第二章 染色体与DNA

分子生物学

Chapter 2 The DNA and Chromosome

基因工程 教研室

分子生物学

Content: 2.1

DNA 2.2 Chromosome Structure

分子生物学

2.1 DNA

分子生物学

Discovery of DNA FunctionA. Discovery of DNA1. Friedrich Miescher first isolated from cells in 1868 2. unknown function 3. Nuclein contains deoxyribonucleic acid (DNA).

At the end of 19th century, chemists learned the structure of DNA and a related compound, ribonucleic acid (RNA) Both DNA and RNA are long polymers -- chains of small units called nucleotides.

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D.Gierer & Schramm Tobacco Mosaic Virus (TMV) Experiment - 1956 Fraenkel-Conrat & Singer - 1957 Used 2 viral strains to demonstrate RNA is the genetic material of TMV

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Discovery of DNA StructureA. Components of DNA 1. DNA consists of a sugar, base and phosphates 2. one of 4 possible bases: adenine (A) cytosine (C) guanine (G) thymine (T)

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B: Chargaff’s Rules

Erwin Chargaff observed in 1950 that the content of purines was always roughly equal to the content of pyrimidines. Furthermore, the amounts of adenine and thymine were always roughly equal, as were the amounts of guanine and cytosine.

A=T, C=G, but what does this mean?

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B. Chargaff experiments 1. the amount of A = amount of T 2. the amount of C = amount of G 3. ratios vary by species 4. Thus… if an organism has 20% T, how much C is present in the organism? since % T = % A and % G = % C, then if T=20, A=20 and G+C = 60 thus C=30%

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DNA vs RNA(Deoxyribonucleic Acid)Sugar Deoxyribose (Ribonucleic Acid) Ribose

Base

Adenine Cytosine Guanine Thymine

Adenine Cytosine Guanine Uracil

分子生物学

分子生物学

Nucleotides = monomers of nucleic acid polymer Nucleotide = Base + sugar + single phosphate group Nucleoside = Base + sugar

分子生物学

分子生物学

分子生物学

C. Franklin experiments , 1952 1. used X-rays to examine structure of DNA 2. observed that a. DNA is helical b. DNA exists in long strands c. DNA is highly repetitive

What does this interesting pattern mean?

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D. Watson-Crick analysis 1. used data from Franklin & Chargaff experiments to construct a model of DNA structure 2. phosphate groups from backbone 3. double-stranded molecule 4. base=pairing of A-T and G-C 5. helical shape

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Double Helix Model of DNA: Six main features1. Two polynucleotide chains wound in a right-handed (clockwise) double-helix.

2.3. 4.

Nucleotide chains are antiparallel:

5’ 3’ 3’ 5’

Sugar-phosphate backbones are on the outside of the double helix, and the bases are oriented towards the central axis. Complementary base pairs from opposite strands are bound together by relatively weak hydrogen bonds. A pairs with T (2 H-bonds), and G pairs with C (3 H-bonds).

e.g., 5’-TATTCCGA-3’ 3’-ATAAGGCT-3’5. 6. Base pairs are 0.34 nm apart. One complete turn of the helix requires 3.4 nm (10 bases/turn). Sugar-phosphate backbones are not equally-spaced, resulting in major and minor grooves.

分子生物学

分子生物学

1962: Nobel Prize in Physiology and Medicine

Francis Crick Maurice Wilkins

James Waston

Rosalind Franklin

分子生物学

分子生物学

Watson and Crick submitted a 900 word manuscript

on their DNA structure prediction to Nature. In their paper, they wrote: “It has not escaped our notice that the specific base pairing we have proposed immediately suggests a possible copying mechanism for the genetic material.” This basically serves as a guideline on how DNA can be faithfully replicated or copied into the next generation.

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Two DNA strands can be separated simply by heating, a process called DNA denaturation or DNA melting. The temperature at which the DNA strands are half denatured is called the melting temperature, or Tm. The amount of DNA strand separation can be measured by the absorbance of the DNA solution at 260 nm light.

Tm of a DNA is largely determined by its G/C%.

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