Final answer:
The telocentric chromosome is the type with the centromere placed at or very near the end. Chromosomes become visibly lined up on the metaphase plate during metaphase of mitosis and meiosis, and chromosome 21's nondisjunction causes Down Syndrome.
Step-by-step explanation:
The type of chromosome with the centromere at or very near the end is the telocentric chromosome.
In humans, chromosomes are categorized based on the position of their centromere, which is critical for the movement of chromosomes during cell division. Four types of chromosomes are identified based on centromere location: metacentric (middle), submetacentric (slightly off center), acrocentric (near one end), and telocentric (at the very end). Each chromosome consists of two identical copies called sister chromatids, attached at the centromere.
During mitosis, chromosomes become visible under a light microscope at prophase, and they line up at the cell's equator during metaphase. Sister chromatids are separated during anaphase. Additionally, homologous chromosomes undergo crossover and are held together by the synaptonemal complex during meiosis, specifically during prophase I. The nondisjunction of chromosome 21 is known to cause Down Syndrome.