Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 16
Текст из файла (страница 16)
A molecule of DNA consists of two long chains ofnucleotide subunits twisted around one another to form a right-handed helix. Each nucleotide subunit contains anyone of four bases: A (adenine), T (thymine), G (guanine), or C (cytosine). The bases are paired in the two strandsof a DNA molecule.
Wherever one strand has an A, the partner strand has a T, and wherever one strand has a G,the partner strand has a C. The base pairing means that the two paired strands in a DNA duplex molecule havecomplementary base sequences along their lengths. The structure of the DNA molecule suggested that geneticinformation could be coded in DNA in the sequence of bases. Mutations (changes in the genetic material) couldresult from changes in the sequence of bases, such as by the substitution of one nucleotide for another or by theinsertion or deletion of one or more nucleotides. The structure of DNA also suggested a mode of replication inwhich the two strands of the parental DNA molecule separate and each individual strand serves as a template forthe synthesis of a new complementary strand.Most genes code for proteins.
More precisely stated, most genes specify the sequence of amino acids in apolypeptide chain. The transfer of genetic information from DNA into protein is a multistep process that includesseveral types of RNA (ribonucleic acid). Structurally, an RNA strand is similar to a DNA strand except that the"backbone" contains a different sugar (ribose instead of deoxyribose) and RNA contains the base uracil (U) insteadof thymine (T).
Also, RNA is usually present in cells in the form of single, unpaired strands. The initial step ingene expression is transcription, in which a molecule of RNA is synthesized that is complementary in basesequence to whichever DNA strand is being transcribed. In polypeptide synthesis, which takes place on a ribosome,the base sequence in the RNA transcript is translated in groups of three adjacent bases (codons). The codons arerecognized by different types of transfer RNA (tRNA) through base pairing.
Each type of tRNA is attached to aparticular amino acid, and when a tRNA base-pairs with the proper codon on the ribosome, the growing end of thepolypeptide chain is attached to the amino acid on the tRNA. There are special codons that specify the "start" and"stop" of polypeptide synthesis. The most probable reason why various types of RNA are an intimate part oftranscription and translation is that the earliest forms of life used RNA for both genetic information and enzymecatalysis.A mutation that alters one or more codons in a gene may change the amino acid sequence of the resulting proteinsynthesized in the cell. Often the altered protein is functionally defective, so an inborn error of metabolism results.The particular manner in which an inborn error of metabolism is expressed can be very complex, becausemetabolism consists of an intricate branching network of biochemical pathways.
Most visible traits of organismsresult from many genes acting together in combination with environmental factors. The relationship between genesand traits is often complex because (1) every gene potentially affects many traits (hence a gene may showpleiotropy), (2) every trait is potentially affected by many genes (hence two different genes may interact, or showepistasis), and (3) many traits are significantly affected by environmental factors as well as by genes.
Manycomplex traits include unrecognized subtypes that differ in their genetic or environmental causation. Progress ingenetics has often resulted from finding ways to distinguish the subtypes.All living creatures are united by sharing many features of the genetic apparatus (for example, transcription andtranslation) and many metabolic features. The unity of life results from common ancestry and is one of theevidences for evolution.
There is also great diversity among living creatures. The three major kingdoms oforganisms are the bacteria (which lack a membrane-bounded nucleus), the archaea (which share features with botheukarya and bacteria but form a distinct group), and eukarya (all "higher" organisms, whose cells have amembrane-bounded nucleus containing DNA organized into discrete chromosomes).
The bacteria and archaeacollectively are often called prokaryotes.The ultimate source of diversity among organisms is mutation. However, natural selection is the process by whichmutations that are favorable for survival and reproduction are retained and mutations that are harmful areeliminated. Natural selection, first proposed by Charles Darwin, is therefore the primary mechanism by whichorganisms become progressively more adapted to their environments. It is not always easy to determine how (orwhether) a particular trait is adaptive.Key Termsadenine (A)basecolonyamino acidbiochemical pathwaycomplementaryArchaeacentral dogmacytosine (C)Bacteriachromosomedeoxyribonucleic acid (DNA)bacteriophagecodondouble-stranded DNAPage 26duplex DNAmessenger RNAsickle-cell anemiaenzymemRNAsingle-stranded DNAepistasismutationtemplateEukaryanatural selectionthymine (T)eukaryotenucleotidetraitevolutionphagetranscriptfavismpleiotropic effecttranscriptiongenespleiotropytransfer RNAgeneticspolypeptidetransformationglucose-6-phosphatedehydrogenaseprokaryotetranslationG6PDreplicationtRNAG6PD deficiencyribonucleic aciduracil (U)guanine (G)ribosomal RNAWatson-Crick pairinghemoglobinribosomewildtypeinborn error of metabolismRNAjust-so storyrRNAReview the Basics• What is a trait? Give five examples of human traits.
How could you determine whether each of these traits wasgenetically transmitted?• How is it possible for a trait to be determined by both heredity and environment? Give an example of such a trait.• How did understanding the molecular structure of DNA give clues to its ability to replicate, to code for proteins,and to undergo mutations?• Why is pairing of complementary bases a key feature of DNA replication? What is the process of transcriptionand in what ways does it differ from DNA replication?• How is the sequence of amino acids in a protein encoded in the sequence of nucleotides in a messenger RNA?• What is "an inborn error of metabolism"? How did this concept serve as a bridge between genetics andbiochemistry?• What does it mean to say that any gene potentially affects more than one trait? What does it mean to say that onetrait is potentially affected by more than one gene?• If a species A is more closely related evolutionarily to species B than it is to species C, would you expect theDNA sequence of B to be closer to that of A or that of C? Why?Guide to Problem SolvingProblem 1: A double-stranded DNA molecule has the sequence5'-ATGCTTCATTTCAGCTCGAATTTTGCC-3'3'-TACGAAGTAAAGTCGAGCTTAAAACGG-5'When this molecule is replicated, what is the base sequence of the new partner strand that is synthesized to pairwith the upper strand? What is the base sequence of the new partner strand that pairs with the lower strand?Answer: Both new strands have a nucleotide sequence that follows the Watson-Crick base-pairing rules of A withT and G with C.
Therefore, the newly synthesized partner of the upper strand has a base sequence identical to thatof the old lower strand, including the same 3'5' polarity. Similarly, the newly synthesized partner of the lowerstrand has a base sequence identical to that of the old upper strand.Problem 2: Certain enzymes isolated from bacteria can recognize specific, short DNA sequences in duplex DNAand cleave both strands. The enzyme AluI is an example. It recognizes the sequence 5'-AGCT-3' in double-strandedDNA and cleaves both strands at the chemical bond connecting the G and C nucleotides.
If the DNA duplex inWorked Problem 1 were cleaved with AluI, what DNA fragments would result?Answer: There is only one 5'-AGCT-3' site in the duplex, so both strands would be cleaved once at the positionbetween the G and the C in this sequence. Each cleavage generates a new 5' end and a new 3' end, which mustmaintain the polarity of the strand cleaved. Therefore, the resulting double-stranded DNA fragments are5'-ATGCTTCATTTCAG-3'3'-TACGAAGTAAAGTC-5'5'-CTCGAATTTTGCC-3'3'-GAGCTTAAAACGG-5'Problem 3: Suppose that one strand of the DNA duplex in Worked Problem 1 is transcribed from left to right asthe molecule is illustrated.
Which strand is the one transcribed? What is the sequence of the resulting transcript?(Hint: The 5' end of the RNA transcript is synthesized first.)Page 27Chapter 1 GeNETics on the webGeNETics on the web will introduce you to some of the most important sites for finding genetic information onthe Internet. To complete the exercises below, visit the Jones and Bartlett home page athttp://www.jbpub.com/geneticsSelect the link to Genetics: Principles and Analysis and then choose the link to GeNETics on the web. You willbe presented with a chapter-by-chapter list of highlighted keywords.GeNETics EXERCISESSelect the highlighted keyword in any of the exercises below, and you will be linked to a web site containing thegenetic information necessary to complete the exercise. Each exercise suggests a specific, written report thatmakes use of the information available at the site. This report, or an alternative, may be assigned by yourinstructor.1.
James D. Watson once said that he and Francis Crick had no doubt that their proposed DNA structure wasessentially correct, because the structure was so beautiful it had to be true. At an internet site accessed by thekeyword DNA, you can view a large collection of different types of models of DNA structure. Some modelshighlight the sugar-phosphate backbones, others the A—T and G—C base pairs, still others the helical structureof double-stranded DNA. If assigned to do so, pick one of the models that appeals to you. Make a sketch of themodel (or, alternatively, print the model), label the major components, and write a paragraph explaining why youfind this representation appealing.2. One of the first inborn errors of metabolism studied by Archibald Garrod (1902) was a condition calledalkaptonuria.
Use this keyword to learn about the symptoms of this condition and its molecular basis. Whatenzyme is defective in alkaptonuria? What substance is present in the urine of patients that causes it to turn darkupon standing? If assigned to do so, write a 200-word summary of what you have learned.3. Perhaps surprisingly, the history of the bacteriophage T2 that figures so prominently in the experiments ofHershey and Chase is clouded in mystery. Use the keyword T2 to learn what is known about its origin and thesleuthing required to find it out. If assigned, prepare a timeline (chronology) of T2 phage from the time of its firstisolation (under a different name) and its passage from researcher to researcher until it received its "final" name,phage T2.MUTABLE SITE EXERCISESThe Mutable Site Exercise changes frequently. Each new update includes a different exercise that makes use ofgenetics resources available on the World Wide Web.
Select the Mutable Site for Chapter 1, and you will belinked to the current exercise that relates to the material presented in this chapter.PIC SITEThe Pic Site showcases some of the most visually appealing genetics sites on the World Wide Web. To visit theshowcase genetics site, select the Pic Site for Chapter 1.Answer: Because the 5' end of the RNA transcript is synthesized first, and transcription proceeds from left to rightas the template molecule is drawn, the transcribed strand must be the lower strand. This is necessary so that thetemplate DNA strand and the RNA transcript will have opposite polarities.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
