Chapter 12

DNA, RNA, Gene function,

Gene regulation,and

Biotechnology

DNA

• Double helix• “Rungs” are base pairs joined by hydrogen bonds• Adenine (A) pairs with thymine (T)• Cytosine (C) pairs with guanine (G)• Complementary strands• Strands oriented in opposite

directions– 5’ to 3’ or 3’ to 5’

– Relationship between nucleic acids and proteins is a flow of information

– Part 1: Transcription – cell templates DNA to RNA

– Part 2: Translation – RNA information used to manufacture proteins

– Developed by Watson & Crick in 1950’s

The “central dogma”

So let’s compare DNA with RNA…

DNA vs. RNA

• 3 types of RNA– Messenger RNA (mRNA) – carries info.

specific to a protein, 3 RNA bases form a codon specifying an amino acid

– Ribosomal RNA (rRNA) – combines with proteins to form a ribosome

– Transfer RNA (tRNA) – carries specific amino acid to ribosome

RNA

Transcription1. Initiation

– Enzymes unwind DNA exposing template strand

– RNA polymerase binds to promoter

2. Elongation– RNA polymerase moves 3’ to 5’

3. Termination– RNA polymerase reaches

terminator sequence at end of gene

– RNA separates – may be mRNA, tRNA or rRNA (for translation to occur, it must be mRNA)

– DNA reforms helix

Translation• Genetic code

– mRNA codon with 3 bases specifies amino acid– Also contains start and stop codons

• Translation requires these players:

– mRNA – genetic information specifying amino acid order in codons

– tRNA – brings specific amino acid to ribosome by pairing anticodon to mRNA codon

– Ribosome – rRNA and proteins

tRNA vs. rRNA

• 3 steps in translation1. Initiation

– mRNA start codon binds to small ribosomal subunit – 1st tRNA binds to mRNA codon

2. Elongation– Large ribosomal subunit attaches– tRNA corresponding to 2nd codon attaches– Covalent bond forms between amino acids– Ribosome release empty 1st tRNA– Ribosome shift down one codon allowing 3rd tRNA to

bind– Polypeptide grows one amino acid at a time

3. Termination – Stop codon reached– New polypeptide released

• Protein folding– Must achieve final functional shape – some

regions attract or repel, enzymes catalyze bonding, “chaperone” proteins stabilize

– Errors in folding can lead to illness– Some proteins must be altered

• Insulin has amino acids removed• Hemoglobin has 4 separate polypeptides

After Translation

Regulation

• Protein synthesis is fast and efficient

• Tremendous ATP requirement

• Cells save energy by not producing unneeded proteins

Mutations

• Change in cell’s DNA sequence

• Can be good, bad, or silent

• Point mutations– Substitute one DNA

base for another– “Silent” is same amino

acid specified (no change caused by mutation)

– May cause disease – sickle cell anemia

• Base insertions and deletions– Frameshift mutation

caused by addition or deletion by any number other than a multiple of 3

– Expanding repeat – number of copies of 3 or 4 nucleotide sequence increases over several generations

• Causes of mutations1. Spontaneous – DNA

replication error

2. Meiotic error – duplication or deletion

3. Chromosome inversion and translocations

4. Transposons – moveable DNA sequences

5. Mutagen – external agent – radiation, chemicals

• Somatic mutations occur in nonsex cells• All cells derived from that cell carry mutation• Not passed to offspring

• Heritable mutations– Germline mutation

• Heritable – passed in every gamete

• Mutations are important– Create new gene variants (alleles)– Random mutations results in antibiotic resistant

bacteria

Types of Mutations

Human Genome Project

• 3.2 billion base pairs• 25,000 genes produce 400,000 different

proteins– Removing different combinations of introns

makes different proteins

• Only about 1.5% of genome encodes protein– 98.5% encodes regulatory sequences,

pseudogenes, and transposons

• Transgenic organism receives recombinant DNA

• Recombinant DNA – genetic material spiced together from multiple organisms– Transgenic bacteria

make drugs– Transgenic crops

resist disease– Transgenic human

disease models

Transgenic Organisms

Biotechnology

• Gene therapy – replacing faulty genes• Block gene expression to silence harmful

gene or study gene function– Antisense RNA, gene knockouts

• DNA microarrays or DNA chips – use collection of known DNA sequences

• Proteomics – genome changes little but proteins different in different cells and different times