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.