Question

What genetic process is occurring in a puff of a polytene chromosome?

Short answer

Answer: The genetic process occurring in a puff of a polytene chromosome is gene expression.

Step-by-step solution

Understand Polytene Chromosomes

Polytene chromosomes are a type of chromosome found in certain cells of certain organisms, such as the salivary glands of the fruit fly (Drosophila melanogaster). These chromosomes are formed by repeated rounds of DNA replication without cell division (endoreplication), resulting in many copies of the chromosome lying side by side, giving the appearance of a thick, banded structure when viewed under a microscope.

Identify the Puff

In the polytene chromosome, puffs are the regions where significant gene activity takes place. They appear as swollen, less-condensed regions along the chromosomes, which usually correspond to active transcription and loading of specific RNA polymerase complexes on the DNA.

Explain the Genetic Process Occurring in a Puff

The genetic process that occurs in a puff of a polytene chromosome is gene expression. It is a process in which genetic information in a gene (DNA) is transcribed into messenger RNA (mRNA) and subsequently translated into functional proteins. Puffs indicate regions where transcription is active, and genes are being expressed, producing specific RNAs for cellular functions such as protein synthesis or other cellular processes. In conclusion, the genetic process occurring in a puff of a polytene chromosome is gene expression. The puff represents an area of active transcription and translation, where genetic information in the DNA is being converted into functional proteins to support various cellular processes.

Key concepts

Gene Expression

Gene expression is the process by which genetic information stored in DNA is converted into functional products, such as proteins. This process is fundamental to the functioning of living cells because the gene products, often proteins, carry out most cellular activities. Every cell in an organism contains the same DNA; however, not all genes are expressed in all cells. This selective gene expression is what allows cells to perform specific functions.
The regulation of gene expression ensures that genes are expressed at the right time, in the right cell type, and in the appropriate amount. The control of gene expression can occur at multiple stages, including transcription, RNA processing, translation, and post-translational modifications.

• Gene regulation is vital for cellular differentiation, development, and adaptation to environmental changes.
• Examples of processes affected by gene expression include growth, metabolism, and immune response.

In polytene chromosomes, puffs are regions with high gene expression because they are areas of active transcription.

Transcription

Transcription is the first step in gene expression where a specific segment of DNA is copied into RNA, particularly messenger RNA (mRNA), by the enzyme RNA polymerase. This process is essential because it determines which proteins are synthesized in a cell at any given time. During transcription, the DNA unwinds, allowing one of the strands to be used as a template for RNA synthesis.
The process can be divided into three main stages:

• Initiation: RNA polymerase binds to a specific region called the promoter. This triggers the unwinding of DNA and the start of RNA synthesis.
• Elongation: The RNA polymerase moves along the DNA, adding nucleotides to the growing RNA chain.
• Termination: Once the entire gene is transcribed, RNA synthesis stops, and the RNA molecule detaches from the DNA.

In polytene chromosomes, visible puffs demonstrate transcription activity because swollen puff regions indicate sites of active RNA production.

Endoreplication

Endoreplication, also known as endoreduplication, is a unique replication process where cells undergo multiple rounds of DNA replication without subsequent cell division. This results in cells with increased genetic content, such as polytene chromosomes, where many copies of DNA are aligned side by side. This is common in certain tissues like the salivary glands of fruit flies.
Endoreplication leads to the formation of large, easily observable chromosomes with distinct banding patterns.

• These chromosomes facilitate enhanced gene expression by providing multiple copies for transcription.
• Endoreplication plays a role in growth and development of some tissues, allowing cells to increase in size and function without dividing.
• This process is also useful for research, as the large chromosomes allow for detailed genetic studies.

In summary, endoreplication contributes to the generation of polytene chromosomes, providing the structural basis for the observable gene expression activity found in puffs.