Segments of Dna Transferred From Parent to Offspring Are Called

Possibly the nearly cardinal property of all living things is the ability to reproduce. All organisms inherit the genetic data specifying their structure and function from their parents. Also, all cells arise from preexisting cells, so the genetic material must be replicated and passed from parent to progeny cell at each cell division. How genetic information is replicated and transmitted from cell to cell and organism to organism thus represents a question that is primal to all of biology. Consequently, elucidation of the mechanisms of genetic transmission and identification of the genetic cloth as Dna were discoveries that formed the foundation of our current agreement of biology at the molecular level.

Genes and Chromosomes

The classical principles of genetics were deduced by Gregor Mendel in 1865, on the ground of the results of breeding experiments with peas. Mendel studied the inheritance of a number of well-defined traits, such as seed color, and was able to deduce general rules for their transmission. In all cases, he could correctly interpret the observed patterns of inheritance by bold that each trait is determined past a pair of inherited factors, which are now called genes. One factor copy (chosen an allele) specifying each trait is inherited from each parent. For example, breeding 2 strains of peas—i having yellow seeds, and the other green seeds—yields the post-obit results (Figure iii.ane). The parental strains each accept two identical copies of the factor specifying yellow (Y) or green (y) seeds, respectively. The progeny plants are therefore hybrids, having inherited i cistron for yellow seeds (Y) and ane for dark-green seeds (y). All these progeny plants (the outset filial, or F1, generation) have yellow seeds, so yellow (Y) is said to be dominant and green (y) recessive. The genotype (genetic composition) of the Fane
peas is thus
Yy, and their phenotype (physical appearance) is yellow. If 1 Fane
offspring is bred with another, giving rise to F2
progeny, the genes for xanthous and green seeds segregate in a characteristic fashion such that the ratio between F2
plants with yellow seeds and those with green seeds is 3:1.

Figure three.1

Inheritance of dominant and recessive genes.

Mendel’s findings, apparently ahead of their time, were largely ignored until 1900, when Mendel’due south laws were rediscovered and their importance recognized. Shortly thereafter, the part of chromosomes as the carriers of genes was proposed. It was realized that most cells of higher plants and animals are diploid—containing two copies of each chromosome. Germination of the germ cells (the sperm and egg), yet, involves a unique type of jail cell division (meiosis) in which only one member of each chromosome pair is transmitted to each progeny jail cell (Figure three.2). Consequently, the sperm and egg are haploid, containing only one re-create of each chromosome. The union of these 2 haploid cells at fertilization creates a new diploid organism, now containing ane member of each chromosome pair derived from the male and one from the female parent. The behavior of chromosome pairs thus parallels that of genes, leading to the decision that genes are carried on chromosomes.

Figure 3.2. Chromosomes at meiosis and fertilization.

Figure 3.ii

Chromosomes at meiosis and fertilization. Two chromosome pairs of a hypothetical organism are illustrated.

The fundamentals of mutation, genetic linkage, and the relationships between genes and chromosomes were largely established by experiments performed with the fruit wing,
Drosophila melanogaster.
Drosophila
can exist hands maintained in the laboratory, and they reproduce about every two weeks, which is a considerable advantage for genetic experiments. Indeed, these features go on to make
Drosophila
an organism of choice for genetic studies of animals, particularly the genetic analysis of evolution and differentiation.

In the early 1900s, a number of genetic alterations (mutations) were identified in
Drosophila, usually affecting readily observable characteristics such as centre color or wing shape. Breeding experiments indicated that some of the genes governing these traits are inherited independently of each other, suggesting that these genes are located on unlike chromosomes that segregate independently during meiosis (Figure 3.3). Other genes, however, are frequently inherited together equally paired characteristics. Such genes are said to be linked to each other by virtue of existence located on the aforementioned chromosome. The number of groups of linked genes is the aforementioned as the number of chromosomes (four in
Drosophila), supporting the idea that chromosomes are carriers of the genes.

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Figure 3.3. Gene segregation and linkage.

Figure 3.3

Cistron segregation and linkage. (A) Segregation of two hypothetical genes for shape (A/a
= square/round) and color (B/b
= red/blue) located on different chromosomes. (B) Linkage of two genes located on the aforementioned chromosome.

Linkage between genes is not consummate, however; chromosomes exchange cloth during meiosis, leading to recombination between linked genes (Effigy 3.four). The frequency of recombination between two linked genes depends on the altitude between them on the chromosome; genes that are close to each other recombine less frequently than do genes further apart. Thus, the frequencies with which different genes recombine can exist used to determine their relative positions on the chromosome, allowing the construction of
genetic maps
(Figure 3.5). By 1915, nearly a hundred genes had been defined and mapped onto the four chromosomes of
Drosophila, leading to full general credence of the chromosomal basis of heredity.

Figure 3.4. Genetic recombination.

Figure 3.four

Genetic recombination. During meiosis, members of chromosome pairs exchange cloth. The effect is recombination between linked genes.

Figure 3.5. A genetic map.

Figure 3.5

A genetic map. Three genes are localized on a hypothetical chromosome based on frequencies of recombination between them (one% recombination between
a
and
b; 3% between
b
and
c; iv% betwixt
a
and
c). The frequencies of recombination are approximately proportional (more…)

Genes and Enzymes

Early genetic studies focused on the identification and chromosomal localization of genes that control readily observable characteristics, such as the eye colour of
Drosophila. How these genes pb to the observed phenotypes, however, was unclear. The first insight into the relationship between genes and enzymes came in 1909, when it was realized that the inherited human disease phenylketonuria (run into Molecular Medicine in Affiliate ii) results from a genetic defect in metabolism of the amino acrid phenylalanine. This defect was hypothesized to result from a deficiency in the enzyme needed to catalyze the relevant metabolic reaction, leading to the general suggestion that genes specify the synthesis of enzymes.

Clearer bear witness linking genes with the synthesis of enzymes came from experiments of George Beadle and Edward Tatum, performed in 1941 with the mucus
Neurospora crassa. In the laboratory,
Neurospora
can be grown on minimal or rich media similar to those discussed in Chapter 1 for the growth of
E.
coli. For
Neurospora, minimal media consist merely of salts, glucose, and biotin; rich media are supplemented with amino acids, vitamins, purines, and pyrimidines. Beadle and Tatum isolated mutants of
Neurospora
that grew unremarkably on rich media merely could not grow on minimal media. Each mutant was found to crave a specific nutritional supplement, such as a particular amino acrid, for growth. Furthermore, the requirement for a specific nutritional supplement correlated with the failure of the mutant to synthesize that particular chemical compound. Thus, each mutation resulted in a deficiency in a specific metabolic pathway. Since such metabolic pathways were known to be governed by enzymes, the decision from these experiments was that each cistron specified the structure of a single enzyme—the
ane factor-one enzyme hypothesis. Many en-zymes are now known to consist of multiple polypeptides, so the currently accepted argument of this hypothesis is that each gene specifies the construction of a unmarried polypeptide chain.

Identification of DNA every bit the Genetic Material

Agreement the chromosomal basis of heredity and the relationship between genes and enzymes did not in itself provide a molecular explanation of the gene. Chromosomes comprise proteins as well every bit DNA, and it was initially idea that genes were proteins. The starting time evidence leading to the identification of Deoxyribonucleic acid every bit the genetic material came from studies in leaner. These experiments correspond a prototype for electric current approaches to defining the function of genes by introducing new DNA sequences into cells, equally discussed subsequently in this chapter.

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The experiments that defined the part of DNA were derived from studies of the bacterium that causes pneumonia (Pneumococcus). Virulent strains of
Pneumococcus
are surrounded past a polysaccharide sheathing that protects the bacteria from attack by the allowed system of the host. Because the capsule gives bacterial colonies a smoothen appearance in culture, encapsulated strains are denoted S. Mutant strains that take lost the ability to make a sheathing (denoted R) form rough-edged colonies in civilization and are no longer lethal when inoculated into mice. In 1928 it was observed that mice inoculated with nonencapsulated (R) bacteria plus rut-killed encapsulated (S) bacteria developed pneumonia and died. Importantly, the bacteria that were and so isolated from these mice were of the S type. Subsequent experiments showed that a cell-free extract of Due south bacteria was similarly capable of converting (or transforming) R bacteria to the Southward state. Thus, a substance in the South extract (called the transforming principle) was responsible for inducing the genetic
transformation
of R to S leaner.

In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty established that the transforming principle was DNA, both by purifying it from bacterial extracts and by demonstrating that the activity of the transforming principle is abolished by enzymatic digestion of Deoxyribonucleic acid but non by digestion of proteins (Figure 3.6). Although these studies did not immediately lead to the acceptance of Dna every bit the genetic material, they were extended within a few years by experiments with bacterial viruses. In detail, it was shown that, when a bacterial virus infects a cell, the viral DNA rather than the viral protein must enter the prison cell in order for the virus to replicate. Moreover, the parental viral DNA (but not the poly peptide) is transmitted to progeny virus particles. The concurrence of these results with continuing studies of the action of Dna in bacterial transformation led to acceptance of the idea that Deoxyribonucleic acid is the genetic material.

Figure 3.6. Transfer of genetic information by DNA.

Figure iii.6

Transfer of genetic information past DNA. Dna is extracted from a pathogenic strain of
Pneumococcus, which is surrounded past a capsule and forms smooth colonies (Due south). Improver of the purified South DNA to a culture of nonpathogenic, nonencapsulated leaner (R (more…)

The Structure of DNA

Our understanding of the threedimensional structure of DNA, deduced in 1953 by James Watson and Francis Crick, has been the basis for nowadays-24-hour interval molecular biology. At the time of Watson and Crick’southward work, Dna was known to be a polymer composed of four nucleic acid bases—ii purines (adenine [A] and guanine [M]) and two pyrimidines (cytosine [C] and thymine [T])—linked to phosphorylated sugars. Given the central part of DNA as the genetic material, elucidation of its three-dimensional structure appeared critical to understanding its function. Watson and Crick’s consideration of the problem was heavily influenced past Linus Pauling’s description of hydrogen bonding and the α helix, a common element of the secondary structure of proteins (run across Chapter two). Moreover, experimental data on the structure of DNA were bachelor from X-ray crystallography studies by Maurice Wilkins and Rosalind Franklin. Assay of these data revealed that the DNA molecule is a helix that turns every 3.4 nm. In improver, the data showed that the distance between adjacent bases is 0.34 nm, and then in that location are ten bases per turn of the helix. An important finding was that the diameter of the helix is approximately 2 nm, suggesting that it is composed of non one simply two Deoxyribonucleic acid chains.

From these data, Watson and Crick built their model of Dna (Figure 3.7). The central features of the model are that DNA is a double helix with the saccharide-phosphate backbones on the outside of the molecule. The bases are on the inside, oriented such that hydrogen bonds are formed between purines and pyrimidines on reverse chains. The base pairing is very specific: A ever pairs with T and One thousand with C. This specificity accounts for the earlier results of Erwin Chargaff, who had analyzed the base composition of various DNAs and institute that the amount of adenine was always equal to that of thymine, and the amount of guanine to that of cytosine. Because of this specific base of operations pairing, the two strands of a Dna molecule are complementary: Each strand contains all the information required to specify the sequences of bases on the other.

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Figure 3.7. The structure of DNA.

Replication of Deoxyribonucleic acid

The discovery of complementary base of operations pairing between Dna strands immediately suggested a molecular solution to the question of how the genetic material could directly its own replication—a process that is required each time a cell divides. It was proposed that the two strands of a DNA molecule could separate and serve every bit templates for synthesis of new complementary strands, the sequence of which would exist dictated by the specificity of base pairing (Figure 3.8). The process is called
semiconservative replication
because i strand of parental DNA is conserved in each progeny DNA molecule.

Figure 3.8. Semiconservative replication of DNA.

Figure 3.8

Semiconservative replication of Deoxyribonucleic acid. The two strands of parental Deoxyribonucleic acid split up, and each serves every bit a template for synthesis of a new daughter strand by complementary base pairing.

Straight support for semiconservative Deoxyribonucleic acid replication was obtained in 1958 as a consequence of elegant experiments, performed by Matthew Meselson and Frank Stahl, in which DNA was labeled with isotopes that altered its density (Effigy iii.9).
E.
coli
were first grown in media containing the heavy isotope of nitrogen (15North) in place of the normal light isotope (14N). The DNA of these bacteria consequently contained
15Northward and was heavier than that of bacteria grown in
14N. Such heavy DNA could exist separated from DNA containing
14North by equilibrium centrifugation in a density slope of CsCl. This ability to separate heavy (xvN) DNA from light (14N) Deoxyribonucleic acid enabled the study of DNA synthesis.
E.
coli
that had been grown in
fifteenNorth were transferred to media containing
fourteenN and allowed to replicate i more than time. Their DNA was so extracted and analyzed by CsCl density gradient centrifugation. The results of this analysis indicated that all of the heavy Dna had been replaced by newly synthesized DNA with a density intermediate between that of heavy (15N) and that of light (fourteenNorth) Deoxyribonucleic acid molecules. The implication was that during replication, the two parental strands of heavy Dna separated and served as templates for newly synthesized progeny strands of lite DNA, yielding double-stranded molecules of intermediate density. This experiment thus provided direct evidence for semiconservative DNA replication, clearly underscoring the importance of complementary base pairing between strands of the double helix.

Figure 3.9. Experimental demonstration of semiconservative replication.

Figure iii.9

Experimental sit-in of semiconservative replication. Bacteria grown in medium containing the normal isotope of nitrogen (14N) are transferred into medium containing the heavy isotope (15N) and grown in this medium for several generations. They (more…)

The ability of DNA to serve every bit a template for its own replication was further established with the demonstration that an enzyme purified from
Due east.
coli
(DNA polymerase) could catalyze Deoxyribonucleic acid replication
in vitro. In the presence of Dna to human action as a template, DNA polymerase was able to direct the incorporation of nucleotides into a complementary DNA molecule.

Segments of Dna Transferred From Parent to Offspring Are Called

Source: https://www.ncbi.nlm.nih.gov/books/NBK9944/