From Gene to Protein pp 344-361 Discover Biology: C15 From Gene to Protein pp 344-361

How Genes Work DNA’s nucleotide sequence determines 1o structure of RNA & protein DNA is transcribed into a messenger RNA (mRNA) copy DNA encodes ribosomal RNA (rRNA) DNA encodes transfer RNA (tRNA) RNA directs synthesis of proteins rRNA combines with cytoplasmic proteins to form ribosomes. mRNA’s sequence, 3 bases/ codon, is translated into amino acids proteins tRNA transports amino acids, specific to mRNA codon, to ribosomes Proteins are multifunctional Structure Locomotion Defense Enzymes (Biological catalysts)

Genes Affecting Phenotype 1902: Archibald Garrod & William Bateson Alkaptonuria caused by defective enzyme Lack of proper enzyme causes urine to be black Genes control production of enzymes Major contribution by Garrod & Bateson Noncoding genes DNA codes for RNA Protein-coding Genes DNA codes for all proteins, including enzymes Gene = any DNA transcribed into RNA

RNA versus DNA DNA = Deoxyribonucleic acid RNA = Ribonucleic acid Double-stranded a-helix in nucleus only Sugar (deoxyribose) – phosphate backbone Nucleotides attached to 3’ or 5’ hydroxyl of sugar Nucleotides hydrogen-bond to each other to join 2 strands Adenine to Thymine (2 H-bonds) Guanine to Cytosine (3 H-bonds) RNA = Ribonucleic acid Single-stranded; cytoplasmic Sugar (ribose) – phosphate backbone Nucleotides include Adenine, Guanine, Cytosine, & Uracil Less stable than DNA; limited life in cytoplasm

DNA vs. RNA

From DNA to Protein

Transcription vs. Replication Creates single-stranded mRNA via RNA polymerase Only 1 strand of DNA, the template strand, is “read” Only 1 section of DNA, the gene, is copied. Prokaryotic transcription ceases at hairpin loops in mRNA/ DNA terminator Replication Creates 2 semi-conserved double-stranded copies of DNA DNA polymerase binds promoter and reads/ writes/ checks DNA Both the leading & lagging strands of DNA are copied The entire chromosome is copied

Transcription DNA is unwound by helicase Nuclear binding proteins stabilize DNA strands RNA polymerase binds promoter on DNA template strand RNA polymerase makes RNA copies of DNA’s nucleotide sequence Polymerase adds base pairs complementary to DNA’s nucleotides DNA’s adenine is matched with uracil, thymine with adenine, cytosine with guanine, & guanine to cytosine Pre-mRNA is processed into mRNA Nucleotides coding for introns are excised; exons are spliced together Alternate splicing creates different arrangements of exons  new proteins mRNA has additional nucleotides added as a “cap” and as a “poly-A tail”

Translation and the Genetic Code

Genetic Code Universal Unambiguous Redundant All living creatures thus recognized have the same genetic code Unambiguous Each codon specifies 1 amino acid There are 20 essential amino acids Redundant Several different codons may call for the same amino acid There are 64 total codons (43 = 64) Wobble effect: Sometimes the 3rd nucleotide is nonspecific for the amino acid

Translation: On the Ribosomes

tRNA

Genetic Mutations Point Mutation A single nucleotide is changed A different nucleotide is substituted = Substitution Not all substitutions lead to phenotype mutations Introns Wobble effect in codons A nucleotide is added = Insertion A nucleotide is removed - Deletion Insertion & Deletion mutations disrupt the original sequence Frameshift mutations arise with insertions & deletions Entirely new codon sequence is created; therefore, new amino acids linked. More deleterious if mutation is near the front of the gene Can create a stop codon & truncate the protein

Protein Structure Primary structure Secondary structure Amino acid sequence Secondary structure Regional folding of amino acid chain Alpha helix (spiral) & Beta sheets (ridges & valleys) Tertiary structure Interactions among distal segments Ionic & covalent bonds (disulfide bonds between cysteines) Quaternary structure Between different polypeptides (e.g., hemoglobin)

Form Follows Function If the structure is changed, so is the behavior of protein Proteins/ enzymes depend on their 3-D shape to elicit effects Denaturation Typically permanent High or low pH pH = - log [H+] = log 1 / [H+] Extreme Temperatures High Salinity