Molecular Structure & Function
of Genetic Material
Professor Janaki Natalie Parikh
profjnp@gmail.com
D.N.A. Structure
• D.N.A.: deoxyribonucleic acid: is a double stranded
(dbl helix) polymer of a nucleotides
• Resides in the nucleus (eukaryotes)
• Made of 3 molecules:
• Phosphate, Sugar &
nitrogenous base
DNA
•
•
•
•
•
•
4 bases in D.N.A.:
Adenine, Guanine, Thymine & Cytosine
Rules for pairing bases together:
Adenine  Thymine
Guanine  Cytosine
Our DNA is composed of literally billions of bases!
Genes are long sections (segments) of D.N.A.
D.N.A. Function
• 1. D.N.A. can make a copy of itself, handy during?
• Mitosis & meiosis
• 2. D.N.A. contains the code for protein synthesis,
the manufacture of proteins
• Problem, where does protein synthesis take place?
• Ribosomes, located? Outside the nucleus. D.N.A.
can’t leave the nucleus.
So how does this get done?
R.N.A.
• R.N.A.: ribonucleic acid, single stranded, free
floating throughout the cell
• Similar bases, w/ 1 important diffc.
• Adenine, Guanine, Uracil & Cytosine
• Adenine  Uracil
Guanine  Cytosine
• R.N.A assists in completing protein synthesis
Protein Synthesis
• Proteins: polymers as well, but difft. components?
• Amino acids. How many are there?
• 20 total. Of these 11 are naturally occuring, the
other 9 must be consumed through food, those are
known as “essential amino acids” (in kids 10 are
essential, 1 loses this status once we produce it)
• How do we get these essential amino acids?
• http://www.glisonline.com/aminoacids.php
• http://www.all-creatures.org/health/plantfoods.html
Protein Synthesis
• Recall our logistical dilemma?
• Making proteins: multi-step task
• Cheesy analogy: outside of the
nucleus is the “hood”, the nucleus: gated commty
• D.N.A.: Doesn’t kNow About the hood
• However, D.N.A.’s cousin, R.N.A. is another story
• R.N.A.: Really kNows About the hood
Steps of Protein Synthesis
• 1.transcription: m.R.N.A. enters nucleus, produces a
transcript of D.N.A. code (in R.N.A. language)
• Let’s try part of a sequence:
• D.N.A. reads: A T A G A G
mRNA?
• m.R.N.A.:
U AU C U C
• 2. translation: t.R.N.A. reads the mRNA transcript &
translates the info one codon at a time
Codons & Genetic Code
• Codons: base triplet that codes for an amino acid
• Notice: genetic redundancy: more than 1 codon
codes for the
same amino acid
(we’ll discuss
signifcance of
this redundancy
subsequently)
Protein Synthesis
• Back to our 2nd step: mRNA: U A U C U C
tRNA: A U A G A G
• Amino acid:
Isoleucine, Glutamic Acid
Genetic Redundancy
• Sometimes mistakes occur in this process (mutation)
• Problem: even 1 incorrect base can render a protein
useless junk (loss of function)
• Remember genetic redundancy? It’s purpose:
• Serves as an built-in security mechanism, reducing
the chance that a base substitution resuls in loss in
protein function
Mutations Overview
• Mutagens: accelerate the rate of mutations
• Mutations are completely random accidents
• Most mutations result in loss in protein function
(junk protein), some are neutral, Rarely: new
protein function producted
• (Ecstasy q: http://www.maps.org/media/mtvclarify.html
•
http://www.shroomery.org/forums/showflat.php/Cat/0/Number/6307333
Mutations
• Point mutations: involve 1 single base
– Base substitution: swapping of nucleotide base
• Can possibly be neutral due to genetic redundancy
– Addition or deletion: extra base insert or a base
is omitted from correct sequence
• Results in a frameshift mutation (affects multiple
amino acids) & can never be neutral
• http://evolution.berkeley.edu/evosite/evo101/IIIC3aT
ypes.shtml
Chromosomal Mutations
• Chromosomal mutations (macrolesions): occur
during meiosis, larger scale of significance since
whole chromosome involved
• Nondisjunction: chromosome pair failed to split
• http://www.biostudio.com/d_%20Meiosis.htm
• http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosi
s%20II.htm
• Results in a gamete w/ too many, or too few
chromosomes
Trisomy & Monosomy
• Trisomy: presence of 3 chromosomes
instead of the normal 2 in a
homologous pair
• Monosomy: presence of 1 chromosome instead of
the normal 2 in a homologous pair
• Examples?
• Down’s syndrome: trisomy of #21
•
http://www.ndss.org/PageFiles/2588/Maternal%20Age%20Chart2.png
Syndromes
• Klinefelter’s Syndrome: XXY or XXXY
• Turner’s Syndrome: X0
• http://www.nichd.nih.gov/health/topics/klinefelter_syndrome.cfm
• http://www.merck.com/mmhe/sec23/ch266/ch266b.html
Mutations & Evolutionary Significance
• In order for a mutation to have an evolutionary
impact, it must be inheritable (happens in the
gametes)
• Next, we’ll examine a specific point mutation that
had a major impact on human populations