Botany · Ch 6 — Cell: The Unit of Life
Nucleus
Nucleus
The nucleus is, in every meaningful sense, the control centre of a eukaryotic cell: it is the single largest organelle in the cell, and it is the structure that both controls the cell's overall activities and physically holds its hereditary information. Its outward shape varies from cell to cell - it may be spherical, cuboidal, ellipsoidal or discoidal. It is enclosed by a double-membrane structure called the nuclear envelope, made up of a smooth inner membrane (which carries no ribosomes) and a rougher outer membrane (studded with ribosomes and, at scattered irregular intervals, physically continuous with the endoplasmic reticulum). This envelope is not a sealed barrier - it is perforated all over by nuclear pores, each one ringed by a circular structure called an annulus, and together a pore and its annulus form what is called the pore complex; it is through these pore complexes that essential macromolecules such as messenger RNA, ribosomal subunits and various proteins pass in and out of the nucleus. The narrow space that separates the inner and outer nuclear membranes is called the perinuclear space. Inside the envelope, the whole nuclear interior is filled with a gelatinous matrix called the nucleoplasm, within which sit the loosely organised chromatin network and one or more small, dense, non-membrane-bound nucleoli. Functionally, the nucleus does three connected jobs: it controls all of the cell's activities; it stores the organism's genetic or hereditary information; and it holds the genetic code that the cell uses to actually produce enzymes and pr …
What this figure shows. A rounded organelle bounded by a double-membrane nuclear envelope (outer membrane studded with ribosomes and continuous with the ER, inner membrane smooth) with nuclear pores (each a pore-plus-annulus complex) piercing both membranes; the perinuclear space is shown between the two membranes; inside, the nucleoplasm holds a diffuse, thread-like chromatin network and one or more dens …
Giant Chromosomes
Beyond ordinary chromosomes, certain highly specialised tissues in a few organisms produce unusually large chromosomes, collectively called giant chromosomes, of which two named types are recognised. Polytene chromosomes were first seen by C. G. Balbiani in 1881, in the salivary glands of Drosophila (the fruit fly) larvae; they also occur in the suspensor cells of some plant embryos. In the larvae of many flies, midges (Diptera) and certain other insects, the interphase chromosomes repeatedly duplicate and re-duplicate without any accompanying nuclear division - a process called endomitosis - so that a single original chromosome ends up present as many identical copies bundled together, with the resulting daughter chromatids aligning side by side without ever actually separating; this whole multi-copy bundle is what is called a polytene chromosome, and it is large enough to see under an ordinary light microscope. Polytene chromosomes are genetically active and show a striking, alternating pattern of dark bands (where roughly 95% of the chromosome's DNA is concentrated) and lighter inter-bands (holding only about 5% of the DNA); gene expression, transcription and active RNA synthesis specifically happen at these bands. At certain points, a band can swell outward into an especially large, ballooned structure called a Balbiani ring, or chromosome puff - a visible marker of a site of unusually intense RNA synthesis. Because polytene chromosomes occur specifically in the salivary gland, they are also known as salivary gland chromosomes, and, when the two parental (maternal and paternal) homologous chromosomes stay closely associated side by side within the tissue, that pairing is called somatic pairing. Lampbrush chromosomes are the second named type of giant chromosome, first observed by Walther Flemming in 1882. They occur specifically at the diplotene stage of the first meiotic prophase, inside the oocytes (developing egg cells) of animals such as the salamander, and also in the giant nucleus of the unicellular alga Acetabularia. Structural …
What this figure shows. Two giant-chromosome diagrams. Polytene chromosome: a thick chromosome built of many aligned, unseparated chromatid copies (endomitosis), showing alternating dark bands (about 95% of the DNA) and lighter inter-bands (about 5%), with one region ballooned outward into a large Balbiani ring / chromosome puff marking a site of intense RNA synthesis. Lampbrush chromosome: a central, paired chromosomal axis made of chromonemata fibres and matrix, from which many thin, paired lateral loops of DNA project outward on either side, giving the chromosome a bristly, brush-like outline; the labels chromomeres (bead-like thickenings along the …