Problem 44 - Objective Biology for NEET

Question

What is a genomic DNA of bacteria? (a) Circular DNA (b) Single stranded DNA (c) Both (a) and (b) (d) ds DNA, linear

Step-by-Step Solution

Answer

The genomic DNA of bacteria is 'circular DNA'. Therefore, the correct answer is (a) 'Circular DNA'.

Step 1: Discard Incorrect Choices

To narrow down the correct answer, first dismiss the options that are evidently incorrect. In this case, we know that DNA is typically double-stranded, not single. So we can disregard Option (b) 'Single stranded DNA'. Some DNA like those present in viruses can be single-stranded, but typically bacterial DNA is double-stranded.

Step 2: Identify Correct Choice

From the remaining options, you should identify the correct one based on your understanding. Option (d) 'ds DNA, linear' is not correct as bacterial DNA is not linear. Option (a) 'Circular DNA' is correct. Bacterial DNA differentiates from eukaryotic DNA in that it is generally circular, non-compartmentalized, and does not have associated proteins. So, Option (a) is the correct choice.

Step 3: Confirm Answer

Upon re-evaluation, it is clear that 'circular DNA' is the right description of bacterial DNA. So, Option (a) is correct and Option (c) 'Both (a) and (b)' also becomes incorrect due to the inclusion of (b).

Key Concepts

Genomic DNA

Genomic DNA refers to the complete set of an organism's DNA, containing all the biological information needed to build and maintain that organism. In bacteria, genomic DNA is particularly unique compared to the genomic DNA found in eukaryotes, like humans or plants. Bacterial genomic DNA is typically found in a single, circular chromosome, which encompasses the organism's entire genetic blueprint. This stands in contrast to the multiple linear chromosomes of eukaryotic cells.

The structure of bacterial genomic DNA, being circular, ensures that it is tightly packed within a relatively small cellular space. This arrangement is efficient and helps bacteria to replicate their DNA rapidly during cell division. As it contains essential genes for survival, any changes or mutations within the genomic DNA can have significant effects on the bacterial cell's functions.

Circular DNA

Circular DNA is a common form of DNA in prokaryotes, such as bacteria. Unlike eukaryotes, which typically have linear DNA within their cell nuclei, bacteria house their DNA in the cytoplasm in a circular form. This circular DNA is also referred to as a plasmid when it contains non-essential genes that can be exchanged between bacteria, offering an adaptive advantage in certain environments.

The circular nature of bacterial DNA is advantageous for several reasons. It allows for more efficient replication due to the absence of 'end problems' faced by linear DNA during replication. In contrast to linear DNA that has ends (telomeres) which can pose problems for the replication machinery, circular DNA does not have ends, allowing the replication enzymes to continuously copy the DNA without interruption. This feature contributes to the high replication speed often observed in bacterial cells.

Double-Stranded DNA

Double-stranded DNA (dsDNA) is the standard structure of DNA, composed of two complementary strands that wind around each other to form a double helix. Each strand is a long polymer made of repeating units called nucleotides, and each nucleotide comprises a sugar, a phosphate group, and a nitrogenous base.

The two strands are held together by hydrogen bonds between the bases: adenine pairs with thymine, and cytosine pairs with guanine. This base-pairing rule is crucial for the process of DNA replication, during which the two strands separate and serve as templates for the creation of two new complementary strands. In bacteria, the double-stranded DNA in its circular chromosome is supercoiled, meaning that the circular DNA helix is further twisted upon itself, making it more compact and easier for the cell to manage during replication and cell division.