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Chapter 1

Chapter 1. Genes are DNA. DNA 是遗传物质 DNA 为双螺旋 DNA 的复制是半保留的 通过碱基配对进行核酸杂交 突变改变了 DNA 的序列 突变集中于热点 顺反子是单个 DNA 片断 多重等位基因的种类 DNA 的物理交换导致重组 遗传密码是三联体 细菌的基因和蛋白是共线性的 顺式作用点和反式作用分子 遗传信息可由 DNA 或者 RNA 提供. 本章主要内容. Figure 1.1 A brief history of genetics. 1.1 Introduction. Genes are DNA.

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Chapter 1

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  1. Chapter 1 Genes are DNA

  2. DNA是遗传物质 DNA为双螺旋 DNA的复制是半保留的 通过碱基配对进行核酸杂交 突变改变了DNA的序列 突变集中于热点 顺反子是单个DNA片断 多重等位基因的种类 DNA的物理交换导致重组 遗传密码是三联体 细菌的基因和蛋白是共线性的 顺式作用点和反式作用分子 遗传信息可由DNA或者RNA提供 本章主要内容

  3. Figure 1.1 A brief history of genetics. 1.1 Introduction

  4. Genes are DNA

  5. Avirulent mutants of a virus have lost the capacity to infect a host cell productively, that is, to make more virus.Transfection of eukaryotic cells is the acquisition of new genetic markers by incorporation of added DNA.Transforming principle is DNA that is taken up by a bacterium and whose expression then changes the properties of the recipient cell. 1.2 DNA is the genetic material

  6. Figure 1.2 The transforming principle is DNA. 1.2 DNA is the genetic material

  7. Figure 1.3 The genetic material of phage T2 is DNA. 1.2 DNA is the genetic material

  8. Figure 1.4 Eukaryotic cells can acquire a new phenotype as the result of transfection by added DNA. 1.2 DNA is the genetic material

  9. 1.3 DNA is a double helix Antiparallel strands of the double helix are organized in opposite orientation, so that the 5′ end of one strand is aligned with the 3′ end of the other strand.Base pairing describes the specific (complementary) interactions of adenine with thymine or of cytosine with thymine in a DNA double helix (the former is replaced by adenine with uracil in double helical RNA).Complementary base pairs are defined by the pairing reactions in double helical nucleic acids (A with T in DNA or with U in RNA, and C with G).Supercoiling describes the coiling of a closed duplex DNA in space so that it crosses over its own axis.

  10. Figure 1.5 A polynucleotide chain consists of a series of 5￠-3￠sugar-phosphate links that form a backbone from which the bases protrude 1.3 DNA is a double helix

  11. Figure 1.6 The double helix maintains a constant width because purines always face pyrimidines in the complementary A-T and G-C base pairs. The sequence in the figure is T-A, C-G, A-T, G-C. 1.3 DNA is a double helix

  12. Figure 1.7 Flat base pairs lie perpendicular to the sugar-phosphate backbone. 1.3 DNA is a double helix

  13. Figure 1.8 The two strands of DNA form a double helix. 1.3 DNA is a double helix

  14. 1.4 DNA replication is semicon-servative DNA polymerases are enzymes that synthesize a daughter strand(s) of DNA (under direction from a DNA template). May be involved in repair or replication.DNAases are enzymes that attack bonds in DNA.Endonucleases cleave bonds within a nucleic acid chain; they may be specific for RNA or for single-stranded or double-stranded DNA.Exonucleases cleave nucleotides one at a time from the end of a polynucleotide chain; they may be specific for either the 5′ or 3′ end of DNA or RNA.Parental strands of DNA are the two complementary strands of duplex DNA before replication.

  15. 1.4 DNA replication is semicon-servative Replication fork is the point at which strands of parental duplex DNA are separated so that replication can proceed.Ribonucleases are enzymes that degrade RNA. Exo(ribo)nucleases work progressively, typically degrading one base at a time from the 3′ end toward the 5 ′ end. Endo(ribo)nucleases make single cuts within the RNA chain.RNA polymerases are enzymes that synthesize RNA using a DNA template (formally described as DNA-dependent RNA polymerases).RNAases are enzymes that degrade RNA.Semiconservative replication is accomplished by separation of the strands of a parental duplex, each then acting as a template for synthesis of a complementary strand.

  16. Figure 1.9 Base pairing provides the mechanism for replicating DNA. 1.4 DNA replication is semicon-servative

  17. Figure 1.10 Replication of DNA is semiconservative. 1.4 DNA replication is semicon-servative

  18. 1.4 DNA replication is semiconservative Figure 1.11 The replication fork is the region of DNA in which there is a transition from the unwound parental duplex to the newly replicated daughter duplexes.

  19. Denaturation of DNA or RNA describes its conversion from the double-stranded to the single-stranded state; separation of the strands is most often accomplished by heating.Hybridization is the pairing of complementary RNA and DNA strands to give an RNA-DNA hybrid.Melting of DNA means its denaturation.Melting temperature of DNA is the mid-point of the transition when duplex DNA to denatured by heating to separate into single strands. Renaturation is the reassociation of denatured complementary single strands of a DNA double helix. 1.5 Nucleic acids hybridize by base pairing

  20. Figure 1.12 Base pairing occurs in duplex DNA and also in intra- and inter-molecular interactions in single-stranded RNA (or DNA). 1.5 Nucleic acids hybridize by base pairing

  21. Figure 1.13 Denatured single strands of DNA can renature to give the duplex form. 1.5 Nucleic acids hybridize by base pairing

  22. 1.5 Nucleic acids hybridize by base pairing Figure 1.14 Filter hybridization establishes whether a solution of denatured DNA (or RNA) contains sequences complementary to the strands immobilized on the filter.

  23. Background level of mutation describes the rate at which sequence changes accumulate in the genome of an organism. It reflects the balance between the occurrence spontaneous mutations and their remomval by repair systems, and is characteristic for any species.Deletions are generated by removal of a sequence of DNA, the regions on either side being joined together.result from the action of a mutagen (which may act directly on the bases in DNA) or indirectly, but in either case the result is a change in the sequence of DNA. are identified by the presence of an additional stretch of base pairs in DNA. 1.6 Mutations change the sequence of DNA

  24. 1.6 Mutations change the sequence of DNA Leaky mutants have some residual function, either because the mutant protein is partially active (in the case of a missense mutation), or because a small amount of wild-type protein is made (in the case of a nonsense mutation).Mutagens increase the rate of mutation by inducing changes in DNA sequence, directly or indirectly.Point mutations are changes involving single base pairs.Revertants are derived by reversion of a mutant cell or organism.Spontaneous mutations occur as the result of natural effects, due either to mistakes in DNA replication or to environmental damage.

  25. Suppression describes the occurrence of changes that eliminate the effects of a mutation without reversing the original change in DNA.Suppressor (extragenic) is usually a gene coding a mutant tRNA that reads the mutated codon either in the sense of the original codon or to give an acceptable substitute for the original meaning.Transition is a mutation in which one pyrimidine is substituted by the other or in which one purine is substituted for the other.Transversion is a mutation in which a purine is replaced by a pyrimidine or vice versa. 1.6 Mutations change the sequence of DNA

  26. Figure 1.15 Mutations can be induced by chemical modification of a base. 1.6 Mutations change the sequence of DNA

  27. Figure 1.16 Mutations can be induced by the incorporation of base analogs into DNA. 1.6 Mutations change the sequence of DNA

  28. Back mutation reverses the effect of a mutation that had inactivated a gene; thus it restores wild type.Forward mutations inactivate a wild-type gene.Hotspot is a site at which the frequency of mutation (or recombination) is very much increased.Modified bases are all those except the usual four from which DNA (T, C, A, G) or RNA (U, C, A, G) are synthesized; they result from postsynthetic changes in the nucleic acid.Neutral substitutions in a protein are those changes of amino acids that do not affect activity.Silent mutations do not change the product of a gene. 1.7 Mutations are concentrated at hotspots

  29. Figure 1.17 Spontaneous mutations occur throughout the lacI gene of E. coli, but are concentrated at a hotspot. 1.7 Mutations are concentrated at hotspots

  30. 1.7 Mutations are concentrated at hotspots Figure 1.18 The deamination of 5-methylcytosine produces thymine (causing C-G to T-A transitions), while the deamination of cytosine produces uracil (which usually is removed and then replaced by cytosine).

  31. Figure 1.15 Mutations can be induced by chemical modification of a base. 1.7 Mutations are concentrated at hotspots

  32. Cistron is the genetic unit defined by the cis/trans test; equivalent to gene.Complementation group is a series of mutations unable to complement when tested in pairwise combinations in trans; defines a genetic unit (the cistron).Gene (cistron) is the segment of DNA involved in producing a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons).One gene : one enzyme hypothesis is the basis of modern genetics: that a gene is a stretch of DNA coding for a single polypeptide chain. 1.8 A cistron is a single stretch of DNA

  33. Figure 1.19 Genes code for proteins; dominance is explained by the properties of mutant proteins. A recessive allele does not contribute to the phenotype because it produces no protein (or protein that is nonfunctional). 1.8 A cistron is a single stretch of DNA

  34. Figure 1.20 The cistron is defined by the complementation test. Genes are represented by bars; red stars identify sites of mutation. 1.8 A cistron is a single stretch of DNA

  35. 1.9 The nature of multiple alleles Gain-of-function mutation represents acquisition of a new activity. It is dominant.Leaky mutants have some residual function, either because the mutant protein is partially active (in the case of a missense mutation), or because a small amount of wild-type protein is made (in the case of a nonsense mutation).Loss-of-function mutation inactivates a gene. It is recessive.Null mutation completely eliminates the function of a gene, usually because it has been physically deleted.Polymorphism refers to the simultaneous occurrence in the population of genomes showing allelic variations (as seen either in alleles producing different phenotypes or-for example-in changes in DNA affecting the restriction pattern).

  36. Figure 1.19 Genes code for proteins; dominance is explained by the properties of mutant proteins. A recessive allele does not contribute to the phenotype because it produces no protein (or protein that is nonfunctional). 1.9 The nature of multiple alleles

  37. 1.9 The nature of multiple alleles Figure 1.21 The w locus has an extensive series of alleles, whose phenotypes extend from wild-type (red) color to complete lack of pigment.

  38. Figure 1.22 The ABO blood group locus codes for a galactosyltransferase whose specificity determines the blood group. 1.9 The nature of multiple alleles

  39. 1.10 Recombination occurs by physical exchange of DNA Bivalent is the structure containing all four chromatids (two representing each homologue) at the start of meiosis.Breakage and reunion describes the mode of genetic recombination, in which two DNA duplex molecules are broken at corresponding points and then rejoined crosswise (involving formation of a length of heteroduplex DNA around the site of joining).Chiasma (pl. chiasmata) is a site at which two homologous chromosomes appear to have exchanged material during meiosis.Crossing-over describes the reciprocal exchange of material between chromosomes that occurs during meiosis and is responsible for genetic recombination.Hybrid DNA is another term for heteroduplex DNA.

  40. 1.10 Recombination occurs by physical exchange of DNA Figure 1.23 Chiasma formation is responsible for generating recombinants.

  41. 1.10 Recombination occurs by physical exchange of DNA Figure 1.24 Recombination involves pairing between complementary strands of the two parental duplex DNAs.

  42. 1.10 Recombination occurs by physical exchange of DNA Figure 1.13 Denatured single strands of DNA can renature to give the duplex form.

  43. 1.11 The genetic code is triplet-- Key terms Codon is a triplet of nucleotides that represents an amino acid or a termination signal.Frameshift mutation results from an insertion or deletion that changes the phase of triplets, so that all codons are misread after the site of mutation.Genetic code is the correspondence between triplets in DNA (or RNA) and amino acids in protein.Initiation codon is a special codon (usually AUG) used to start synthesis of a protein.ORF is an open reading frame; presumed likely to code for a protein.>Reading frame is one of three possible ways of reading a nucleotide sequence as a series of triplets.Suppressor (extragenic) is usually a gene coding a mutant tRNA that reads the mutated codon either in the sense of the original codon or to give an acceptable substitute for the original meaning.Termination codon is one of three (UAG, UAA, UGA) that causes protein synthesis to terminate.

  44. 1.11 The genetic code is triplet Figure 1.25 Frameshift mutations show that the genetic code is read in triplets from a fixed starting point.

  45. 1.11 The genetic code is triplet Figure 1.26 An open reading frame starts with AUG and continues in triplets to a termination codon. Blocked reading frames may be interrupted frequently by termination codons.

  46. 1.12 The relationship between coding sequences and proteins Coding region is a part of the gene that represents a protein sequence.Leader of a protein is a short N-terminal sequence responsible for passage into or through a membrane.RNA splicing is the process of excising the sequences in RNA that correspond to introns, so that the sequences corresponding to exons are connected into a continuous mRNA.Trailer is a nontranslated sequence at the 3´ end of an mRNA following the termination codon.Transcription is synthesis of RNA on a DNA template.Translation is synthesis of protein on the mRNA template.

  47. 1.12 The relationship between coding sequences and proteins Figure 1.27 The recombination map of the tryptophan synthetase gene corresponds with the amino acid sequence of the protein.

  48. 1.12 The relationship between coding sequences and proteins Figure 1.28 RNA is synthesized by using one strand of DNA as a template for complementary base pairing.

  49. 1.12 The relationship between coding sequences and proteins Figure 1.29 The gene may be longer than the sequence coding for protein.

  50. 1.12 The relationship between coding sequences and proteins Figure 1.30 Gene expression is a multistage process.

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