B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 68
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In other words, if we designate the two DNA strands as S and Sʹ, strand_5′ endObasesminorgroovemajorgroove_OP OOO P_O OOO_O P O(A)5′ end(B)3′OOP OOO5′OOP O_O OOG0.34 nmOGOCOO5′2 nmO3′ end_OCPOOTOOOChydrogen bond3′ O3′ end_OOsugarAGPO_P OOphosphodiesterbondFigure 4–5 The DNA double helix.(A) A space-filling model of 1.5 turns ofthe DNA double helix. Each turn of DNA ismade up of 10.4 nucleotide pairs, and thecenter-to-center distance between adjacentnucleotide pairs is 0.34 nm.
The coiling ofthe two strands around each other createstwo grooves in the double helix: the widergroove is called the major groove, and thesmaller the minor groove, as indicated.(B) A short section of the double helixviewed from its side, showing four basepairs. The nucleotides are linked togethercovalently by phosphodiester bonds thatjoin the 3ʹ-hydroxyl (–OH) group of onesugar to the 5ʹ-hydroxyl group of the nextsugar.
Thus, each polynucleotide strandhas a chemical polarity; that is, its twoends are chemically different. The 5ʹ endof the DNA polymer is by convention oftenillustrated carrying a phosphate group,while the 3ʹ end is shown with a hydroxyl.178Chapter 4: DNA, Chromosomes, and Genomestemplate S strand5′3′S strand3′5′3′S′ strand5′3′5′new S′ strandnew S strand5′3′Figure 4–6 DNA as a template for itsown duplication.
Because the nucleotideA successfully pairs only with T, and Gpairs with C, each strand of DNA can actas a template to specify the sequence ofnucleotides in its complementary strand.In this way, double-helical DNA can becopied precisely, with each parental DNAhelix producing two identical daughter DNAhelices.parental DNA double helix3′template S′ strand5′daughter DNA double helicesS can serve as a template for making a new strand Sʹ, while strand Sʹ can serve as atemplate for making a new strand S (Figure 4–6). Thus, the genetic information inDNA can be accurately copied by the beautifully simple process in which strandS separates from strand Sʹ, and each separated strand then serves as a templatefor the production of a new complementary partner strand that is identical to itsformer partner.The ability of each strand of a DNA molecule to act as a template for producingMBoC6 m4.08/4.08a complementary strand enables a cell to copy, or replicate, its genome beforepassing it on to its descendants.
We shall describe the elegant machinery that thecell uses to perform this task in Chapter 5.Organisms differ from one another because their respective DNA moleculeshave different nucleotide sequences and, consequently, carry different biologicalmessages. But how is the nucleotide alphabet used to make messages, and whatdo they spell out?As discussed above, it was known well before the structure of DNA was determined that genes contain the instructions for producing proteins. If genes aremade of DNA, the DNA must therefore somehow encode proteins (Figure 4–7).As discussed in Chapter 3, the properties of a protein, which are responsible for itsbiological function, are determined by its three-dimensional structure.
This structure is determined in turn by the linear sequence of the amino acids of which it iscomposed. The linear sequence of nucleotides in a gene must therefore somehowspell out the linear sequence of amino acids in a protein. The exact correspondence between the four-letter nucleotide alphabet of DNA and the twenty-letteramino acid alphabet of proteins—the genetic code—is not at all obvious from theDNA structure, and it took over a decade after the discovery of the double helixbefore it was worked out. In Chapter 6, we will describe this code in detail in thecourse of elaborating the process of gene expression, through which a cell convertsthe nucleotide sequence of a gene first into the nucleotide sequence of an RNAmolecule, and then into the amino acid sequence of a protein.The complete store of information in an organism’s DNA is called its genome,and it specifies all the RNA molecules and proteins that the organism will eversynthesize.
(The term genome is also used to describe the DNA that carries thisinformation.) The amount of information contained in genomes is staggering. Thenucleotide sequence of a very small human gene, written out in the four-letternucleotide alphabet, occupies a quarter of a page of text (Figure 4–8), while thecomplete sequence of nucleotides in the human genome would fill more than athousand books the size of this one. In addition to other critical information, itincludes roughly 21,000 protein-coding genes, which (through alternative splicing; see p.
415) give rise to a much greater number of distinct proteins.In Eukaryotes, DNA Is Enclosed in a Cell NucleusAs described in Chapter 1, nearly all the DNA in a eukaryotic cell is sequestered ina nucleus, which in many cells occupies about 10% of the total cell volume. Thiscompartment is delimited by a nuclear envelope formed by two concentric lipidgene Agene Bgene CDNAdoublehelixGENEEXPRESSIONprotein Aprotein B protein CFigure 4–7 The relationship betweengenetic information carried in DNA andproteins.
(Discussed in Chapter 1.)MBoC6 m4.06/4.06CHROMOSOMAL DNA AND ITS PACKAGING IN THE CHROMATIN FIBERFigure 4–7 The nucleotide sequence of the human β-globin gene. Byconvention, a nucleotide sequence is written from its 5ʹ end to its 3ʹ end,and it should be read from left to right in successive lines down the page asthough it were normal English text. This gene carries the information for theamino acid sequence of one of the two types of subunits of the hemoglobinmolecule; a different gene, the α-globin gene, carries the information for theother. (Hemoglobin, the protein that carries oxygen in the blood, has foursubunits, two of each type.) Only one of the two strands of the DNA doublehelix containing the β-globin gene is shown; the other strand has the exactcomplementary sequence.
The DNA sequences highlighted in yellow showthe three regions of the gene that specify the amino acid sequence for theβ-globin protein. We shall see in Chapter 6 how the cell splices these threesequences together at the level of messenger RNA in order to synthesize afull-length β-globin protein.bilayer membranes (Figure 4–9). These membranes are punctured at intervalsby large nuclear pores, through which molecules move between the nucleus andthe cytosol.
The nuclear envelope is directly connected to the extensive systemof intracellular membranes called the endoplasmic reticulum, which extend outfrom it into the cytoplasm. And it is mechanically supported by a network of intermediate filaments called the nuclear lamina—a thin feltlike mesh just beneaththe inner nuclear membrane (see Figure 4–9B).The nuclear envelope allows the many proteins that act on DNA to be concentrated where they are needed in the cell, and, as we see in subsequent chapters, italso keeps nuclear and cytosolic enzymes separate, a feature that is crucial for theproper functioning of eukaryotic cells.SummaryGenetic information is carried in the linear sequence of nucleotides in DNA.
Eachmolecule of DNA is a double helix formed from two complementary antiparallelstrands of nucleotides held together by hydrogen bonds between G-C and A-T basepairs. Duplication of the genetic information occurs by the use of one DNA strandas a template for the formation of a complementary strand. The genetic informationstored in an organism’s DNA contains the instructions for all the RNA molecules andproteins that the organism will ever synthesize and is said to comprise its genome.In eukaryotes, DNA is contained in the cell nucleus, a large membrane-bound compartment.CHROMOSOMAL DNA AND ITS PACKAGING IN THECHROMATIN FIBERThe most important function of DNA is to carry genes, the information that specifies all the RNA molecules and proteins that make up an organism—includinginformation about when, in what types of cells, and in what quantity each RNAmolecule and protein is to be made. The nuclear DNA of eukaryotes is divided upinto chromosomes, and in this section we see how genes are typically arranged oneach chromosome.
In addition, we describe the specialized DNA sequences thatare required for a chromosome to be accurately duplicated as a separate entityand passed on from one generation to the next.We also confront the serious challenge of DNA packaging. If the double helicescomprising all 46 chromosomes in a human cell could be laid end to end, theywould reach approximately 2 meters; yet the nucleus, which contains the DNA, isonly about 6 μm in diameter. This is geometrically equivalent to packing 40 km (24miles) of extremely fine thread into a tennis ball. The complex task of packagingDNA is accomplished by specialized proteins that bind to the DNA and fold it,generating a series of organized coils and loops that provide increasingly higherlevels of organization, and prevent the DNA from becoming an unmanageabletangle.
Amazingly, although the DNA is very tightly compacted, it neverthelessremains accessible to the many enzymes in the cell that replicate it, repair it, anduse its genes to produce RNA molecules and proteins.179180Chapter 4: DNA, Chromosomes, and Genomesendoplasmic reticulumheterochromatinheterochromatinDNA and associatedproteins (chromatin),plus many RNA andprotein moleculesnuclearenvelopenucleoluscentrosomenucleolusmicrotubulenuclear laminanuclear poreouter nuclear membrane(A)(B)inner nuclear membranenuclear envelope2 µmFigure 4–9 A cross-sectional view of a typical cell nucleus. (A) Electron micrograph of a thin section through the nucleus ofa human fibroblast. (B) Schematic drawing, showing that the nuclear envelope consists of two membranes, the outer one beingcontinuous with the endoplasmic reticulum (ER) membrane (see also Figure 12–7).
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