DNA Replication

The replication of a DNA molecule begins at special sites called origins of replication Proteins recognize a stretch of DNA having a specific sequence of nucleotides These proteins attach to the DNA and separate the two strands and open up a replication “bubble”

DNA Replication Once the two strands of the double helix have separated, two replication forks (a Y-shaped where the new strands of the DNA are elongating) form at the end of a replication “bubble” As each new strand forms, new bases are added following the rules of base pairing

DNA Replication Elongation of new DNA at the replication fork is catalyzed by enzymes called DNA polymerases (enzymes that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain)

DNA Replication The DNA molecule _______, or unzips, into two strands ↓ Each strand of the DNA molecule serves as a _______, or model, to produce the new strands ↓ Two new ___________ strands are produced, following the rules of _________ separates template complimentary Base pairing

DNA Replication

The two strands of DNA are anti-parallel (their sugar phosphate backbones run in opposite directions) In the double helix, the two sugar-phosphate backbones are upside down relative to each other

DNA Replication The 5’ to 3’ direction of one strand runs counter to the 5’ to 3’ of the other

DNA Replication DNA polymerases only add nucleotides to the free 3’ end of a growing DNA strand, never to the 5’ end A new DNA strand can only elongate in the 5’ to the 3’ direction

DNA Replication DNA polymerase adds nucleotides to a complimentary strand as the replication fork progresses The DNA strand made by this mechanism is called the leading strand

DNA Replication To elongate the other new strand of DNA, DNA polymerase works along the other template strand in the direction away from the replication fork DNA synthesized in this direction is called the lagging strand