1 The Genetic Code of Genes and Genomes

DNA: Molecule of Heredity Inherited traits are affected by genes that are transmitted from parents to offspring in reproduction Genes are composed of the chemical deoxyribonucleic acid = DNA Fig. 1.6

DNA: Molecule of Heredity DNA was discovered by Friedrich Miescher in 1869 In 1920s microscopic studies with special stains showed that DNA is present in chromosomes In 1944 Avery, McLeod and McCarty provided the first evidence that DNA is the genetic material

Avery, McLeod, McCarty Experiment Avery, McLeod and McCarty identified the chemical substance responsible for changing rough, nonvirulent cells of Streptococcus pneumoniae (R) into smooth encapsulated infectious cells (S): Transforming activity was destroyed by DNAse, not RNAse or protease Conclusion: transforming factor that converts R cells to S cells is DNA

Fig. 1.3

Hershey-Chase Experiment In 1952 Hershey and Chase showed that DNA, not protein, is responsible for phage activity in bacterial cells: Radioactive phage DNA enters bacteria after attachment, but protein coat of virus remains outside Phage DNA directs the reproduction of virus in infected bacterial cells

DNA Structure: Double Helix In 1953 Watson and Crick proposed the three dimensional structure of DNA Molecular structure of DNA is a double-stranded helix comprised of a linear sequence of paired subunits = nucleotides Each nucleotide contains any one of four bases = adenine, thymine, guanine and cytosine Pairing between nucleotides of the double helix is complementary: adenine pairs with thymine guanine pairs with cytosine

DNA Structure: Double Helix DNA backbone forms right-handed helix Each DNA strand has polarity = directionality The paired strands are oriented in opposite directions = antiparallel Fig. 6.7

DNA Replication Watson-Crick model of DNA replication: The strands of the original (parental) duplex separate Each parental strand serves as a template for the production of a complementary daughter strand by means of A-T and G-C base pairing

Genes and Proteins The genetic information contained in the nucleotide sequence of DNA specifies a particular type of protein Enzymes = proteins that are biological catalysts essential for metabolic activities in the cell Metabolites = small molecules upon which enzymes act In 1908 Archibald Garrod proposed that enzyme defects result in inborn errors of metabolism = hereditary diseases

Genes and Proteins Garrod studied alkaptonuria and identified abnormal excreted substance = homogentisic acid Alkaptonuria results from a metabolic defect that blocks the conversion of a substrate molecule to a product molecule in a biochemical pathway due to absence of required enzyme = metabolic block In case of alkaptonuria, a defective homogentisic acid 1,2 dioxygenase is unable to converts homogentisic acid into 4-maleylacetoacetic acid in the pathway for the breakdown of phenylalanine and thyrosine

Genes and Proteins Another defective enzyme in the same pathway, phenylalanine hydroxylase (PAH), leads to phenylalanine accumulation which causes the condition known as phenylketonuria (PKU) Incidence of PKU, characterized by severe mental retardation, is about one in 8000 among Caucasian births. A defective enzyme results from a mutant gene

Central Dogma Central Dogma of molecular genetics: DNA RNA Protein DNA is the informational molecule which does not code for protein directly but rather acts through RNA intermediate DNA codes for RNA = transcription RNA codes for protein = translation

Fig. 1.14

Transcription Transcription is the production of an RNA strand that is complementary in base sequence to a DNA template = messenger RNA (mRNA) RNA contains the base uracil in place of thymine and the sugar ribose instead of deoxyribose RNA is synthesized from template DNA following strand separation of the double helix Fig. 1.15

Base pairing in DNA and RNA Complementary base pairing specifies the linear sequence of bases in RNA Adenine pairs with uracil; thymine pairs with adenine; guanine pairs with cytosine

Translation The sequence of bases in mRNA codes for the sequence of amino acids in a polypeptide The mRNA is translated in nonoverlapping group of three bases = codons that specify the sequence of amino acids in proteins Each codon specifies one amino acid Transfer RNAs (tRNA) contain triplet base sequences = anticodons, which are complementary to codons in mRNA

Fig. 1.16

Translation Translation occurs at the ribosomes which contain several types of ribosomal RNA (rRNA) tRNAs participate in translation by carrying amino acids and positioning them on ribosomes Translation results in the synthesis of a polypeptide chain composed of a linear sequence of amino acids whose order is specified by the sequence of codons in mRNA

Mutations Mutation refers to any heritable change in a gene The change may be: substitution of one base pair in DNA for a different base pair; deletion or addition of base pairs Any mutation that causes the insertion of an incorrect amino acid in a protein can impair its function

Genes and Environment One gene can affect more than one trait = pleiotropy Any trait can be affected by more than one gene as well as environment Most complex traits are affected by multiple genetic and environmental factors Often several genes are involved in genetic disorders and the severity of a disease may depend upon genetic status and environmental factors

Evolution All creatures on Earth share many features of the genetic apparatus and many aspects of metabolism Groups of related organisms descend from a common ancestor Evolution occurs whenever a population of organisms with a common ancestry gradually changes in genetic composition over time

Fig. 1.21

Evolution The totality of DNA in a single cell = genome The complete set of proteins encoded in the genome = proteome Genes or proteins that derive from a common ancestral sequence via gene duplication = paralogs Genes that share a common ancestral gene via speciation = orthologs The molecular unity of life is seen in comparisons among genomes and proteomes