Download Transcription Factors

AP Biology—Unit 6
Molecular Genetics Notes
Name:
I.
Major Experiments and Contributing Scientists—See Chart
II.
DNA
A. Structure
i. Nucleotide—basic building block molecule made of 3 parts
 Always pair a purine with a pyrimidine
o A—T
o G—C
 Differ from other organisms in order and number of bases
ii. Double Helix
1. Sides of the Ladder:
2. Hydrogen Bonds:
3. Antiparallel Strands:
NOTE: DNA polymerase can only add nucleotides to 3’end
iii. DNA Replication—done for mitosis and meiosis (During S of
Interphase)
III.
Protein Synthesis—making of proteins for…structure and controlling
chemical reactions
A. Transcription:
i. Initiation:
ii. RNA elongation: Unzip DNA—
iii. Termination:
iv. Each group of 3 bases on mRNA = 1codon
(Complementary to DNA)
v. Other notes:
Amino Acids- Building block molecules of proteins—20 AA
Start Codon- AUG (A.A. = methionine)
Stop Codons- UAA, UGA, UAG—no amino acids associated
B. Translation: Occurs at ribosome; Use mRNA to assemble protein
i. Ribosome: made from rRNA and some proteins; has 2 subunits;
can be free or bound on ER or nuclear membrane
ii. tRNA (transfer RNA)
Two jobs:
1.
2.
Note: Anticodon on tRNA complements mRNA codon; other end of
tRNA has attachment site for amino acid
iii. Steps
1. Initiation:
a. Initiator tRNA (with 1st AA = methionine) attaches to
small ribosomal subunit at P site.
b.
iv. Chain Elongation
1. Codon Recognition
2. Peptide bonds
3. Translocation
4. Repeat steps 1-3
v. Chain Termination
1. Reach a stop codon with no amino acid—detach polypeptide
2. Ribosome splits into subunits—mRNA is degraded
C. Summary / Overview
IV. Gene Regulation—Purpose: Don’t waste energy—don’t make things that
aren’t needed
A. Prokaryotes: Bacteria do transcription/translation at same time
i. Inducible Operon—Lac Operon: Normally repressed—but genes
are turned on (induced) when bacteria needs enzymes to break
down lactose
Lactose is the inducer / lac repressor is innately active.
ii. Repressible Operon—Trp Operon: Normally on…but genes that
make enzymes to make the amino acid tryptophan can be turned
off (repressed) when tryptophan is present
iii. Activators: proteins that bind to DNA making it easier for RNA
polymerase to bind to promoter (turn on faster)
B. Eukaryotes:
i. Differential Gene Expression—
Examples:
-Some genes are never used in a particular cell
-Some genes are only used when needed
-Some genes are used all the time
ii. Chromatin Structure Regulation—location of promoters in relation
to histones and the nuclear lamina can regulate whether or not
transcription occurs.
1. Histone Acetylation
2. DNA Methylation
3. Epigenetics
iii. Controls
1. Transcriptional Controls
a. Promoters—
b. Control Elements—segments of noncoding DNA that
act as binding sites for transcription factors
c. Transcription Factors—
d. Activators (transcription factors that activate) and
Enhancers (distal control elements)—work together to
turn on non-adjacent genes
2. Post-Transcriptional Controls (RNA Processing)
a. Cap / Tail—
b. Gene Splicing—
3. Translational Controls
a. mRNA Degradation—how long mRNA lasts…depends
on organism and mRNA
b. Other proteins bind to inhibit ribosome—sometimes
ribosomes are blocked so that mRNA can’t be used
4. Post-translational Controls
a. Protein Processing
 Cut polypeptides—into functional units to do
jobs on their own
 Combine polypeptides—quaternary structure
b. Save / Breakdown proteins—determine how long
protein lasts before breakdown (enzymes can come
break it up)