Transcript Biol 178 Lecture 27

Bio 178 Lecture 27
Genetics and DNA
Reading
•
Chapters 13 & 14
Quiz Material
•
Questions on P 276-278 & 300
•
Chapters 13 & 14 Quizzes on Text Website
(www.mhhe.com/raven7)
Outline
• Genetics
 Human Genetics (cntd)
 Chromosomes and Genetics
• DNA
 Experiments - DNA is hereditary material
 DNA Structure
 DNA Replication
Nondisjunction (Cntd.)
• Nondisjunction of Sex Chromosomes
Generally, the effects are not as severe as nondisjunction of
autosomes.
1. X Chromosome
(a) Triple X Syndrome
Usually taller than average females, generally “normal”
and most are *fertile.
(b) Klinefelter Syndrome
XX + Y  XXY
Sterile male with female characteristics and sometimes
mildly impaired intelligence.
Klinefelter Syndrome
http://www.carolguze.com/text/442-4chromosome_abnormalities.shtml
Klinefelter calico cat - evidence that Y chromosome
determines maleness.
Nondisjunction of Sex Chromosomes (Cntd.)
(c) Turner Syndrome (Monosomy X)
X + O  XO
Short females with edema
(resulting in webbed neck) that
are almost always sterile.
http://www.carolguze.com/text/442-4chromosome_abnormalities.shtml
Nondisjunction of Sex Chromosomes (Cntd.)
(d) OY
O (egg) + Y  OY
Lethal - genes on the X chromosome are necessary for
survival.
2. Y Chromosome
X + YY (sperm)  XYY
Tall, fertile males.
Genetic Counseling
Read P 274.
Nondisjunction
Nondisjunction
Abnormal F Meiosis
ity
I
F Meiosis M Meiosis M Meiosis
II
I
II
Trisomies
75-95%
5-25%
45, X
20%
80%
3N
~25%
0-25%
4N
0
Nondisjunction
All
Spontaneous
Abortion (%)
50
Trisomy 16
7.5
Trisomy 13, 18, 21
4.5
XXX, XXY, XYY
0.3
All other trisomies
13.8
45, X
8.7
3N
6.4
4N
2.4
Structural Abnormalities
2.0
Chromosome Abnormality
Pedigree Example
The above pedigree is for a rare kidney disease. Deduce
the inheritance (autosomal, sex-linked, dominant,
recessive).
Which Part of the Chromosome is
Responsible for Heredity?
• Scientific View up to 1940s
1. Human chromosomes are 60% protein.
2. Proteins are more heterogeneous than nucleic acids (20
amino acids compared to 2 bases).
 Genetic material assumed to be protein.
• Griffith (1928)
Hereditary material can be passed between cells
(transformation).
Read P 282.
Griffith’s Experiment
Which Part of the Chromosome is the
Genetic Material (Cntd.)?
• Hershey and Chase (1952)
DNA (not protein) is the hereditary material.
Read P 283.
Write a QHP for
this experiment and
hand in for extra
credit.
What is the Structure of DNA?
• Levene (1920s)
DNA is a polymer of 5-C sugar, phosphate, & 4 bases (A, T,
C, G). Believed bases in equal proportions. Read P 284285.
• Chargaff (1947) - Chargaff’s Rules
(a) The amounts of the 4 bases are
not equal.
(b) A=T and C=G
What is the Structure of DNA (Cntd.)?
• Franklin (1953)
Method
X-ray diffraction of DNA.
Results
(a) DNA is a double helix with a sugar-phosphate
backbone.
(b) Used the photographs to deduce the dimensions of
DNA.
Rosalind Franklin
What is the Structure of DNA (Cntd.)?
• Watson and Crick (1953)
Method
Built models to determine the structure of DNA (using
Chargaff’s rules and Franklin’s data).
Read Watson and Crick, 1953 and P 286-287.
Structure of DNA
Numbering the C Atoms in a Nucleotide
http://bca.cryst.bbk.ac.uk/bca/cnews/books/Jun00.html
How Does DNA Replicate?
Complementarity suggested a copying mechanism.
Eg. 5´-ACGTAAT-3´ pairs with 3´-TGCATTA-5´
There were 3 hypotheses:
Conservative, dispersive, & semiconservative.
• Meselson and Stahl (1958)
The semiconservative hypothesis is correct.
Read P 288-289 & predict what results would have been
obtained for the other hypotheses (hand this in also).
Meselson-Stahl Experiment
Meselson-Stahl Results