DNA Chapter 12 DNA

Mendel’s work and experiments created more questions and sparked scientists to start researching heredity with a more molecular approach.

DNA – Deoxyribonucleic Acid How do we know this is the genetic material? Genes consist of DNA – Proven by a series of experiments 1. Frederick Griffith in 1928 did the following experiment. He treated mice with 2 different strains of pneumonia bacteria.

Griffith’s Experiments Two different strands of pneumonia bacteria S (smooth)Bacteria – virulent- able to cause disease – Has a protective coat around it R (rough) Bacteria – does not cause disease – Does not have a protective coat around it

A culture taken from the dead mouse showed a mixture of living rough and smooth bacteria

Griffith’s Conclusions Harmless R bacteria became harmful when mixed with the S bacteria that caused disease. A process where the harmless bacteria is changed into the harmful bacteria is called transformation The cause of transformation was still was not known which leads us to the next experiment…

2. Oswald Avery, in 1944, with two colleagues at Rockefeller University did an experiment and discovered that DNA was the material responsible for the transformation.

He extracted DNA from the coated bacteria (S) and put it into the uncoated bacteria (R). Some of the uncoated bacteria grew coats In a second experiment it was found that if the DNA was destroyed, transformation to new bacteria did not occur.

Scientists were still skeptical, knew little about DNA and still believed that protein carried genetic material until the Hershey- Chase experiment.

3. Hershey-Chase Experiment In 1952 Alfred Hershey and Martha Chase used viruses that infect bacteria Bacteriophage – a virus that infects bacteria Protein Coat Nucleic Acid

Conclusions Hershey and Chase concluded that the DNA of viruses was injected into bacterial cells while most of the viral protein remained outside This confirmed Avery’s results and convinced many scientists that DNA was the genetic material found in genes, not just bacteria and viruses, but in all living cells.

The Role of DNA The DNA that makes up genes must be capable of storing copying transmitting the genetic information in a cell.

The Structure of DNA DNA is a nucleic acid The elements that make up nucleic acids are ___, ___, ___, ___, ___, The monomers of nucleic acids are __________. Nucleotide - made up of a sugar, a nitrogen base, and a phosphate group. DNA is like a twisted ladder and is called a double helix. nucleotides

A nucleotide: many of these make up DNA

1. Adenine 2. Guanine 3. Thymine 4. Cytosine The Four Bases

There are four different nucleotides that make up DNA. They are identical except for their bases.

THE DNA LADDER Uprights = sugars and phosphates Each rung = 2 bases connected by H bonds

The backbone of the ladder is made of the sugar and phosphate of the nucleotides. The rungs are two nitrogen bases held together by hydrogen bonds.

COMPLIMENTARY BASES Adenine ALWAYS binds with Thymine. Guanine ALWAYS binds with Cytosine.

Note: because of their molecular structure…. Adenine forms 2 hydrogen bonds with __________. Cytosine forms 3 hydrogen bonds with ____________.

CG G A A C What is the complementary strand? P P P S S S P P P S S S T T SS

Figure 16.5 The double helix

= Adenine = Thymine = Cytosine = Guanine = Phosphate = deoxyribose

Phosphate Sugar Base – H - Base Sugar Phosphate Bonds Adenine and Thymine are Connected by 2 Hydrogen bonds And Cytosine and Guanine are Connected by 3 hydrogen bonds

Note the Hydrogen bonds Between the bases

DNA Structure video: – Watch me! Watch me! – And me! And me!

Discovering DNA’s Structure Erwin Chargaff found that the amount of adenine always equaled the amount of _______________ and that the amount of cytosine always equaled the amount of _____________ in EVERY organism he studied. He found which base pairs paired together

Relative Proportions (%) of Bases in DNA OrganismATGC Human Chicken Grasshopper Sea Urchin Wheat Yeast E. coli

Using Chargaff's observations, in 1952 Rosalind Franklin and Maurice Wilkins used x-rays that indicated that DNA was a tightly coiled helix composed of two strains of nucleotides.

Explanation of x ray diffraction The angle of the X in the picture indicates that the nitrogenous bases are near the center of the DNA molecule

In the 1950’s James Watson and Francis Crick built a model of DNA’s structure using information from Chargaff and Franklin’s work Built a DNA model with the configuration of a double helix a “spiral staircase” of two strands of nucleotides twisting around a central axis

Review 1.Who discovered the base pairing rules? 2. Cytosine will always pair with _______ 3. Nitrogen bases are held together by _________ 4. Who discovered that DNA was in the shape of a helix? 5. The nitrogen base pairs are ___, ___, ___, ___ 6. The backbone of DNA is made of ___________ 7. T or F The sequence of bases on one strand of DNA determines the sequence of bases on the other strand. 8. A nucleotide is made up of _______, _______, and ________

The closer one species DNA code is to another species DNA code, the more closely the two species are related. If two organisms are of the same species, the closer the DNA code is to another organisms DNA code, the closer the family relationship. Interesting fact:

ScientistsExperimentResults Frederick Griffith Oswald Avery Alfred Hershey and Martha Chase Erwin Chargaff Rosalind Franklin and Maurice Wilkins James Watson and Francis Crick

DNA Replication Before the cell divides it duplicates its DNA in a process called replication. Why does DNA need to replicate? When does DNA replicate?

Enzymes play a role: DNA must unwind The enzyme DNA helicase unzips the DNA by breaking the hydrogen bonds that connect the nucleotides. Helicase

DNA polymerase adds nucleotides to the exposed nitrogen bases Two new double helixes are formed Process continues until all DNA is copied Each DNA molecule is composed of one new and one old strand.

Sometimes the wrong nucleotide is added to a new strand of DNA Proof reading enzymes scan the DNA and fix errors that have occurred during the replication process.

Replication fork: The place where DNA starts to unzip. There can be multiple replication forks throughout DNA

Now lets see it… Basic DNA replication DNA replication Real time

How fast is DNA Replication? If there was only one replication fork the replication process would take _________. We know that this is too long due to the fact that cells divide and repair themselves pretty quickly. Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides 33 days

So with multiple replication forks working at the same time, DNA can be copied in about ________. 8 hours

Telomeres The tips of chromosomes are known as telomeres and are difficult to copy. The enzyme telomerase compensates for this problem and adds short, repeated DNA sequences to telomeres, lengthening the chromosomes slightly and making it less likely that important gene sequences will be lost from the telomeres during replication.

DNA Replication in Prokaryotes Regulatory proteins bind to a single starting point on a chromosome Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied.

DNA Replication in Eukaryotes Replication can begin in many places on the DNA molecule, going in both directions until the entire chromosome is copied.