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Bio 178 Lecture 27Genetics 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/442-4-chromosome_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/442-4-chromosome_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
Abnormality
F Meiosis I
F Meiosis II
M Meiosis I
M Meiosis II
Trisomies 75-95% 5-25% 0
45, X 20% 80%
3N ~25% 0-25%
4N
Nondisjunction
Chromosome AbnormalitySpontaneous Abortion (%)
All 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
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 1940s1. 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 284-285.
• 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