DNA and the Language of Life Chapter 10 and 11

Virus Experiments Provide More Evidence Hershey and Chase – Used radioactive isotopes on virus components to further confirm that it was DNA that transformed cells and contain hereditary information

DNA by the numbers Each cell has about 2 m of DNA. The average human has over a trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only m.

DNA DNA is often called the blueprint of life. In simple terms, DNA contains the instructions for making proteins within the cell.

Why do we study DNA? We study DNA for many reasons its central importance to all life on Earth medical benefits such as cures for diseases better food crops

Nucleotides Building blocks of nucleic acid polymers Structure 1.Ring-shaped sugar 2.Phosphate group 3.Nitrogenous base

Nucleotides Pyrimidines –Single ring 1.Thymine 2.Cytosine Purines –Double ring 1.Adenine 2.Guanine

Sugar-phosphate backbone

Discoverers of DNA Structure 1950s Rosalind Franklin and Maurice Wilkins James Watson and Francis Crick Nobel Prize in 1953

The Shape of the Molecule DNA is a very long polymer. The basic shape is like a twisted ladder or zipper. This is called a double helix.

Complimentary Base Pairs Individual structures of the nitrogenous bases determine very specific pairings between the two strands Adenine pairs with Thymine (A is complimentary with T) Guanine pairs with Cytosine (G is complimentary with C) The sequence of nucleotides along the length of one side determine the sequence of nucleotides along the length of the other side

C C C C N N O N C C C C N N O N N N C Hydrogen Bonds The bases attract each other because of hydrogen bonds. Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA. (The bonds between cytosine and guanine are shown here.)

DNA Replicates Replication – makes a copy of itself Occurs just before cell division Necessary so that each new cell gets a copy of the parent cell’s DNA Occurs in nucleus during interphase

Replication Steps in DNA replication: 1. Double helix unwinds, then unzips. 2.Nucleotides on each of two single strands are now available to form base pairs with their complement from a new free-floating nucleotide. 3.Both original strands can be copied, making two double helices from one original. 4.The new helices are composed of half old (original) and half new nucleotides.

Replication is complex, but catalyzed by enzymes a) The addition and matching of new bases is catalyzed by DNA polymerase. b) DNA ligase permanently attaches short sections together to make one long chromosome.

DNA is now copied so mitosis or meiosis can take place.

George Beadle and Edward Tatum Orange bread mold The function of an individual gene is to dictate the production of a specific enzyme

Let’s Review! Where is the DNA stored? What is the membrane around that organelle called? What are the openings in that membrane that lead to other parts of the cell called? What organelle makes proteins?

While DNA has the information for all the cell’s activities, it is not directly involved in the day to day operations of the cell. –Proteins are responsible for implementing the instructions contained in DNA Each gene along a DNA molecule directs the synthesis of a specific type of messenger RNA molecule (mRNA). The mRNA interacts with the protein- synthesizing machinery to direct the ordering of amino acids in a polypeptide. Steps from DNA to Proteins

RNA= ribonucleic acid Ribonucleic acid Ribose= Single stranded Needed for the production of proteins Nitrogenous base uracil instead of thymine

Steps from DNA to Proteins 1.DNA is copied or transcribed into a single strand of mRNA (messenger RNA) 2.The mRNA exits the nucleus through a nuclear pore 3.The mRNA then is read by Ribosomal RNA. 4.In the ribosome, the mRNA is read and amino acids attached to tRNA (transfer RNA) are assembled to make proteins.

DNA -> RNA -> Protein

Genetic code DNA contains a triplet code Every three bases on a DNA strand code for one amino acid Each three-letter unit on mRNA is called a codon Some amino acids can have more than one codon The code is nearly universal to all living organisms –All animals, plants, fungi, bacteria, archaea, and viruses use this same genetic code. –All these organisms evolved from the same chemical basis, so the same chemical process continued through evolution.

Transcription= process in which DNA’s nucleotide sequence is converted to the form of a single-stranded RNA molecule mRNA= messenger RNA –RNA molecule transcribed from a DNA template DNA molecule unzips RNA bases pair with the complementary DNA bases RNA has uracil instead of thymine RNA polymerases links the RNA nucleotides together Transcription

RNA polymerase  Enzyme found in the nucleus  Separates the two DNA strands by breaking the hydrogen bonds between the bases  Then moves along one of the DNA strands and links RNA nucleotides together

In prokaryotes, the mRNA transcribed from a gene is the direct messenger molecule needed to make a protein In eukaryotes, the mRNA has to be modified before it leaves the nucleus Noncoding nucleotides interrupt the nucleotide sequences Internal noncoding sequences are called introns Researchers are still trying to determine their function The coding regions, or the parts of the gene that will be expressed are called exons Before mRNA leaves the nucleus, the introns are removed and the exons are connected together in a process called RNA splicing

Translation tRNA= transfer RNA, translates the three-letter codons of mRNA to the amino acids that make up proteins (an “interpreter”) Picks up the appropriate amino acid floating in the cytoplasm Transports amino acids to the mRNA Have anticodons that are complementary to mRNA codons Recognizes the appropriate codons on the mRNA and bonds to them with Hydrogen bonds (there is a different version of tRNA molecule that matches each codon)

amino acid attachment site UAC anticodon

Ribosome Coordinates the functioning of mRNA and tRNA Consists of two subunits made up of proteins and rRNA (ribosomal RNA) –Small subunit with a binding site for mRNA –Large subunit with a binding site for tRNA rRNA= made in nucleolus, and functions to decode the mRNA into amino acids The subunits of the ribosome act as a vise holding the mRNA and tRNA close together The ribosome connects the newly arrived amino acid to the growing polypeptide chain

Mutations A. Mutation = permanent alteration in cell’s DNA base sequence B. Almost all cancers begin as a mutation that is passed along at replication. 1.In somatic cells (body cells) 2.Mutation rate is low, but after decades of accumulated mutations, cells can become malignant.

C.Heritable mutations occur in germ-line cells (cells that divide to make sperm and eggs). D.Heritable mutations also create genetic diversity.

Accidents Happen With Some “Accidents” (Base Mismatches) Leading to Mutation A mutation is a heritable change in DNA sequence. X rays, ultraviolet light, and radioactive substances can change the chemical nature of DNA, and are classified as mutagens.