C HAPTER 10 T HE S TRUCTURE AND F UNCTION OF DNA CM Lamberty General Biology

B IOLOGY AND S OCIETY Flu Virus The influenza virus is one of the deadliest pathogens in the world. Each year in the United States, over 20,000 people die from influenza infection. In the flu of 1918–1919, about 40 million people died worldwide. Vaccines against the flu are the best way to protect public health. Because flu viruses mutate quickly, new vaccines must be created every year.

DNA S TRUCTURE AND R EPLICATION Molecular Biology Was known to be a chemical in cells by the end of the nineteenth century Has the capacity to store genetic information Can be copied and passed from generation to generation

DNA AND RNA S TRUCTURE Nucleotides Polynucleotides Sugar-phosphate backbone DNA: 4 nucleotides Thymine, T Cytosine, C Adenine, A Guanine, G Uracil instead of thymine in RNA

W ATSON AND C RICK ( AND F RANKLIN ) Determination of DNA double helix structure From photo of X-ray image taken by Rosalind Franklin: diameter of helix is uniform, thickness suggested 2 polynucleotide strands Combined info from x-ray photos and known data Backbones allowed for the swivel in the interior Base-pairs for support originally AA or CC but that would cause bulges Became apparent for double ring with single ring Hydrogen bonding to link pairs dasd

R OPE L ADDER A NALOGY The model of DNA is like a rope ladder twisted into a spiral. The ropes at the sides represent the sugar-phosphate backbones. Each wooden rung represents a pair of bases connected by hydrogen bonds.

DNA R EPLICATION DNA Known to be a chemical in cells at end of 19 th century Has capacity to store genetic information Can be copied and passed from generation to generation DNA and RNA are nucleic acids Consist of chemical unit called nucleotides Nucleotides joined by sugar-phosphate backbone

DNA R EPLICATION When a cell reproduces, a complete copy of the DNA must be passed on Watson & Crick’s model suggested DNA replicates by a template mechanism

DNA R EPLICATION DNA can be damaged by UV light DNA polymerase: Are enzymes Make the covalent bond b/t nucleotides of a new DNA strand Are invovled in repairing damaged DNA DNA repliation in eukaryotes: Begins at specific sties on a double helix Proceeds in both directions

F LOW OF G ENETIC I NFORMATION DNA functions as the inherited directions for a cell or organism How are the directions carried out?

H OW G ENOTYPE DETERMINES P HENOTYPE An organism’s genotype is its genetic makeup, the sequence of nucleotide bases in DNA The phenotype is the organism’s physical traits, which arise from the actions of a wide variety of proteins DNA specifies synthesis of proteins in 2 stages: Transcription, the transfer of genetic information from DNA into an RNA molecule Translation, the transfer of information from RNA into a protein

DNA Cytoplasm Nucleus Figure

RNA TRANSCRIPTION DNA Cytoplasm Nucleus Figure

TRANSLATION Protein RNA TRANSCRIPTION DNA Cytoplasm Nucleus Figure

H OW G ENOTYPE DETERMINES P HENOTYPE The function of a gene is to dictate the production of polypeptide A protein may consist of two or more different polypeptides

F ROM N UCLEOTIDES TO A MINO A CIDS Genetic information in DNA is Transcribed into RNA then Translated into polypeptides What is the language of nucleic acids? In DNA, it is the linear sequence of nucleotide bases A typical gene consists of 1000s of nucleotides A single DNA molecule may contains 1000s of genes When DNA is transcribed, the result is an RNA RNA is then translated into sequence of amino acids into a polypeptide What are the rules for translating RNA message? A codon is a triplet of bases which codes for one amino acid

T HE G ENETIC C ODE Genetic code is the set of rules relating to nucleotide sequence to amino acid sequence Is shared by all organisms Of the 64 triplets 61 code for amino acids 3 are stop codons, indicating the end of the polypeptide

T RANSCRIPTION : FROM DNA TO RNA Transcription Makes RNA from a DNA template Uses a process that resembles DNA replication Substitutes uracil (U) for thymine (T) RNA nucleotides are linked by RNA polymerase Initiation The “start transcribing” signal is a nucleotide sequence called a promoter The 1 st phase of transcription is initiation in which RNA polymerase attaches to the promoter RNA synthesis begins

T RANSCRIPTION : FROM DNA TO RNA RNA Elongation 2 nd phase RNA grows longer RNA strand peels away form the DNA template Termination 3 rd phase RNA polymerases reaches a sequence of DNA bases called terminator Polymerases detaches from RNA The DNA strands rejoin

T HE P ROCESSING OF E UKARYOTIC RNA After transcription Eukaryotic cells process RNA Prokaryotic cells do not RNA processing include Adding a cap and tail Removing introns Splicing exons together to form messenger RNA (mRNA)

T RANSLATION : T HE P LAYERS Translation is the conversion from the nucleic acid language to the protein language Translation requires mRNA ATP Enzymes Ribosomes Transfer RNA (rRNA)

T RANSFER RNA ( T RNA) Acts as a molecular interpreter Carries amino acids Matches amino acids with codons in mRNA using anticodons

R IBOSOMES Organelles that Coordinate the functions of mRNA and tRNA Are made of two protein strands Contain ribosomal RNA (rRNA) A fully assembled ribosome holds tRNA and mRNA for using in translation

T RANSLATION : T HE P ROCESS Divided into three phases 1. Initiation Brings together mRNA The first aa, Met, with its attached tRNA 2 subunits of ribosome mRNA has cap and tail to help it bind to ribosome Occurs in two steps mRNA moleucle binds to a small ribosomal subunti then an initiator tRNA binds to the start codon Large ribosomal subunit binds, creating a funcitonal ribosome

T RANSLATION : T HE P ROCESS Divided into three phases 2. Elongation Occurs in 3 steps Codon recognation: the anticodon of an incomoing tRNA pairs w/ mRNA codon at the A site of the ribosome Peptide bond formation: Polypeptide leaves the tRNA in the P site and attaches to the aa on the tRNA in the A site The ribosome catalyzes the bond formation between 2 aa Translocation The P site tRNA leaves the ribosome The tRNA carrying the polypeptide moves from the A to the P site

T RANSLATION : T HE P ROCESS Divided into three phases 3. Termination Elongation continues until Ribosomes reaches a stop codon The completed polypeptide is freed The ribosome splits into its subunits

M UTATIONS A mutation is any change in the nucleotide sequence of DNA. Mutations can change the amino acids in a protein. Mutations can involve: Large regions of a chromosome Just a single nucleotidepair, as occurs in sickle cell anemia Mutations within a gene can occur as a result of: Base substitution, the replacement of one base by another Nucleotide deletion, the loss of a nucleotide Nucleotide insertion, the addition of a nucleotide Insertions and deletions can: Change the reading frame of the genetic message Lead to disastrous effects Mutations may result from: Errors in DNA replication Physical or chemical agents called mutagens Although mutations are often harmful, they are the source of genetic diversity, which is necessary for evolution by natural selection.

V IRUSES AND O THER N ONCELLULAR INFECTIOUS A GENTS Viruses exhibit some, but not all, characteristics of living organisms. Viruses: Possess genetic material in the form of nucleic acids Are not cellular and cannot reproduce on their own.

B ACTERIOPHAGES Bacteriophages, or phages, are viruses that attack bacteria. Phages have two reproductive cycles. (1) In the lytic cycle : Many copies of the phage are made within the bacterial cell, and then The bacterium lyses (breaks open) (2) In the lysogenic cycle : The phage DNA inserts into the bacterial chromosome and The bacterium reproduces normally, copying the phage at each cell division

P LANT V IRUSES Viruses that infect plants can: Stunt growth Diminish plant yields Spread throughout the entire plant Viral plant diseases: Have no cure Are best prevented by producing plants that resist viral infection

A NIMAL V IRUSES Viruses that infect animals are: Common causes of disease May have RNA or DNA genomes Some animal viruses steal a bit of host cell membrane as a protective envelope. The reproductive cycle of an enveloped RNA virus can be broken into seven steps.

HIV, THE AIDS V IRUS HIV is a retrovirus, an RNA virus that reproduces by means of a DNA molecule. Retroviruses use the enzyme reverse transcriptase to synthesize DNA on an RNA template. HIV steals a bit of host cell membrane as a protective envelope. The behavior of HIV nucleic acid in an infected cell can be broken into six steps. AIDS (acquired immune deficiency syndrome) is: Caused by HIV infection and Treated with drugs that interfere with the reproduction of the virus

V IROIDS AND P RIONS Two classes of pathogens are smaller than viruses: Viroids are small circular RNA molecules that do not encode proteins Prions are misfolded proteins that somehow convert normal proteins to the misfolded prion version Prions are responsible for neurodegenerative diseases including: Mad cow disease Scrapie in sheep and goats Chronic wasting disease in deer and elk Creutzfeldt-Jakob disease in humans

E VOLUTION C ONNECTION : E MERGING V IRUSES Emerging viruses are viruses that have: Appeared suddenly or Have only recently come to the attention of science Avian flu: Infects birds Infected 18 people in 1997 Since has spread to Europe and Africa infecting 300 people and killing 200 of them If avian flu mutates to a form that can easily spread between people, the potential for a major human outbreak is significant. New viruses can arise by: Mutation of existing viruses Spread to new host species