Transcript DNA structure

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
RNA
• Sugar is ribose
– Has –H
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Bases are A,C, G, and T
Double-stranded helix
Only in nucleus
Modified only by mutations
1 type
– Has -OH
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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 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.