Transcript DNA - Ellis Benjamin

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’
The “central dogma”
– 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
So let’s compare
DNA with RNA…
DNA vs. RNA
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
Transcription
1. 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 translation
1. 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
After Translation
• 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
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 mutations
1. 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
Types of Mutations
• 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
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 Organisms
• 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
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