Bacteriophages are significantly smaller than the bacteria they infect. In terms of volumetric comparison, a typical bacteriophage particle occupies approximately 1/1000th of the volume of its host bacterial cell, reflecting the extreme size disparity between these obligate intracellular parasites and their prokaryotic hosts.
842
How many distinct replication cycles are commonly identified for bacteriophages?
Bacteriophages primarily exhibit two distinct replication strategies: the lytic cycle and the lysogenic cycle. In the lytic cycle, the virus replicates rapidly and kills the host cell. In the lysogenic cycle, the viral genome integrates into the host genome and remains dormant, replicating only when the host cell divides, until environmental triggers induce a switch to the lytic cycle.
843
What happens to the T-lymphocyte count when an individual is affected by HIV?
HIV specifically targets and infects CD4+ T-lymphocytes, which are crucial components of the human immune system. As the virus replicates within these cells, it leads to their destruction. Consequently, the progressive decline in the count of CD4+ T-lymphocytes is a primary indicator of HIV disease progression and the eventual development of AIDS.
844
Which structure is produced by bacteria and located external to the cell wall?
The capsule is an extracellular structure secreted by many bacteria that lies outside the cell wall. It provides protection against desiccation, environmental toxins, and host immune responses, and it is distinct from the plasma membrane which is located internal to the cell wall.
845
Which plant organ is primarily responsible for providing structural support to the leaves?
The stem serves as the primary axis of the plant, providing structural support for leaves, flowers, and fruits. It also acts as a conduit for the transport of water and minerals from the roots to the leaves, and organic nutrients from the leaves to other parts of the plant.
846
What structure gives rise to the branches of a plant?
The trunk or main stem acts as the primary axis from which lateral branches emerge. These branches develop from axillary buds located at the nodes of the stem, allowing the plant to expand its canopy and optimize light interception for photosynthesis.
847
Which of the following plant parts typically grows underground?
A rhizome is a modified underground stem that grows horizontally. It serves as a storage organ for starch and proteins, allowing the plant to survive adverse conditions and facilitating vegetative reproduction by producing new shoots and roots from its nodes.
848
At what temperature is the embryo of a seed typically considered to be injured?
Thermal injury to plant embryos varies by species, but 54 degrees Celsius is often cited in agricultural studies as a critical threshold where protein denaturation and metabolic disruption lead to significant embryo damage in many common seeds.
849
Which category of plants is characterized by seeds containing a single cotyledon?
Monocotyledonous plants, or monocots, are a major group of flowering plants defined by the presence of a single embryonic seed leaf, known as a cotyledon. This feature is a primary taxonomic characteristic used to distinguish them from dicotyledonous plants, which possess two cotyledons. Monocots also typically exhibit parallel leaf venation and floral parts in multiples of three.
850
Which of the following terms describes a seed that possesses two embryonic leaves?
Dicotyledonous seeds, or dicots, are characterized by the presence of two cotyledons within the embryo. This is a primary taxonomic feature used to distinguish them from monocotyledons, which possess only a single cotyledon.
