1. DNA Structure
Chapter 10

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

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

12. 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

13. 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)

14. 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

15. 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

16. 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°)

17. 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.