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 SyndromeXX + Y XXYSterile 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 CounselingRead 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 Abnormality Spontaneous Abortion (%)

All 50Trisomy 16 7.5

Trisomy 13, 18, 21 4.5XXX, XXY, XYY 0.3All other trisomies 13.8

45, X 8.73N 6.44N 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