dna structure chapter 10. nucleic acids polymers made of nucleotides sugar-phosphate backbone...
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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 • Sugar is deoxyribose
– Has –H
• Bases are A,C, G, and T• Double-stranded helix• Only in nucleus• Modified only by mutations• 1 type
RNA• 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)– 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
– 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 Biologytranscribed 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.