北大生物化学完整课件chapter7
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Nucleotides Deoxyribonucleic Acids (DNA)
Ribonucleic Acids (RNA)
Daochun Kong The College of Life Sciences
Peking University
Deoxyribonucleic Acids (DNA)
Virulent bacteria
Injection Mouse dies
Nonvirulent bacteria
Mouse lives
Avery OT, MacLeod CM, McCarty M (1944) J. Exp. Med. 79:137.
Nonvirulent bacteria DNA from virulent bacteria
CLeabharlann Baidunclusion:
DNA carried a genetic message from one type of cells could be assimilated and expressed by another type of cells.
Argument:
At that time, not everyone accepted Avery et al’s conclusion. The critics attacked that,
2. DNA, the Genetic Substance:
1). DNA was suspected to be genetic material, but, there was more reason to assign this role to proteins. Proteins were thought to be larger, more complex (composed of twenty different amino acids), and therefore were more suitable for a genetic role. DNA was thought to be just a repeating polymer of one kind of tetranucleotide unit.
1. The Discovery:
In 1868, Friedrich Miescher, a Swiss biologist, isolated a phosphorus-containing substance from the nuclei of pus cells (leukocytes), which he called “nuclein”. He further found that the “nuclein” consisted of two portions. One portion is acidic, which we now know as DNA. The other portion is basic, which are mostly histone proteins. Thereofre, the “nuclein” Miescher isolated was actually chromatin. Later, he found that a similar substance in the heads of salmon sperm cells.
3). In 1944, Avery, MacLeod, and McCarty found that DNA prepared from a virulent strain of pneumococcus could transform a nonvirulent strain of this organism into a virulent one.
Conclusion: some chemical in cells of S strain is stable in heat treatment, retains genetic information, is able to transmit that information to R-strain bacteria, convert them to S.
It was just about seventy years ago, in1930s, DNA was widely believed to exist only in animal cells, and RNA was in plant cells. Plant and animal cells were sometimes distinguished based on this chemical feature.
a. The DNA was possibly contaminated by slight amount of proteins that was responsible for the observed transformation.
2). In 1928, Frederick Griffith discovered the phenomenon of transformation in bacteria
Streptococcus pneumoniae has two forms,
Slimy colonies (virulent S strain) forms mucous capsules, it kills mice. But, heat-killed strain does not kill mice. Rough colonies (non-virulent R strain) lacks capsules, it does not kill mice. Mixed heat-killed S-strain with live R-strain, resulting organisms killed mice.
Ribonucleic Acids (RNA)
Daochun Kong The College of Life Sciences
Peking University
Deoxyribonucleic Acids (DNA)
Virulent bacteria
Injection Mouse dies
Nonvirulent bacteria
Mouse lives
Avery OT, MacLeod CM, McCarty M (1944) J. Exp. Med. 79:137.
Nonvirulent bacteria DNA from virulent bacteria
CLeabharlann Baidunclusion:
DNA carried a genetic message from one type of cells could be assimilated and expressed by another type of cells.
Argument:
At that time, not everyone accepted Avery et al’s conclusion. The critics attacked that,
2. DNA, the Genetic Substance:
1). DNA was suspected to be genetic material, but, there was more reason to assign this role to proteins. Proteins were thought to be larger, more complex (composed of twenty different amino acids), and therefore were more suitable for a genetic role. DNA was thought to be just a repeating polymer of one kind of tetranucleotide unit.
1. The Discovery:
In 1868, Friedrich Miescher, a Swiss biologist, isolated a phosphorus-containing substance from the nuclei of pus cells (leukocytes), which he called “nuclein”. He further found that the “nuclein” consisted of two portions. One portion is acidic, which we now know as DNA. The other portion is basic, which are mostly histone proteins. Thereofre, the “nuclein” Miescher isolated was actually chromatin. Later, he found that a similar substance in the heads of salmon sperm cells.
3). In 1944, Avery, MacLeod, and McCarty found that DNA prepared from a virulent strain of pneumococcus could transform a nonvirulent strain of this organism into a virulent one.
Conclusion: some chemical in cells of S strain is stable in heat treatment, retains genetic information, is able to transmit that information to R-strain bacteria, convert them to S.
It was just about seventy years ago, in1930s, DNA was widely believed to exist only in animal cells, and RNA was in plant cells. Plant and animal cells were sometimes distinguished based on this chemical feature.
a. The DNA was possibly contaminated by slight amount of proteins that was responsible for the observed transformation.
2). In 1928, Frederick Griffith discovered the phenomenon of transformation in bacteria
Streptococcus pneumoniae has two forms,
Slimy colonies (virulent S strain) forms mucous capsules, it kills mice. But, heat-killed strain does not kill mice. Rough colonies (non-virulent R strain) lacks capsules, it does not kill mice. Mixed heat-killed S-strain with live R-strain, resulting organisms killed mice.