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Chapter 11: Problem 4

Describe how giant polytene chromosomes are formed.

Short Answer

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Answer: Polytene chromosomes are formed to cope with an increased demand for gene expression within specific cells. They are created through a process called endoreplication, where a cell undergoes multiple rounds of replication without cell division. This results in multiple copies of each chromosome within a single cell, which align themselves in parallel arrays to form giant polytene chromosomes. These large chromosomes have distinct banding patterns and increased gene expression capacity, thereby supporting essential processes such as growth, development, and cellular differentiation.

Step by step solution

01

(Understanding Polytene Chromosomes)

Polytene chromosomes are large chromosomes found in certain cells, like the salivary glands of Drosophila larvae. They are formed through a series of replications of a single chromosome without cell division (endoreplication), resulting in many copies of the chromosome arranged in parallel arrays. These thick chromosome structures are visible under the light microscope, allowing researchers to study them in detail. #Step 2: Replication Process#
02

(Formation through Endoreplication)

Polytene chromosomes are formed when there is an increase in the demand for gene expression within a specific cell. To cope with this demand, the cell undergoes multiple rounds of replication without cell division, known as endoreplication. This results in multiple copies of each chromosome being present within a single cell. These copies then align themselves in parallel arrays, which eventually give rise to giant polytene chromosomes. #Step 3: Chromosome Structure and Banding Pattern#
03

(Structure and Banding Pattern)

Each polytene chromosome is composed of many chromatids (up to 1024 in some cases), which are packed together tightly in a parallel arrangement. This high number of chromatids creates a thick, ribbon-like structure that can be easily visualized under a light microscope. Due to the tight packing, alternating bands of more and less condensed chromatin can be observed across the length of polytene chromosomes. These bands and interbands have distinct patterns that are specific to each polytene chromosome and can be used to identify specific regions and genes. #Step 4: Function and Gene Expression#
04

(Function and Gene Expression)

Polytene chromosomes are particularly useful for studying gene expression, as their large size and distinct banding patterns make it easier to visualize specific regions of the chromosome. In addition, the high number of chromatids in polytene chromosomes ensures that multiple copies of each gene are present, thereby increasing the overall gene expression capacity of the cell. This increased gene expression allows the specific tissues that harbor polytene chromosomes to function more efficiently, supporting essential processes such as growth, development, and cellular differentiation.

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Most popular questions from this chapter

While much remains to be learned about the role of nucleosomes and chromatin structure and function, recent research indicates that in vivo chemical modification of histones is associated with changes in gene activity. For example, Bernstein and others (2000. Proc. Natl. Acad. Sci. USA 97: 5340-5345) determined that acetylation of \(\mathrm{H} 3\) and \(\mathrm{H} 4\) is associated with 21.1 percent and 13.8 percent increase in yeast gene activity, respectively, and that yeast heterochromatin is hypomethylated relative to the genome average. Speculate on the significance of these findings in terms of nucleosome- -DNA interactions and gene activity.

Describe the basic structure of a nucleosome. What is the role of histone H1?

In this chapter, we focused on how DNA is organized at the chromosomal level. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: (a) How do we know that viral and bacterial chromosomes most often consist of circular DNA molecules devoid of protein? (b) What is the experimental basis for concluding that puffs in polytene chromosomes and loops in lampbrush chromosomes are areas of intense transcription of RNA? (c) How did we learn that eukaryotic chromatin exists in the form of repeating nucleosomes, each consisting of about 200 base pairs and an octamer of histones? (d) How do we know that satellite DNA consists of repetitive sequences and has been derived from regions of the centromere?

In an article entitled "Nucleosome Positioning at the Replication Fork," Lucchini and others (2002. EMBOJ. 20: 7294-7302) state, "both the 'old' randomly segregated nucleosomes as well as the 'new' assembled histone octamers rapidly position themselves (within seconds) on the newly replicated DNA strands." Given this statement, how would one compare the distribution of nucleosomes and DNA in newly replicated chromatin? How could one experimentally test the distribution of nucleosomes on newly replicated chromosomes?

A particular variant of the lambda bacteriophage has a DNA double-stranded genome of 51,365 base pairs. How long would this DNA be?
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