DNA Structure Chapter 10

Nucleic Acids Polymers made of nucleotides Sugar-phosphate backbone (sides) Nitrogenous bases face in (rungs) Purines (2 rings) G and A Pyrimidines (1 ring) C, T, and U

Nucleic Acid Types DNA RNA Sugar is deoxyribose Has –H Bases are A,C, G, and T Double-stranded helix Only in nucleus Modified only by mutations 1 type Sugar is ribose Has -OH Bases are A, C, G, and U Single-stranded Not confined to nucleus Lots of processing and modifications 3 types

RNA Types Ribosomal RNA (rRNA) Messenger RNA (mRNA) Combines with proteins to form ribosomes Synthesize polypeptides Messenger RNA (mRNA) Complimentary DNA sequence Carries DNA message from the nucleus to ribosomes Transfer RNA (tRNA) Transfers amino acids to ribosomes Build polypeptide chains

James Watson and Francis Crick Nobel prize for DNA double-helix model Rope ladder with antiparallel sides 5’ to 3’ ends Pyrimidines and purines A with T form 2 bonds G with C form 3 bonds Supported by Chargaff’s rules

Semiconservative model Each strand of original DNA serves as a template Nucleotides match to template according to base pairing rules (complementary strand) 1 ‘parent’ DNA strand produces 2 new ‘daughter’ strands Occurs rapidly, both strands simultaneously Humans with 6 billion pairs a few hours, with only about 1 error every 10 billion nucleotides

DNA Replication Helicase DNA polymerase DNA ligase Unzips and separates strands DNA polymerase link nucleotides to growing daughter strands Can only bind to 3’ New strands can only grow 5’ to 3’ Leading strand - toward fork (continuous) Lagging strand – away from fork (fragmented) DNA ligase Links fragments together Roles in maintenance, proofreading, and repair Video 1

Central Dogma of Biology transcribed translated DNA  RNA  protein Francis Crick Genes instruct, but don’t build Transcription (same language) in nucleus Translation (new language) in cytoplasm mRNA codes for polypeptides

Transcription In the nucleus RNA polymerase binds to 1 strand with promoter Many work at once RNA nucleotides added Bind to 3’ end only Builds 5’  3’ Separates DNA strands Unstable complex = immediate release Terminator sequence releases RNA polymerase Release pre-mRNA

mRNA Processing Before leaving nucleus Initially has introns (filler) and exons (code) Nucleotide sequences added to either end 5’ cap and Poly A tail Introns removed and exons rejoined Creates mRNA

Decoding Codons Only 4 nucleotide bases to specify 20 amino acids Genetic instructions are based on triplet code called codons 42 = 16 (not enough); 43 = 64 (plenty) Demonstrates redundancy, but not ambiguity Nearly universal across species

Translation Within ribosomes mRNA has codon message from DNA Translated by tRNA Anticodon and amino acid on opposite ends Ribosomes facilitate addition of tRNA to mRNA

Ribosomes Coordinate mRNA and tRNA Composed of proteins and ribosomal RNA (rRNA) Actually make polypeptides 2 subunits, large and small Small locks mRNA Large has 2 sites P site holds growing polypeptide A site holds new tRNA molecule (amino acid)

Building Ribosomes Small subunit binds mRNA at a start codon (AUG) 1st tRNA enters the P site carrying the amino acid met Anticodon is what? Large subunit binds to create a ribosome Met is in the P site A site is empty

Translating mRNA 2nd tRNA molecule into A site Polypeptide in P site breaks off and attaches to amino acid in A site P site tRNA leaves Ribosome translocates Shifts 5’ to 3’ A site tRNA to P site Repeats

A New Polypeptide Stop codon sequence signifies the end of a polypeptide chain Enters A site, doesn’t carry amino acid Polypeptide released from P site tRNA Ribosome splits Polypeptide assumes level of structure (1° to 4°)

Mutations Changes to the genetic information of a cell Ultimate source of diversity because ultimate source of new genes Point mutation Replace 1 nucleotide with another Effect depends on codon Base insertions and deletions Changes reading frame Most often deleterious effects E.g. The cat ate the rat.