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Chapter 12: Problem 6

Describe the structure of lampbrush chromosomes. Where are they located?

Short Answer

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Answer: The primary function of lampbrush chromosomes during the meiotic prophase of oocyte nuclei is to synthesize large amounts of RNA required for the early development of the embryo after fertilization.

Step by step solution

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1. Definition of Lampbrush Chromosomes

Lampbrush chromosomes are a specific type of chromosome found during the meiotic prophase of oocyte nuclei in animals. They are named "lampbrush chromosomes" due to their appearance, which is similar to a test tube brush or a lamp cleaner.
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2. Structure of Lampbrush Chromosomes

Lampbrush chromosomes consist of chromatin fibers that are organized into loops called chromomeres. These chromomeres extend out from the chromosome axis, forming a brush-like structure. Each chromomere loop consists of DNA, RNA, and proteins. In each loop, transcription takes place, meaning that the DNA in the loop is used as a template to synthesize RNA. This process produces numerous RNA transcripts along the loop, which leads to the formation of structures called RNP-matrix. The RNP-matrix consists of RNA and proteins and is responsible for the lampbrush appearance.
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3. Loops and Chromomeres

Loops of lampbrush chromosomes can be categorized as either simple or complex loops. Simple loops consist of a single chromomere, while complex loops are composed of two or more chromomeres. The size and shape of the loops can vary, with some being short and thick, and others being long and thin.
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4. Functions of Lampbrush Chromosomes

Lampbrush chromosomes play a significant role in the synthesis of RNA during the meiotic prophase of oocyte nuclei. They enable the oocyte to produce vast amounts of RNA that will be required for the early development of the embryo after fertilization. This high level of transcriptional activity is unique to lampbrush chromosomes compared to other types of chromosomes.
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5. Location of Lampbrush Chromosomes

Lampbrush chromosomes are located within the oocyte nucleus during the meiotic prophase I in the germinal vesicle of animals, including amphibians, birds, and some mammals. They are not present in somatic cells, and their appearance is mostly limited to oocytes that are in the process of development.

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Most popular questions from this chapter

Mammals contain a diploid genome consisting of at least \(10^{9}\) bp. If this amount of DNA is present as chromatin fibers, where each group of 200 bp of DNA is combined with 9 histones into a nucleosome and each group of 6 nucleosomes is combined into a solenoid, achieving a final packing ratio of \(50,\) determine (a) the total number of nucleosomes in all fibers, (b) the total number of histone molecules combined with DNA in the diploid genome, and (c) the combined length of all fibers.

Tandemly repeated DNA sequences with a repeat sequence of one to six base pairs-for example, (GACA) \(_{n}\) -are called microsatellites and are common in eukaryotes. A particular subset of such sequences, the trinucleotide repeat, is of great interest because of the role such repeats play in human neurodegenerative disorders (Huntington disease, myotonic dystrophy, spinal- bulbar muscular atrophy, spinocerebellar ataxia, and fragile-X syndrome). Following are data (modified from Toth et al., 2000 ) regarding the location of microsatellites within and between genes. What general conclusions can be drawn from these data? $$\begin{array}{lcc}\text { Taxonomic Group } & \text { Within Genes } & \text { Between Genes } \\ \text { Primates } & 7.4 & 92.6 \\ \text { Rodents } & 33.7 & 66.3 \\ \text { Arthropods } & 46.7 & 53.3 \\ \text { Yeasts } & 77.0 & 23.0 \\ \text { Other fungi } & 66.7 & 33.3\end{array}$$

Examples of histone modifications are acetylation (by histone acetyltransferase, or HAT), which is often linked to gene activation, and deacetylation (by histone deacetylases, or HDACs), which often leads to gene silencing typical of heterochromatin. Such heterochromatinization is initiated from a nucleation site and spreads bidirectionally until encountering boundaries that delimit the silenced areas. Recall from earlier in the text (see Chapter 4 ) the brief discussion of position effect, where repositioning of the \(w^{+}\) allele in Drosophila by translocation or inversion near heterochromatin produces intermittent \(w^{+}\) activity. In the heterozygous state \(\left(w^{+} / w\right),\) a variegated eye is produced, with white and red patches. How might one explain position-effect variegation in terms of histone acetylation and/or deacetylation?

Describe the transitions that occur as nucleosomes are coiled and folded, ultimately forming a chromatid.

What chemical and structural properties of histones enable them to successfully package eukaryotic DNA? What is chromatin remodeling, and how is it controlled within eukaryotic cells?
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