Final answer:
A mature erythrocyte is characterized by lacking a nucleus, not having mitochondria, and having a biconcave disc shape which facilitates gas exchange.
Step-by-step explanation:
The characteristic of a mature erythrocyte, often referred to as a red blood cell, is that it lacks a nucleus. During their maturation process in the red bone marrow, erythrocytes extrude their nucleus to make room for more hemoglobin, which is crucial for their primary function of transporting oxygen and carbon dioxide. A mature erythrocyte also does not have mitochondria, relying instead on anaerobic respiration, which ensures that they deliver all the oxygen they carry to the tissues without using any themselves. The shape of the erythrocyte is a biconcave disc which maximizes the surface area to volume ratio, aiding in efficient gas exchange and allowing the cells to deform as they pass through narrow capillaries.