PSYC 3102: Introduction to Behavioral Genetics
Lecture 4
Neurotransmission continued…
1) Opens Ion Channels
Ions go in and out of postsynaptic neuron
Changes electric potential
Stimulates or Inhibits
2) Second Messenger System
Binding has a sentinel role
Chemical communication
Initiation of cascade of chemical reactions by receptor binding
Down the line—gene regulation (turns expression on or off)
Remember Lemonade analogy; Environmental stimulus  Genetic reaction
Gene Regulation – proteins attach and make it easier or harder for DNA to give info for
the production of other proteins
3 Points to Know:
1. Genes are the blueprints for proteins
2. All receptors are proteins or protein complexes (contain protein segments)
3. All enzymes are proteins
Enzyme
 Protein that speeds up chemical reactions
 Start with 2 molecules: substrate & enzyme
 Enzyme will not act on anything, only what fits it
 Where the enzyme interacts with the substrate is called the active site
 A chemical reaction occurs: slice or splice
 The enzyme will cut something off the substrate or add something to it
 End with a product
(see following figure)
All you have to know about Cell Division:
Mitosis
 Ordinary cell division; how your cells normally divide
 Make a carbon copy of the cell
 Daughter cells have the same type/amount of DNA
Meiosis
 Gamete production
 Reduces the amount of DNA (or else the amount of DNA would double with every
generation)
 Have to recognize that cell division has to occur with an incredible degree of fidelity
(precision)
 Mechanisms involving copy errors can result in cancers, there are ‘proofreading’
genes to stop this (tumor suppressor genes)
DNA
Deoxyribonucleic Acid
Packaging
 Physical unit of inheritance is the chromosome (not the gene)
 Humans receive 23 ‘packets’ (chromosomes) of DNA from each parent
 A chromosome is a very, very, very, very, very long molecule of DNA wrapped
around a series of protein scaffolds
 The importance of this is that the way the DNA is packaged can allow genes to be
turned on, or keep them turned off
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Double helix makes a ‘thread’ that wraps around histone complexes
This complex with DNA is called the nucleosome
o There is growing evidence that these are functional in ways wer don’t
understand (may influence if genes are turned on or off)
These complexes then wrap around themselves
Then there is more and more scaffolding
DNA Molecule
 Double helix (spiral staircase)
 DNA unwound would look like a ladder
 The backbones to the ladder are sugars and phosphates
 The rungs are the most important
 Each rung is made up of 2 chemical components called nucleotides or base pairs
 There are four nucleotides: (KNOW NAMES)
o A - Adenine
o T - Thymine
o C – Cytosine
o G – Guanine
Complimentary base pairing (VERY IMPORTANT MECHANISM OF GENE ACTION)
 A always pairs with T
 C always pairs with G
 If you know one strand of the helix, you can always predict the other
 i.e. ACGT MUST pair with TGCA
Sequencing of DNA
 Finding out the ordering of nucleotides in a certain section of DNA
 There are 3.2 billion nucleotides x 4 strands (helices) in human cell
 Human Genome Project only had to sequence 3.2 billion
 1 cell: 2 copies of each chromosome, but can throw out 1 copy, then halve the other
copy
 So, 3.2 billion nucleotides reflects ¼ of the nucleotides found in a cell
 Project was a huge undertaking; storing data and searching it is difficult
Bioinformatic field deals with this
For DNA to copy itself
 Enzymes are needed to break bonds (zipper)
o Two sides will separate
 A molecule (polymerase) latches on to a strand
o This ‘reads’ the strand
o Grabs a molecule that is complimentary
o Makes new strand as it moves along the original strand
RNA
Ribonucleic Acid
 Differs from DNA in 3 important ways:
o RNA is single-stranded while DNA is double-stranded
o The nucleotide Uracil (U) replaces Thymine in RNA
 complementary base pairing takes place as usual except A-U pairs
instead of A-T
o There are several types of RNA, but only one type of DNA
 mRNA – messenger RNA
 tRNA – transfer RNA
 rRNA – ribosomal RNA
Protein Synthesis
5 steps
1. Transcription (Photocopying)
2. Post-transcriptional Modification (Editing)
3. Transportation (Transportation)
4. Translation (Translation)
5. Post-translational modification (Final assembly)
Analogy – Building a Garage
 DNA = Blueprints
 Blueprints in library, can’t come out, so you PHOTOCOPY
 90% of DNA may be junk
o Distinction between junk and garbage
o Junk is stuff you don’t need at this time, so you throw it in the attic
o Garbage is stuff you don’t want or need, so you throw it out of the house
 So, then you EDIT the photocopied blueprint
 Then you TAKE it to the manufacturing site
 You TRANSLATE the blueprints into the actual foundation, walls, etc.
 You put on the finishing touches with the FINAL ASSEMBLY
Step 1: Transcription
 DNA strands separate
 Nuclear RNA is synthesized from on of the DNA strands using complimentary
pairing
 This is a series of chemical reactions mediated and performed by collection enzymes
(transcription stuff)
 This does not happen haphazardly
 Not all of the DNA is transcribed, just a small amount
 There are recognition sites along the DNA that signal starting points and ending
points
 Starting points are called ‘promoters’
 End points are don’t have an official name, but are sometimes called ‘termination
regions’