KEY CONCEPT DNA structure is the same in all organisms.

To truly understand genetics, scientists realized they had to discover the chemical nature of the gene. If the molecule that carries genetic information could be identified, it might be possible to understand how genes control the inherited characteristics of living things. The discovery of the chemical nature of the gene began in 1928 with British scientist Frederick Griffith, who was trying to figure out how certain types of bacteria produce pneumonia.

Griffith’s Experiments Griffith isolated two different strains of the same bacterial species. Both strains grew very well in culture plates in Griffith’s lab, but only one of the strains caused pneumonia. The disease-causing bacteria (S strain) grew into smooth colonies on culture plates, whereas the harmless bacteria (R strain) produced colonies with rough edges.

Griffith injected mice with the disease causing bacteria and the mice developed pneumonia and died. He then injected mice with a combination of heat killed bacteria and harmless bacteria. The harmless bacteria had transformed (changed permanently) into another form, the disease causing bacteria. He concluded that the transforming factor had to be a gene, because it was inherited by the offspring of the transformed bacteria.

Oswald Avery wanted to continue Griffith’s work and determine which molecule was the most important for the transformation. Avery and his team extracted a mixture of various molecules from the heat =killed bacteria. They treated the mixture with enzymes that destroyed proteins, lipids, carbohydrates and some other molecules, including RNA. Transformation still occurred. They finally used an enzyme that would break down DNA, when they destroyed the DNA in the mixture, transformation did not occur. This lead them to the explanation that the DNA was the transforming factor.

Alfred Hershey and Martha Chase collaborated in studying viruses and conducted most important experiment relating to Avery’s team’s research to further verify Avery’s results. Hershey and Chase studied bacteriophages (viruses that only attack bacteria) that was composed of a DNA core and a protein coat. They wanted to determine which part of the virus entered the bacterial cell. They grew viruses in cultures with radioactive isotopes that could be used as markers to help the scientists identify exactly which molecules actually enter the bacteria. Their results showed that nearly all of the radioactivity in the bacteria was the P-32 the marker found in the DNA.

DNA is short for deoxyribonucleic acid It must be capable of storing, copying and transmitting genetic information in a cell. 7

DNA is composed of four types of nucleotides. DNA is made up of a long chain of nucleotides. Each nucleotide has three parts. a phosphate group a deoxyribose sugar a nitrogen-containing base phosphate group deoxyribose (sugar) nitrogen-containing base

2 categories of nitrogenous bases: The nitrogen containing bases are the only difference in the four nucleotides. 2 categories of nitrogenous bases: Pyrimidines: single ring, ex. thymine and cytosine Purines: double ring, ex. adenine and guanine 9

Chargaff’s rule states that the amount of thymine = adenine and that the amount of cytosine = guanine in DNA. He discovered that the percentages of A and T are almost equal in any sample of DNA and that the same is true for C and G. 10

In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule. X-ray diffraction revealed an X-shaped pattern showing that the strands in DNA are twisted around each other like the coils of a spring. The angle of the X-shaped pattern suggested that there are two strands in the structure. Other clues suggest that the nitrogenous bases are near the center of the DNA molecule.

Franklin’s x-ray images suggested that DNA was a double helix of even width. Watson and Crick’s discovery built on the work of Rosalind Franklin and Erwin Chargaff.

Watson and Crick determined the three-dimensional structure of DNA by building models. They realized that DNA is a double helix that is made up of a sugar-phosphate backbone on the outside with bases on the inside.

Nucleotides always pair in the same way. The base-pairing rules show how nucleotides always pair up in DNA. A pairs with T C pairs with G Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width. C G T A

Bonding in DNA Phosphate group ↓ Deoxyribose Sugar → hydrogen bond covalent bond Deoxyribose Sugar → The DNA backbone: alternating deoxyribose (sugar) and phosphate group is connected by strong covalent bonds. The nitrogen bases are connected by hydrogen bonds.

hydrogen bond covalent bond T G A C T A | | | | | | A C T G A T 16

8.3 KEY CONCEPT DNA replication copies the genetic information of a cell. 17

Replication copies the genetic information. A single strand of DNA serves as a template for a new strand. The rules of base pairing direct replication. DNA is replicated during the S (synthesis) stage of the cell cycle. Each body cell gets a complete set of identical DNA. 18

Proteins carry out the process of replication. DNA serves only as a template. Enzymes and other proteins do the actual work of replication. DNA helicase unzip the double helix. The site of this is called the replication fork. Free-floating nucleotides form hydrogen bonds with the template strand. nucleotide The DNA molecule unzips in both directions. 19

DNA polymerase enzymes bond the nucleotides together to form the double helix. Polymerase enzymes form covalent bonds between nucleotides in the new strand. DNA polymerase new strand nucleotide 20

DNA replication is semiconservative. Two new molecules of DNA are formed, each with an original strand and a newly formed stran DNA replication is semiconservative. original strand new strand Two molecules of DNA 21

Replication is fast and accurate. DNA replication starts at many points in eukaryotic chromosomes. There are many origins of replication in eukaryotic chromosomes. DNA polymerases can find and correct errors. 22

Prokaryotic DNA Replication In most prokaryotes, DNA replication does not start until regulatory proteins bind to a single starting point on the chromosome. This triggers the beginning of DNA replication. Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied.

Prokaryotic DNA Replication Often, the two chromosomes produced by replication are attached to different points inside the cell membrane and are separated when the cell splits to form two new cells.

Eukaryotic DNA Replication Eukaryotic chromosomes are generally much bigger than those of prokaryotes. In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied.

8.4-8.5 CONCEPT Transcription and Translation 26

RNA carries DNA’s instructions. The central dogma states that information flows in one direction from DNA to RNA to proteins. 27

The central dogma includes three processes. Replication Transcription Translation replication transcription translation RNA is a link between DNA and proteins. 28

RNA differs from DNA in three major ways. RNA has a ribose sugar. RNA has uracil instead of thymine. RNA is a single-stranded structure. 29

Transcription copies a specific gene in DNA to make a strand of mRNA. RNA polymerase and other proteins form a transcription complex. start site nucleotides transcription complex 30

RNA polymerase moves along the DNA RNA nucleotides pair with one strand of the DNA. RNA polymerase bonds the nucleotides together into a messenger RNA (mRNA) molecule. The DNA helix winds again as the gene is transcribed. DNA RNA polymerase moves along the DNA 31

The mRNA strand detaches from the DNA once the gene is transcribed The mRNA strand detaches from the DNA once the gene is transcribed. The mRNA then exits the nucleus to begin translation. RNA 32

Amino acids are coded by mRNA base sequences. Translation converts mRNA messages into proteins. A codon is a sequence of three nucleotides that codes for an amino acid. Regardless of the organism, codons code for the same amino acid. codon for methionine (Met) leucine (Leu) 33

The genetic code matches each codon to its amino acid or function. The genetic code matches each RNA codon with its amino acid or function. three stop codons one start codon, codes for methionine 34

Three types of RNA are used in translation: Messenger RNA (mRNA) carries the message that will be translated to form a protein. Ribosomal RNA (rRNA) forms part of ribosomes where proteins are made. Transfer RNA (tRNA) brings amino acids from the cytoplasm to a ribosome. 35

For translation to begin, mRNA enters into a ribosome. 36

Ribosomes consist of two subunits. The large subunit has three binding sites for tRNA. The small subunit binds to mRNA. 37

tRNA binds to a start codon and signals the ribosome to assemble tRNA binds to a start codon and signals the ribosome to assemble. The tRNA must match the start codon on the mRNA. 38

An anticodon carried by a tRNA is a set of three nucleotides that is complementary to an mRNA codon. 39

A complementary tRNA molecule binds to the exposed codon, bringing its amino acid close to the first amino acid. 40

The ribosome helps form a polypeptide bond between the amino acids. The ribosome pulls the mRNA strand the length of one codon. 41

The now empty tRNA molecule exits the ribosome. A complementary tRNA molecule binds to the next exposed codon. Once the stop codon is reached, the ribosome releases the protein and disassembles. 42

8.6 KEY CONCEPT Gene expression is carefully regulated in both prokaryotic and eukaryotic cells. 43

Transcription is controlled by regulatory DNA sequences and protein transcription factors. 44

Prokaryotic cells turn genes on and off by controlling transcription. An operon includes a promoter, an operator, and one or more structural genes that code for all the proteins needed to do a job. A promotor is a DNA segment that allows a gene to be transcribed. An operator is a part of DNA that turns a gene “on” or ”off.” 45

mRNA processing includes three major steps. RNA processing is also an important part of gene regulation in eukaryotes. mRNA processing includes three major steps. 46

A change in the order in which codons are read changes the resulting protein. 47

A mutation is a change in an organism’s DNA. Some mutations affect a single gene, while others affect an entire chromosome. mutated base 48

A point mutation substitutes one nucleotide for another. Gene Mutations: A point mutation substitutes one nucleotide for another. mutated base 49

A frameshift mutation inserts or deletes a nucleotide in the DNA sequence. 50

Chromosomal mutations affect many genes. Chromosomal mutations may occur during crossing over Chromosomal mutations affect many genes. 51

Inversion results from the reversal of DNA segments on the same chromosome. Duplication results from the duplication of one or more segments on the same chromosomes Deletion results from the deletion of one or more segments on the same chromosomes Translocation results from the exchange of DNA segments between nonhomologous chromosomes. 52

Mutations may or may not affect phenotype. Chromosomal mutations tend to have a big effect. Some gene mutations change phenotype. A mutation may cause a premature stop codon. A mutation may change protein shape or the active site. A mutation may change gene regulation. blockage no blockage 53

Some gene mutations do not affect phenotype. A mutation may be silent meaning it occurs in a noncoding region. A mutation may not affect protein folding or the active site. 54

Mutations in body cells do not affect offspring. Mutations in sex cells can be harmful or beneficial to offspring. Natural selection often removes mutant alleles from a population when they are less adaptive. 55

Mutations can be caused by several factors. Replication errors can cause mutations. Mutagens, such as UV ray and chemicals, can cause mutations. Some cancer drugs use mutagenic properties to kill cancer cells. 56