BIO 1012 Study Guide Chapters 10 12 Chapter 10 Molecular Biology of the Gene History Molecular biology the study of heredity at the molecular level Until 1940s thought that proteins made up genetic material Frederick Griffith 1928 discovered transforming factor o Harmless bacteria converted to disease causing bacteria by exposure to heat killed Alfred Hershey and Martha Chase 1952 used bacteriophages to show DNA is genetic material of pathogenic bacteria T2 a virus that infects E coli o Bacteriophages phages viruses that infect bacterial cells Labeled with radioactive sulfur to detect proteins Labeled with radioactive phosphorus to detect DNA Phosphorus labeled DNA was detected inside cell and produced new bacteriophages with radioactivity in DNA but not in protein o Demonstrated that genetic material was most like DNA James Watson and Francis Crick 1953 deduced secondary structure of DNA o Used x ray crystallography data o Observed amount of adenine equal to amount of thymine guanine equal to cytosine o Reported DNA consisted of 2 polynucleotide strands wrapped into double helix o A pairs with T 2 hydrogen bonds form o G pairs with C 3 hydrogen bonds form o Gave new meaning to the words genes and chromosomes o Genetic information in a chromosome is encoded in nucleotide sequence of DNA DNA Nucleic acid Uses deoxyribose Nucleotide polymer chain composed of 3 parts o 1 Nitrogenous base Adenine A Thymine T Cytosine C Guanine G o 2 Five carbon sugar o 3 Phosphate group o Joined to one another by sugar phosphate backbone Thymine Cytosine together form pyrimidines Adenine Guanine together form purines RNA Also a nucleic acid Uses sugar ribose Has nitrogenous base uracil U instead of thymine DNA Replication Ensures that all somatic cells in multicellular organism carry same genetic information Follows a semiconservative model 2 DNA strands separate complementary strand of each is produced using specific base pairing each new DNA helix has one old strand and one new Occurs in the 5 to 3 direction o Continuous on 3 to 5 template o Discontinuous on 5 to 3 template forming short segments 2 key proteins o DNA ligase joins small fragments into continuous chain o DNA polymerase adds nucleotides to chain proofreads corrects improper pairing o Both repair DNA damaged by radiation and toxic chemicals Flow of Genetic Information from DNA RNA Protein DNA genotype is expressed as proteins which provide molecule basis for phenotypic traits DNA specifies traits by dictating protein synthesis Molecular chain of command DNA nucleus RNA cytoplasm Protein Transcription synthesis of RNA under direction of DNA o DNA RNA o Takes place in nucleus Translation synthesis of proteins under direction of RNA o RNA Protein o Takes place in cytoplasm o Involves switching from nucleotide language to amino acid language Sequence of nucleotides in DNA provides a code for constructing a protein o Requires conversion of nucleotide sequence amino acid sequence o Transcription rewrites DNA code into RNA using same nucleotide language Flow of information from gene to protein is based on a triplet code Genetic instructions for amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping three base words called codons Each amino acid is specified by a codon o 64 possible codons o Some amino acids have more than one possible codon Characteristics of the genetic code o 3 nucleotides specify 1 amino acid language of DNA and RNA 61 codons correspond to amino acids AUG codes for methionine signals start of transcription 3 stop codons signal end of translation UAA UAG UGA Genetic code is o Redundant more than one codon for some amino acids o Unambiguous any codon for one amino acid does not code for any other amino acid o Nearly universal genetic code is shared by organisms from simplest bacteria to the most complex plants and animals o Without punctuation codons are adjacent to each other with no gaps in between Transcription overview strand using DNA replication RNA molecule is transcribed from a DNA template by process that resembles synthesis of a DNA RNA polymerase transcription enzyme that links RNA nucleotides Specific sequence marks where transcription beings and ends Start transcribing signal AUG is called a promoter First phase begins with initiation as RNA polymerase attaches to promoter Second phase elongation RNA grows longer as RNA peels away DNA strands rejoin Third phase termination RNA polymerase reaches a sequence of bases in the DNA template called a terminator which signals the end of a gene Polymerase molecule detaches from RNA molecule and the gene In eukaryotic cells occurs in the nucleus and the mRNA must travel from nucleus to cytoplasm Messnger RNA mRNA Encodes amino acid sequence Conveys genetic messages from DNA to the translation machinery of the cell o In prokaryotes occurs in same place that mRNA is made o In eukaryotes mRNA must exit the nucleus via nuclear pores to enter the cytoplasm Eukaryotic mRNA o Contains Introns interrupting sequences that separate Exons coding regions How mRNA is made o Undergoes processing before leaving the nucleus RNA splicing removes introns and joins exons to produce a continuous coding sequence Cap and tail of extra nucleotides are added to the ends of the mRNA to Facilitate the export of the mRNA from the nucleus Protect the mRNA from attack by cellular enzymes Help ribosomes bind to the mRNA Transfer RNA tRNA Function as a language interpreter o Convert genetic message mRNA into language of proteins Perform their interpreter task by o Picking up appropriate amino acid o Using a special triplet of bases called an anticodon to recognize the appropriate codons in the mRNA Ribosomes Translation occurs on their surface They coordinate the functioning of mRNA tRNA and ultimately the synthesis of polypeptides Have 2 subunits small and large o Each is composed of ribosomal RNAs and proteins o Come together during translation Having binding sites for mRNA and tRNAs Can be divided into the same 3 phases as transcription Translation o Initiation o Elongation o Termination Initiation o Brings together mRNA tRNA bearing first amino acid 2 subunits of a ribosome o Establishes where translation will begin o Occurs in 2 steps mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the start codon Start codon reads AUG and codes for methionine First tRNA has the anticodon uAC Large ribosomal subunit joins the small