Question

Prokaryotes contain a chromosome, and eukaryotes contain chromosomes. a. single-stranded circular; single-stranded linear b. single-stranded linear; single-stranded circular c. double-stranded circular; doublestranded linear d. double-stranded linear; double-stranded circular

Short answer

Prokaryotes contain a double-stranded circular chromosome, and eukaryotes contain double-stranded linear chromosomes. The correct answer is option (c).

Step-by-step solution

Identifying the type of chromosome in Prokaryotes

Prokaryotes, such as bacteria and archaea, contain their genetic material in a region of the cell called the nucleoid. This genetic material is usually organized as a single, double-stranded and circular chromosome. Therefore, for prokaryotes, the chromosome is double-stranded and circular.

Identifying the type of chromosome in Eukaryotes

Eukaryotes, such as human beings, animals, plants, and fungi, keep their genetic material within a nucleus, which is enclosed by a membrane. Inside the nucleus, DNA is packaged into several chromosomes, which are linear, rather than circular. Moreover, these chromosomes are double-stranded.

Selecting the right option

By comparing options (a) to (d) with the facts determined in Steps 1 and 2, the correct answer is found to be option (c): Prokaryotes contain a double-stranded circular chromosome, and Eukaryotes contain double-stranded linear chromosomes. Hence, option (c) fills the blanks in the statement of the exercise correctly.

Key concepts

Prokaryotic Chromosome

Prokaryotic organisms, such as bacteria, have a distinctive type of chromosome that sets them apart from eukaryotic cells. Their genetic material is organized as a single, circular chromosome that is double-stranded. Understandably, this may seem quite foreign compared to what we usually picture – the linear chromosomes found within the human body.

Now, this circular chromosome is not sprawled haphazardly within the cell. It's neatly held in what's known as the nucleoid region, which, although not an enclosed nucleus, serves a similar purpose in storing and managing the organism's genetic information. This more simplistic chromosome arrangement reflects the generally smaller size and less complex requirements of prokaryotic cells. Moreover, such an arrangement is efficient for prokaryotes, allowing them to rapidly replicate their DNA and divide through processes such as binary fission.

Eukaryotic Chromosome

In contrast to their prokaryotic counterparts, eukaryotic cells, which are found in organisms like humans, animals, plants, and fungi, have a more elaborate chromosome structure. Let's dissect this: eukaryotic chromosomes are linear, not circular, and there are usually multiple copies within a cell. This linearity and abundance reflect the complex nature of eukaryotic life forms.

The term 'double-stranded' is key here as well, delineating the DNA's structure with two complementary strands wound around each other in the iconic double helix. But that's not all. These linear strands of DNA are intricately packaged around proteins called histones forming structures known as nucleosomes. This ensures efficient management of the vast amount of genetic material which, in this meticulous format, can fit within the cell nucleus and support the complex functionalities of eukaryotes.

Nucleoid Region

Focusing on the nucleoid region, we find the distinguishing 'headquarters' of a prokaryotic cell's DNA. Unlike a eukaryotic cell, prokaryotes do not have a defined nucleus enclosed by a membrane. Instead, the nucleoid region is an area within the cell where the chromosome is localized.

This region is not randomly assigned; it's actually dictated by the intricate cellular machinery that’s responsible for compacting the DNA molecule into a supercoiled state. This compaction is essential; without it, the DNA would not fit inside the small prokaryotic cell. Even though it lacks a surrounding membrane, the nucleoid region is super effective, allowing the cell to access the genetic code needed for daily functions and quickly respond to environmental changes. Essentially, this area is a masterpiece of efficiency, wrapping the crucial genetic instructions into a neat little package.

DNA Packaging

DNA packaging is a fascinating process across both kingdoms of life. Imagine trying to fit 20 miles of fishing line into a basketball without any tangles; that's essentially what cells must do with DNA. In prokaryotes, DNA packaging involves the coiling and supercoiling of their single chromosome so that it condenses into the nucleoid region. Proteins play a role here but not to the same complexity as in eukaryotes.

In eukaryotic cells, the challenge of DNA packaging is amplified. Histones, around which DNA winds, are like spools that help in organizing DNA into nucleosomes - fundamental units of chromatin, the material chromosomes are made of. Further folding and coiling of chromatin shapes the distinct chromosome structures visible under a microscope during cell division. This meticulous packaging system is vital as it not only condenses the DNA to fit into the nucleus but also plays a crucial role in gene regulation. Without it, the precise orchestration of gene expression and cellular function would be impossible.