Dna Replication In Plants

John A. Bryant

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315892412
• eBook ISBN: 9781351088411
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Biología

This texts discusses DNA replication in plants including chapters on; functional chromosomal structure, the biochemistry of DNA replication, Control of DNA replication, Replication of plant organelle DNA, replication of DNA viruses in plants, and DNA damage, repair, and mutagenesis.

Chapter 1
Functional Chromosomal Structure: The Replicon

Jack Van’t Hof

1. Introduction
2. Plant Chromosomal DNA Replication
   1. Plant Chromosomes Have Very Long DNA Molecules
   2. Plant Chromosomal DNA Is Replicated Simultaneously at Multiple Sites
   3. Three Replicon Properties of Plant Chromosomal DNA Seen by DNA Fiber Autoradiography
   4. Hierarchical Organization of Temporally Ordered Plant Chromosomal DNA Replication
   5. Plant Chromosomal DNA Replicates Stepwise
   6. Chromosomal DNA Maturation Is Influenced by the Size of the Thymidine Pool
   7. Replicon Termination Sites
   8. Rate of Replication Fork Movement in Higher Plants
3. Origins of Replication
   1. A. Prokaryotes
   2. B. Higher Plants
   3. C. Chromatin Structure at the Replication Origins
4. Structural Organization of Replicon Domains
5. General Comments

Added in Proof

Acknowledgment

References

I. Introduction

Historically, plant breeding, classical plant genetics, and plant cytogenetics have established many fundamental rules of inheritance for both plants and animals. Similarly, contemporary work on plants is making leading contributions in genetic engineering, transformation, gene expression, somatic cell hybridization, and the use of sophisticated culture procedures to clone, develop, and propagate cells and plants, as well as adding to our knowledge of concepts applicable to plants and animals. Two basic biological processes essential to both classical and molecular plant research are cell division and chromosomal DNA replication. Should either of these two processes fail to function all experiments, whether classical or molecular, also would fail. Yet, despite their importance, too little is known about cell division and chromosomal DNA replication in higher plants. The objective of this chapter, and indeed, of the entire book, is to encourage more research in these fields.

The review is a synopsis of the current knowledge of chromosomal DNA replication in higher plants. It attempts to unfold the story from its historical beginnings, highlighting areas of ongoing research where questions are posed and answers are still forthcoming, and presents a short summary suggesting where future work with higher plants can contribute to the general principles governing how chromosomes are made and how replication of DNA is tied to functions peculiar to plants.

The information presented is from plants and from other organisms. Work with nonplant systems is well advanced and evidence from bacteria, protozoa, simple fungi, and mammalian cells offers helpful clues about how to approach similar problems in plants.

II. Plant Chromosomal Dna Replication

A. Plant Chromosomes Have Very Long DNA Molecules

Eukaryotic chromosomes viewed at metaphase are neatly and reliably structured. They are the classical hallmarks for species identification; their numbers, dimensions, and shape are commonly used by cytogeneticists and cytotaxonomists alike. More than 40 years ago it was recognized that simple treatments such as chilling changed the structure of plant metaphase chromosomes so predictably that the constrictions produced served as markers for species identification.¹,² Metaphase chromosomes, seen with an electron microscope, are a mass of nucleoprotein fibers ranging in thickness from 100 to 200 Å.³ Evidence from Drosophila,⁴ yeast,⁵ and a basidiomycete fungus, Schizophyllum,⁶ indicates that the fibrillar structure is the package in which the DNA duplex molecules are compacted, one molecule per chromatid. In higher plants, assuming uninemy is characteristic of all species, the length of a single DNA duplex molecule can be enormous and the difference in length between species is great. Trillium grandiflorum S., for example, has about 9 pg or 284 cm of DNA per chromatid, while Arabidopsis thaliana with the same number of chromosomes has only 0.04 pg or 1.2 cm per chromatid.^7,8

B. Plant Chromosomal DNA Is Replicated Simultaneously at Multiple Sites

Clues about how plants replicate long chromosomal DNA molecules were gathered over 20 years ago, in the early days of high-resolution autoradiography.^9,10 Cells, labeled with tritiated thymidine at a given time in S phase and subsequently viewed at metaphase, had clusters of silver grains scattered along the length of the chromatids. This finding established two rules governing the replication of chromosomal DNA. First, it showed that chromosome DNA is replicated simultaneously at many but not at all sites. Second, it showed that, at a given time within a given diploid chromosomal complement, segments of different chromosomes are replicated coordinately.

Additional rules governing chromosomal DNA replication awaited the development of new methods with improved resolution. A silver grain produced by the β-ray emitted from tritium has an average diameter of 1 μm. A grain of this size above a metaphase chromosome covers 29 to 37 mm of DNA or about 9 × 10⁷ base pairs (bp).¹¹ A better look at the DNA hidden beneath a silver grain was achieved by autoradiography of tritium-labeled isolated DNA fibers.¹² DNA fibers from lysed nuclei spread on the surface of a glass microscope slide could be viewed by light microscopy.^13–15 This simple procedure improved the resolution of DNA molecules 30,000-fold. A silver grain of 0.8 to 1 μm in diameter located above a DNA fiber covers roughly 3 × 10³ bp. Application of DNA fiber autoradiography to replicating plant chromosomes was delayed because there was no simple quick procedure for isolating undamaged nuclei from plant tissue. This needed procedure now exists and it provides clean, undamaged nuclei that give long, undegraded molecules of chromosomal DNA.^16–18

C. Three Replicon Properties of Plant Chromosomal DNA Seen by DNA Fiber Autoradiography

Taylor applied the term “replicon” to replication units of eukaryotes.¹⁹ The term, first introduced by bacteriologists in reference to the replication unit of the bacterial chromosome,²⁰ emphasizes the similarities of replication units despite differences in phylogeny. The term serves this purpose provided it is recognized that replicons of bacteria and those of higher organisms have one fundamental difference. In bacteria, the entire chromosome constitutes a single replicon but the chromosome of eukaryotes has a multitude of tandem replicons along its longitudinal axis.^13,21-23 All replicons have three properties: an origin where replication begins, two replication forks that diverge from the origin in opposite directions while forming new daughter DNA chains, and a replication rate (chain elongation rate) determined by the speed at which a fork moves while copying the parental molecule. Evidence for these three properties is seen in DNA fiber autoradiograms of replicating plant DNA.

The analysis of grain arrays on an autoradiogram of labeled DNA fibers is summarized diagrammatically in Figure 1. The lines noted as A, B, and C represent DNA duplex molecules with origins (the “O’s”) spaced at regular intervals along their lengths. Each row of arrows immediately below each line indicates arrays of contiguous silver grains produced by the incorporation of tritiated thymidine into newly replicated DNA. The arrowheads give the direction of fork movement. The space between the arrowshafts of diverging forks represents DNA replicated before the radioactive pulse. Where no space exists between arrows and where the arrow has two heads, replication began during the pulse. In this instance, a single array of contiguous grains represents the movement of two diverging forks (line C; the 3 arrows located at the righthand side).

When two neighboring replicons start replication at the same time before the pulse, the movement of the four forks along the parental chains is recorded by four arrays of grains that appear as sequentially aligned nearly symmetrical pairs of labeled segments (Figure 1 ; the arrows beneath lines A and B). The length of the individual labeled segments is determined by the speed at which the forks traveled during the pulse, the pulse duration, and the time of replication initiation. A plot of the average length of each labeled segment of each pair of grain arrays expressed as a function of pulse duration produces a curve with an initial slope corresponding to the average rate of a si...

Table of contents

• Cover Page
• Title page
• Copyright page
• fmchapter
• bio
• fmchapter
• 1 Functional Chromosomal Structure: The Replicon
• 2 The Biochemistry of DNA Replication
• 3 Control of DNA Replication
• 4 Replication of Plant Organelle DNA
• 5 Replication of DNA Viruses in Plants*
• 6 DNA Damage, Repair, and Mutagenesis